研究者データベース

大野 宗一(オオノ ムネカズ)
工学研究院 材料科学部門 マテリアル設計分野
教授

基本情報

所属

  • 工学研究院 材料科学部門 マテリアル設計分野

職名

  • 教授

学位

  • 博士(工学)(北海道大学)

ホームページURL

科研費研究者番号

  • 30431331

ORCID ID

J-Global ID

研究キーワード

  • 計算材料科学   組織制御   シミュレーション   構造用材料   凝固   粒成長   

研究分野

  • ナノテク・材料 / 金属生産、資源生産
  • ナノテク・材料 / 金属材料物性
  • ナノテク・材料 / 材料加工、組織制御

職歴

  • 2019年04月 - 現在 北海道大学 大学院工学研究院 材料科学部門 教授
  • 2009年11月 - 2019年03月 北海道大学 大学院工学研究院 材料科学部門 准教授
  • 2007年10月 - 2009年10月 北海道大学 大学院工学研究院 材料科学部門 助教
  • 2006年02月 - 2007年09月 北海道大学 量子集積エレクトロニクス研究センター 研究員 量子集積エレクトロニクス研究センター 博士研究員
  • 2004年04月 - 2006年01月 クラウスタール工科大学 金属工学研究所 研究員

学歴

  • 2001年04月 - 2004年03月   北海道大学   大学院工学院   物質工学専攻 博士課程
  • 1999年04月 - 2001年03月   北海道大学   大学院工学研究科   物質工学専攻 修士課程
  • 1995年04月 - 1999年03月   北海道大学   工学部

所属学協会

  • 日本機械学会   日本鉄鋼協会   応用物理学会   Materials Research Society   日本金属学会   The Minerals Metals and Materials Society (TMS)   

研究活動情報

論文

  • Eisuke Miyoshi, Munekazu Ohno, Yasushi Shibuta, Akinori Yamanaka, Tomohiro Takaki
    Journal of Materials Science 2022年09月07日
  • Geunwoo Kim, Ryo Yamada, Tomohiro Takaki, Yasushi Shibuta, Munekazu Ohno
    Computational Materials Science 207 2022年05月 
    A machine leaning-based approach is proposed for the inverse analysis of the anisotropy parameters of solid–liquid interfacial free energy. The interface shape distribution (ISD) map, which characterizes the details of the dendrite morphology, was selected as the input of a convolutional neural network (CNN). The ISD maps for a free-growing dendrite during the isothermal solidification of a model alloy system were obtained by quantitative phase-field simulations and used as the training and test data for the CNN. Two anisotropy parameters were estimated with errors of less than 5%, which can be further improved by increasing the size of the training dataset.
  • Ryo Yamada, Mikihiro Kudo, Geunwoo Kim, Tomohiro Takaki, Yasushi Shibuta, Munekazu Ohno
    Computational Materials Science 204 2022年03月 
    Dendrite morphology has a significant effect on solute segregation and fluid flow in bulk metallic materials. Therefore, the detailed morphological evolution of dendrites is important to better understand these processes. Recently, three-dimensional (3D) dendrite morphology has been analyzed using interface shape distribution (ISD) maps that are spanned by the curvedness and shape factor of the local solid−liquid interface in an Al−Cu alloy. Data were collected through in-situ observations using synchrotron radiation imaging techniques [J.W. Gibbs et al.: Sci. Rep., 5 (2015) 11824]. This methodology is quite effective for describing 3D dendrites. In this study, we thoroughly investigated the morphological evolution and the related ISD map of free-growing equiaxed dendrites in an Al−3mass%Cu alloy using a quantitative phase-field model. The ISD was found to differ depending on the degree of undercooling. Importantly, the results indicate the presence of a time-invariant feature after sufficient branching and growth of secondary arms, when the degree of undercooling is substantial enough to produce a bunch of branching. The time invariance is considered a universal feature of equiaxed dendrite growth.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki
    Computational Materials Science 203 111143 - 111143 2022年02月
  • Eisuke Miyoshi, Munekazu Ohno, Yasushi Shibuta, Akinori Yamanaka, Tomohiro Takaki
    Materials and Design 210 2021年11月15日 [査読有り]
     
    Utilizing the data assimilation and multi-phase-field grain growth model, this study proposes a novel framework of measuring anisotropic (nonuniform) grain boundary energy and mobility. The framework can evaluate a large number of boundary properties from typical observations of grain growth without requiring specifically designed experiments or calculations. In this method, by optimizing the multi-phase-field model parameters such that the simulation results are in good agreement with the observation data, the energies and mobilities of multiple individual boundaries are directly and simultaneously estimated. To validate the method, numerical tests on boundary property estimation were performed using synthetic microstructure dataset generated from grain growth simulations with a priori assumed property values. Systematic tests on simple tricrystal systems confirmed that the proposed method accurately estimates each boundary energy and mobility within an error of only several % of their assumed true values even for conditions with strong property anisotropy and grain rotation. Further numerical tests were conducted on a more general multi-grain system, showing that our method can be successfully applied to complicated polycrystalline grain growth. The obtained results demonstrate the potential of the proposed method in extracting a large dataset of grain boundary properties for arbitrary boundaries from actual grain growth observations.
  • Yuhi Nagatsuma, Munekazu Ohno, Tomohiro Takaki, Yasushi Shibuta
    Nanomaterials (Basel, Switzerland) 11 9 2021年09月06日 [査読有り][招待有り]
     
    Temperature dependence of solid-liquid interfacial properties during crystal growth in nickel was investigated by ensemble Kalman filter (EnKF)-based data assimilation, in which the phase-field simulation was combined with atomic configurations of molecular dynamics (MD) simulation. Negative temperature dependence was found in the solid-liquid interfacial energy, the kinetic coefficient, and their anisotropy parameters from simultaneous estimation of four parameters. On the other hand, it is difficult to obtain a concrete value for the anisotropy parameter of solid-liquid interfacial energy since this factor is less influential for the MD simulation of crystal growth at high undercooling temperatures. The present study is significant in shedding light on the high potential of Bayesian data assimilation as a novel methodology of parameter estimation of practical materials an out of equilibrium condition.
  • Ryo Yamada, Haruki Inubushi, Munekazu Ohno
    Journal of Applied Physics 2021年07月07日 [査読有り]
  • Ryo Yamada, Munekazu Ohno, Tetsuo Mohri
    Progress in Materials Science 120 2021年07月 [査読有り]
     
    Cluster Variation Method (CVM) has been widely employed to calculate alloy phase diagrams. The atomistic feature of the CVM is consistent with first-principles electronic structure calculations, and the combination of CVM with electronic structure calculation enables one to formulate a free energy from the first principles. In the conventional CVM, however, local lattice distortion cannot be introduced into a free energy since the local lattice distortion alters the lattice symmetry from one lattice point to another and entropy formula is not justified. Hence, the phase equilibrium determined by the CVM is still under an excited state. In order to circumvent such a deficiency of the conventional CVM, Kikuchi devised the Continuous Displacement Cluster Variation Method (CDCVM) which is capable of introducing local lattice distortion into the theoretical framework of the CVM by viewing an atom displaced to a quasi-lattice point as hypothetical atomic species located at the Bravais lattice point. Then, additional freedom due to the local atomic displacements is converted to configurational freedom of a multicomponent alloy on a rigid lattice. This idea of conversion of a freedom of local atomic displacements into configurational freedom is extended to other internal freedoms to study collective atomic displacements leading to displacive phase transition and magnetic transition. As an example of the former case, we investigated cubic-tetragonal transition of ZrO2, where two kinds of oxygen atoms, one undergoes upward shifting and the other downward shifting associated with the tetragonal transition, are distinguished as different atomic species. This way, the cubic-tetragonal transition is treated as the binary phase equilibria of the two kinds of oxygens on a cubic lattice surrounded by fcc lattice of Zr. In the latter case of magnetic transition, we calculated Curie temperature of collinear magnetic system by viewing various magnetic spin moments of up and down spins as different atomic species. Hence, Curie temperature is determined as the temperature at which long range order of a multicomponent system vanishes. For both the cases, first-principles calculations of the transition temperature are attempted by introducing electronic structure calculations into the free energy, and reasonable values are obtained. Path Probability Method (PPM) has been known as a natural extension of the CVM to a time domain. PPM is utilized to investigate transition, relaxation and steady state kinetics upon the temperature change. Hence, the CVM and PPM are coherent to perform synthetic study from initial non-equilibrium to final equilibrium states. One of the serious problems of PPM is the fact that the number of variables involved in a Path Probability Function, which is corresponding to a free energy of the CVM, becomes formidably large with higher order approximations. This is because PPM deals with transition path from one configuration to another, hence one needs to consider all atomistic paths which connect two configurations at time t and t+Δt. For kinetics studies, three mechanisms should be distinguished; spin-flipping kinetics, direct exchange kinetics and vacancy mediated kinetics. Among these, most studies of PPM have focused on spin flipping kinetics but vacancy mediated kinetics is most commonly observed for metals and alloys. The number of variables to formulate vacancy mediated kinetics is much larger than that of spin flipping and direct exchange kinetics. The authors developed a new formalism of time transition variables, known as path variables, and this facilitates to apply PPM for various vacancy mediated kinetics phenomena. The basic idea behind the new formalism originates from the spirit of CDCVM to convert internal freedom to configurational freedom. And the numerous transition paths from one configuration to another are dealt in the framework of configurational freedom. Among various calculations of relaxation processes, two examples are introduced. One is associated with a cubic-tetragonal displacive phase transition in ZrO2 for which equilibrium states are determined by CDCVM as described above. For this, we employed a high order approximation with spin flipping kinetics. The other is the relaxation processes associated with up- and down-quenching in a Ni3Al ordered phase within the tetrahedron approximation. The vacancy mechanism is fully considered in this case, and the effects of vacancy concentration on the relaxation behavior are investigated. In the present article, recent progress of CDCVM and PPM are reviewed.
  • Jaehoon Lee, Munekazu Ohno, Yasushi Shibuta, Tomohiro Takaki
    Journal of Crystal Growth 558 2021年03月15日 [査読有り]
     
    The steady-state value of primary dendrite arm spacing (PDAS) in the columnar dendrites growing between the converging and diverging grain boundaries is investigated by means of quantitative phase-field simulations. The simulations show that there is a unique value of PDAS under a given solidification condition in the system with grain boundaries. This is in contrast to existence of allowable range of PDAS under a given solidification in a system without the grain boundaries, i.e., an infinitely large columnar grain investigated in many early works. Such a unique value of PDAS depends on the pulling speed and inclination angle of the crystal, but not on the initial condition; that is, it is independent of the history of solidification condition. The dependences of the unique value on the pulling speed and inclination angle qualitatively agree with the theoretical models.
  • Genki Saito, Tianglong Zhang, Norihito Sakaguchi, Munekazu Ohno, Kiyotaka Matsuura, Masayoshi Takeuchi, Taichi Sano, Koki Minoguchi, Takuya Yamaoka
    Materialia 15 2021年03月 [査読有り]
     
    Because abnormal grain growth (AGG) degrades mechanical properties of industrial polycrystalline materials such as steel, understanding and controlling AGG are important. In this study, AGG of Al-Nb-microalloyed low-carbon steel was investigated at 1100°C using in-situ scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). Owing to the pinning particles of AlN and Nb(C,N), fine austenite grains formed initially, and AGG appeared owing to the dissolution of the pinning particles at high temperatures. Relatively large grains invaded the surrounding smaller grains with a size ratio of approximately 0.3, resulting in AGG. We developed the in-situ observation method to investigate the AGG of the carburization process using a diffusion couple of high- and low-carbon steels. The carbon diffusion into the low-carbon steel from the high-carbon steel enhanced the grain growth of the low-carbon steel. Although the detailed mechanism is still unclear, we clearly showed that carburization can reduce the pinning force. When focusing on the misorientation of the grain boundary, grain boundaries with misorientation angles of 50–59° remained during AGG, which is explained by their lower mobility owing to their lower grain boundary energy. These results suggest that the difference in grain-boundary mobility can induce duplex grain growth with a bimodal distribution, resulting in AGG.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki, Charles-Andre Gandin
    MATERIALIA 15 104 - 113 2021年03月 [査読有り][通常論文]
     
    © 2018 Competitive grain growth during the directional solidification of a polycrystalline binary alloy is investigated by performing systematic three-dimensional large-scale phase-field simulations with the GPU supercomputer TSUBAME2.5 at the Tokyo Institute of Technology. Contrary to two-dimensional investigations, in which an unusual growth of unfavorably oriented (UO) grains has been observed frequently in preference to favorably oriented (FO) ones, the grain selection in the present three-dimensional simulations follows essentially the Walton and Chalmers model, in which FO grains are predominant. The UO dendritic grains persist for longer times than the UO cellular grains, and the FO dendritic grains remain smaller than the FO cellular ones. The change in the number of grains follows a power law, with an exponent that is much lower than that of the Kolmogorov's model describing a purely geometrical growth selection.
  • Genki Saito, Norihito Sakaguchi, Munekazu Ohno, Kiyotaka Matsuura, Masayoshi Takeuchi, Taichi Sano, Koki Minoguchi, Takuya Yamaoka
    ISIJ International 61 6 1964 - 1970 2021年 [査読有り]
     
    This study deals with austenite grain growth during high-temperature carburization of an Al- and Nb-microalloyed case-hardening steel. The grain size after carburization-simulated heating for 5 h at 1 050°C decreased with the increase in the cooling rate from hot forging-simulated heating for 1 h at 1 250°C. The increase in cooling rate led to the decreases in the volume fractions and sizes of AlN and Nb(C,N) particles precipitated during cooling, and AlN disappeared when the cooling rate increased to 16°C/min, while Nb(C,N) still slightly exited at 16°C/min. Because of oversaturation caused by cooling within a finite time, further precipitation occurred during the subsequent normalization for 3 h at 1 070°C, resulting in the formation of AlN–Nb(C,N) combined particles. When the cooling rate increased, the volume fraction and number density of these combined particles increased while their size decreased. Therefore, a higher cooling rate causes a larger pinning effect on grain growth during carburization; thus grain size after carburization decreased with the increase in cooling rate. Transmission electron microscopy confirmed the formation of a coherent AlN–Nb(C,N) interface due to good lattice matching between the crystal planes of AlN (1120) and Nb(C,N) (220). This led to the preferential nucleation of AlN on the Nb(C,N) particles, thereby forming stable AlN–Nb(C,N) particles.
  • Eisuke Miyoshi, Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki
    Computational Materials Science 186 2021年01月 [査読有り]
     
    Three-dimensional grain growth behaviors under anisotropic (misorientation-dependent) grain boundary energy and mobility are investigated via phase-field simulations. Based on a multi-phase-field model and parallel graphics-processing unit computing on a supercomputer, very large-scale simulations with more than three million grains are achieved, enabling reliable statistical evaluation of anisotropic grain growth. The anisotropic boundary properties are introduced by the classical Read-Shockley and sigmoidal models; the threshold misorientation angle, Δθh, included in these models is used as a quantity to determine the anisotropy strength of the system. Systematic simulations are performed for different Δθh values, through which the correlations between the anisotropy strength and grain growth characteristics such as grain size and misorientation distributions are examined. The obtained results show that anisotropic grain growth reaches the steady-state regime irrespective of the Δθh value. However, the kinetics and microstructural morphology during the steady-state growth are largely dependent on Δθh. Furthermore, by comparison with the simulated results, the applicability of analytical grain growth theories to anisotropic systems are tested. The tests reveal that the steady-state microstructure in anisotropic growth cannot be well captured by the existing theories, which is likely because the basic assumptions of the theories do not hold for anisotropic systems.
  • Munekazu OHNO
    ISIJ International 60 12 2745 - 2754 2020年12月15日 [査読有り][招待有り]
     
    This review presents the development of quantitative phase-field models for simulating the formation processes of solidification microstructures, with particular attention to the theoretical foundation and progress in modeling. The symmetry of interpolating functions required to reproduce the free-boundary problem in the thin-interface limit and the necessity of antitrapping current in the diffusion equation are discussed. In addition, new cross-coupling in the phase-field equation for two-sided asymmetric diffusion is briefly described. Recent achievements of large-scale simulations using high-performance computing techniques are explained. Furthermore, some important applications of quantitative phase-field simulations such as investigations of cellular and dendritic growth, microsegregation, and peritectic reaction in carbon steel are discussed.
  • Genki Saito, Norihito Sakaguchi, Munekazu Ohno, Kiyotaka Matsuura, Masayoshi Takeuchi, Taichi Sano, Koki Minoguchi, Takuya Yamaoka
    ISIJ International 60 11 2549 - 2557 2020年11月15日 [査読有り][通常論文]
     
    Effects of fine precipitates on the austenite (γ) grain structures were investigated in JIS SCM420-based case-hardening steels with several different concentrations of the micro-alloying elements and hot-forging temperatures. Micro-alloyed steels of 18Al (0.018 mass% Al) and 35Al–32Nb (0.035 mass% Al, 0.032 mass% Nb) were forging-simulated at 1 150°C or 1 250°C, normalized at 1 070°C, and carburized at 1 050°C. When the as-received 18Al steel was normalized and carburized without forging-simulated heating, a uniform γ grain structure was observed with the distribution of fine AlN precipitates. However, coarsening of AlN occurred when the forging-simulated temperature was 1 150°C and it caused abnormal grain growth during carburization. In 35Al–32Nb steel, the same heating did not induce the abnormal grain growth owing to the AlN–Nb(C,N) combined particles. The size of these particles increase with an increase in the forging-simulated temperature. The high forging-simulated temperature caused the dissolution of the fine precipitates, followed by reformation and coarsening of the precipitates during the subsequent cooling and the normalization heating, which resulted in a decreased pinning force and γ grain coarsening. Furthermore, TEM observations revealed that a considerable amount of Nb(C,N) particles exist near large eutectic MnS particles. Thermodynamic calculations based on the Scheil’s condition showed that the formation of these Nb(C,N) particles was due to segregation during solidification. It was suggested that such local concentration of the precipitate particles in the last solidifying region leads to ununiform distribution of the pinning force that may induce the abnormal grain growth.
  • Toko Tokunaga, Kazushi Sotomoto, Munekazu Ohno, Kiyotaka Matsuura
    Journal of Materials Engineering and Performance 29 10 6355 - 6362 2020年10月 [査読有り][通常論文]
     
    Mg alloys have been known as the lightest metal materials. However, their practical use is strictly limited due to their poor corrosion and wear resistances. To improve those resistances simultaneously, the present authors have fabricated an Al-Si alloy coating on a Mg alloy substrate by hot extrusion. The Al-Si alloy for coating material was prepared by powder metallurgy and casting processes. An AZ80 Mg alloy billet, a compacted Al-Si alloy plate or a cast Al-Si alloy plate, a die and rams were put in bottom of an extrusion container, and the Mg alloy and the Al-Si alloy were indirectly extruded together. By the hot extrusion, the Mg alloy was successfully coated with the Al-Si alloys prepared in both the powder metallurgy and the casting. The Al-Si alloy-coated Mg alloys showed a high corrosion resistance in a 1.0 mol/L HCl aqueous solution. Also, the wear resistance of the Mg alloy was significantly improved by the Al-Si alloy coating layers. The highest wear resistance was obtained with a cast Al-20 vol.%Si alloy coating layer. The coating improved the wear resistance by 50% compared with the AZ80 Mg alloy.
  • Chunwen Guo, Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Tetsuo Mohri
    International Journal of Heat and Mass Transfer 160 2020年10月 [査読有り][通常論文]
     
    Herein, three types of converging grain growth were investigated during directional solidification, through two-dimensional phase-field simulations. The three types were distinguished by the orientation of the favorably oriented (FO) dendrites. The growth orientation of the FO dendrites was parallel to the thermal gradient direction in type I, whereas the orientation of FO dendrites was inclined in the same and opposite lateral directions as those of the unfavorably oriented (UO) dendrites in types II and III, respectively. The largest difference between FO and UO grain inclination angles—that is, the threshold angle, for unusual overgrowth in different alloy systems were clearly different, in type I and III cases. In all three types of converging grain growth, overgrowth of UO dendrites by FO dendrites (usual overgrowth) was observed to occur more frequently than that of FO dendrites being overgrown by UO dendrites (unusual overgrowth). The asymmetry of the inclined dendrite diffusion layer caused different solute interactions in the three types of converging grain growths. Based on the different solute interactions, unusual overgrowth behavior occurred most readily in type III cases—and with most difficulty in type II cases. We also saw that the local GB orientation may alternate between FO and UO grain orientations in type III cases.
  • Toshiyuki Koyama, Munekazu Ohno, Akinori Yamanaka, Tadashi Kasuya, Susumu Tsukamoto
    Materials Transactions 61 11 2047 - 2051 2020年07月17日 [査読有り][通常論文]
     
    A simulation system for the phase transformations and microstructure changes in welded area of steels was built with Materials Integration (MI) concepts. We aimed to build an simulation environment suitable not only for conducting research on microstructure developments and performing high-quality simulations but also for integrating practical and academic viewpoints and insights from materials science and engineering. In particular, the methods discussed in this article, such as the coordination of CCT diagrams and phase field (PF) simulations, and combination between PF methods and cellular automaton method, are typical examples of the MI concept. The detail of framework on the simulation system is explained, comprehensively.
  • Y. Mitsuyama, T. Takaki, S. Sakane, Y. Shibuta, M. Ohno
    IOP Conference Series: Materials Science and Engineering 861 1 2020年06月12日 [査読有り][通常論文]
     
    Permeability is a very important parameter determining fluid dynamics in macroscale casting simulations. For a columnar solidification structure, the permeability has anisotropy and becomes a tensor quantity. In our previous study, we proposed a permeability tensor and confirmed its validity. In this study, the applicability of the permeability tensor is shown through a series of permeability computations that are conducted for some columnar dendritic structures with different morphologies.
  • Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki
    Computational Materials Science 178 2020年06月01日 [査読有り][通常論文]
     
    In this study, two-dimensional large-scale simulation of polycrystalline equiaxed solidification is enabled by applying an active parameter tracking and multiple GPUs computation to the phase-field lattice Boltzmann model, which can simulate growth of multiple dendrites with solid motion, liquid flow, collision and coalescence of multiple solids, and subsequent grain growth. It was confirmed that the developed simulation method shows a reasonable parallel efficiency through scalability evaluations. By using the developed method, showering simulations are performed, in which solid nuclei generated at the top of the computational domain settle down with growth into equiaxed dendrite and deposit on the bottom of the computational domain. In the simulations, a massive number of dendrites, up to 350, is successfully treated.
  • Takahisa Kinoshita, Munekazu Ohno
    Computational Materials Science 177 109558 - 109558 2020年05月 [査読有り][通常論文]
     
    Abnormal grain growth of austenite phase often takes place during carburization of steels, originating from carbon concentration dependence of pinning pressure of fine precipitates, the size of which is typically a few orders of magnitude smaller than the austenite grain size. In this study, the phase-field model for simulating the abnormal grain growth during carburization in steels is developed based on a mean field approximation in which the curvature-driven growth with the pinning pressure due to fine particles is reproduced by explicitly introducing the pinning pressure into the time evolution equations of order parameters. This model is applied to simulations of the austenite grain growth during carburization in Nb-added steels. Simulations are carried out to understand the occurrence of abnormal grain growth at different carburization temperatures for different steel compositions. In addition, an effect of non-uniform distribution of fine particles, which often appears as a consequence of microsegregation, on abnormal grain growth is investigated.
  • Munekazu Ohno, Yukimi Oka, Shinji Sakane, Yasushi Shibuta, Tomohiro Takaki
    Physical Review E 101 5 052121 - 052121 2020年05月 [査読有り][通常論文]
     
    Solid-liquid interfacial properties out of equilibrium provide the essential information required for understanding and controlling solidification microstructures in metallic materials. However, few studies have attempted to reveal all interfacial properties out of equilibrium in detail. The present study proposes an approach for simultaneously estimating all interfacial properties in a pure metal below the melting point on the basis of the Bayesian inference theory. The solid-liquid interfacial energy, interfacial mobility, and anisotropy parameters in pure Fe are estimated by combining molecular dynamics simulation with phase-field simulation using an ensemble Kalman filter, which is a data assimilation technique. Furthermore, the temperature dependences of all interfacial parameters are computed and discussed. In summary, the proposed multiscale approach integrates atomistic and microstructural simulations within the framework of data science and it has considerable potential for a wide variety of applications in materials engineering.
  • Yukinobu Natsume, Yukimi Oka, Jota Ogawa, Munekazu Ohno
    International Journal of Heat and Mass Transfer 150 119222 - 119222 2020年04月 [査読有り][通常論文]
     
    We have recently developed a method to estimate the heat transfer coefficient based on data assimilation. To understand its usefulness for estimating the time-dependent heat transfer coefficient, herein we performed unidirectional casting experiments of Al-1mass%Si alloy and obtained the cooling curves during solidification. The experimental data were then used to validate the estimated time-dependent heat transfer coefficient. Consequently, the measured cooling curves could be accurately simulated, and the average errors between measured and simulated cooling curves were below 0.8%. The estimated time-dependent heat transfer coefficient was 29461.5 Wm−2K−1 at the initial stage of cooling, which rapidly decreased to about 6000 Wm−2K−1 and then gradually decreased to about 1000 Wm−2K−1. These values are reasonable as the heat transfer coefficients for castings of Al base alloys. Additionally, the effect of the setting parameters of the data assimilation were evaluated. It was found that the position of the cooling curve(s) used in the estimation was the most important factor and a cooling curve measured at the position near the surface of the mold should be utilized. This method allows the reasonable estimation of the time-dependent heat transfer coefficient between the mold and molten alloy for unidirectional castings without using trial and error, and experimentally measured cooling curves can be accurately reproduced by solidification analysis.
  • Yasumasa Mitsuyama, Tomohiro Takaki, Shinji Sakane, Yasushi Shibuta, Munekazu Ohno
    Acta Materialia 188 282 - 287 2020年04月 [査読有り][通常論文]
     
    Permeability is a very important multiscale parameter providing information of dendritic structures in macroscale simulations of solidification with liquid flow during casting. In our previous study, it was demonstrated that the permeability of liquid flow normal to a columnar dendritic structure can be well approximated by the Kozeny–Carman (KC) equation with a coefficient kc = 9. In this study, the permeability of liquid flow parallel to a columnar dendritic structure is investigated using a method that combines both phase–field and lattice Boltzmann simulations. It is shown that the permeability of parallel flow can also be approximated by the KC equation with kc = 3. A permeability tensor is then proposed for a columnar dendritic structure. Validity of the permeability tensor is confirmed by simulations of liquid flow in arbitrary directions of a columnar dendritic structure.
  • Yukimi Oka, Munekazu Ohno
    International Journal of Heat and Mass Transfer 149 119232 - 119232 2020年03月 [査読有り][通常論文]
     
    It is very important for production of casts with high quality to predict and control the solidification processes of the alloy. Heat transfer analysis has been utilized for understanding solidification processes. However, it is often difficult to obtain values of all input parameters such as thermal conductivity and heat transfer coefficient precisely. In this study, a parameter estimation method in heat transfer analysis is developed based on data assimilation. In the authors’ previous study, the particle filter, a method of data assimilation, was applied to estimation of thermal conductivity and heat transfer coefficient in heat transfer analysis for mold casting, and its applicability was systematically investigated. It was shown that the particle filter is very effective in estimating these parameters. However, the particle filter suffers from a shortcoming called sample degeneracy which often prevents accurate estimation of parameters in phenomena of interest. The present study focuses on a different method of data assimilation called the ensemble Kalman filter and its applicability to the estimation of heat transfer coefficient and thermal conductivity is investigated based on twin experiments. It is shown that thermal conductivity and constant or time-dependent heat transfer coefficient can be accurately estimated independently with three and two cooling curves, respectively. Furthermore, the thermal conductivity and time-dependent heat transfer coefficient can be estimated simultaneously with high accuracy.
  • Shino Sakow, Toko Tokunaga, Munekazu Ohno, Kiyotaka Matsuura
    Journal of Materials Processing Technology 277 116447 - 116447 2020年03月 [査読有り][通常論文]
     
    A heteromorphic extrusion die with right-angle corners in the die hole was designed in this study to give a high local strain to the extrudate metal during hot extrusion of Al-Si-Fe alloys, and the strain distribution in the extrudate metal was evaluated using the finite element method. It was demonstrated that the change in metal flow direction at the right-angle corners is effective for generating high strain in the extrudate metal and refining brittle inclusion particles such as primary Al9Si2Fe2 (β phase) particles and eutectic Si particles. The refinement of these particles led to significant improvements in tensile elongation and plastic deformability in cold rolling of the alloys.
  • Eisuke Miyoshi, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta
    ISIJ International 60 1 160 - 167 2020年 [査読有り][通常論文]
     
    The phase-field method has been widely employed recently for simulating grain growth. Phase-field grain growth models are classified into two types according to their conservation constraints for phase-field variables: the multi-phase-field model and the continuum-field model. In addition, within the multi-phase-field model framework, three models with different formulations exist. These models are reported to accurately simulate grain growth under conditions of isotropic or weakly anisotropic grain boundary energy and mobility. However, for cases of strongly anisotropic grain boundary properties, the accuracy of these models has not yet been examined in detail. In this study, using the continuum-field model and three different multi-phase-field models, systematic grain growth simulations with anisotropic grain boundary energies and mobilities are performed. Through the detailed investigation of the accuracy of the simulated results, the suitability of each model for anisotropic grain growth simulations is revealed. Furthermore, based on the higher-order terms, accuracy improvement of the phase-field models is attempted.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki
    Computational Materials Science 171 2020年01月 [査読有り][通常論文]
     
    The effects of natural convection on dendrite morphologies formed during solidification of a three–dimensional (3D) bulk crystal have not been clarified yet. Although numerical simulation is key to solve the problem, it is not straightforward from a computational cost point of view to simulate the growth of multiple dendrites and liquid flow simultaneously. In this study, we numerically investigate the effects of natural convection on the dendrite morphologies formed during 3D upward directional solidification of an Al–3 wt%Cu alloy, providing a comparison with two–dimensional (2D) simulations. Large–scale simulations of a model coupling phase–field and lattice Boltzmann methods are performed by parallel computation using multi graphics processing units on a supercomputer. As a result of a series of simulations where gravity is changed, similar results are obtained in the 3D and 2D simulations: the average primary arm spacing increases as the gravity decreases in the negative region, the large upward flow causes unstable dendrite growth, and the downward flow enhances the growth of secondary arms. On the other hand, because the natural convection caused under gravity is less in the 3D than the 2D case, the effect of natural convection in the 3D bulk is smaller than in the 2D. In addition, due to differences in flow patterns in front of dendrite tips, there are some distinctions in dendrite growth between 2D and 3D cases.
  • Toko Tokunaga, Munekazu Ohno
    Journal of Alloys and Compounds 805 436 - 443 2019年10月15日 [査読有り][通常論文]
     
    Mg alloys have been gaining attention as the lightest metal material utilized for improvement of fuel efficiency of automobiles. However, their disadvantages, i.e., poor corrosion resistance and poor plastic deformability limit their application fields. The authors have fabricated Al-coated Mg alloy sheets by newly-developed hot extrusion process followed by hot rolling process. In the authors’ previous study, it was demonstrated that the sheet exhibited superplasticity without peeling-off of the Al coating layer. However, the mechanism of superplastic deformation of the Al-coated Mg alloy sheet has not been clarified yet. Especially, the reason why the pure Al coating was able to be superplastically deformed was not been clearly understood yet. In the present study, therefore, the deformation mechanisms of both the Mg alloy substrate and the Al coating layer have been closely investigated by observing the change of a micro-grid structure drawn on the surface of the sheet based on an electron beam lithography technique, and by carrying out electron backscatter diffraction analysis of the crystal orientations. From the micro-grid observation, it has been shown that the dominant deformation mechanism of the Mg alloy substrate is grain boundary sliding, while that of the Al coating layer is intragranular dislocation slip. Additionally, from the crystal orientation investigation, it has been demonstrated that the texture of the Mg alloy substrate is randomized during the superplastic deformation, which is indicative of the occurrence of grain boundary sliding. On the other hand, in the Mg alloy substrate near the Al/Mg interface, it has been observed that twinning occurs to accommodate shear stress generated by the deformation limit of the Mg alloy by the Al coating layer.
  • G. Saito, Tomoya Nakayama, Norihito Sakaguchi, Munekazu Ohno, Kiyotaka Matsuura, Masayoshi Takeuchi, Taichi Sano, Koki Minoguchi, Takuya Yamaoka
    Materialia 7 100409  2019年09月 [査読有り][通常論文]
     
    The austenite (γ) structure reversely transformed from martensite in Fe–0.55C mass% C alloy was investigated at high temperatures using in-situ an electron backscatter diffraction (EBSD) method. When the heating rate was varied, a rapidly heating produced fine γ grains, in contrast, a slow heating induced coarse γ grains having the same crystal orientations as those of the prior γ grains; known as “γ memory”. The most likely mechanism for the γ memory could be variant restrictions owing to cementite (θ). To investigate the effect of θ on the γ memory, the sample was tempered at 700 °C for 24 h and θ was precipitated. The tempered sample exhibited the γ memory irrespective of the heating rate. To clarify the origin of the γ memory, we focused on the reversely transformed γ formed adjacent to the θ and directly observed the orientation relationships (ORs) using the in-situ EBSD. A crystallographic analysis on the between neighboring ferrites (α), θ, and γ revealed the existence of Kurdjumov–Sachs OR between γ and two α (α1 and α2), Isaichev OR between α and θ, and Pitsch OR between θ and γ. In the all possible combinations, the reversely transformed γ satisfying the K–S OR for two α variants was limited to four variants. The θ variant satisfying the Isaichev OR for α1 and α2 was limited to one variant, which was close to one of the four variants by Pitsch OR, which suggests the γ memory is due to these variant restrictions among α-θ-γ.
  • Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta
    Computational Materials Science 164 39 - 45 2019年06月15日 [査読有り][通常論文]
     
    In solidification of metals and alloys under earth's gravity, the growing free equiaxed dendrites either settle or float because of the difference between the solid and liquid densities. In this study, we developed a two-dimensional computational method that can efficiently simulate the growth of free dendrite settling over a long distance. In the developed method, the growth of the dendrite is expressed by a phase-field method, the liquid flow is computed by a lattice Boltzmann method, and the settling of dendrite is expressed by equations of motion. A moving-frame algorithm is employed to track the dendrite settling over a long distance. The computation is accelerated using a multi-level mesh method and GPU computing. The validity of the developed method was evaluated through simulation of the settling of a single circular particle. Specifically, using the developed method, we simulated the growth of a single dendrite in an isothermal undercooled melt of a binary alloy under gravity, and evaluated the time evolutions of the growth velocity of the primary arms, settling velocity, and crystal orientation of the dendrite. The results confirmed that the developed computational method can efficiently express the dendrite growth and settling over a long distance at a reasonable computational cost.
  • Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki
    Modelling and Simulation in Materials Science and Engineering 27 5 054004  2019年05月28日 [査読有り][通常論文]
     
    Thermosolutal convection inevitably occurs during the solidification of alloys owing to the nonuniform distribution of temperature and/or solute concentration, and this can drastically alter the resulting solidification microstructures. In this study, we present a large-scale simulation scheme for the phase-field lattice Boltzmann model, which can express dendrite growth upon considering the solute, heat transport, and liquid flow. A multiple mesh and time step method was employed to reduce computational costs, where different mesh sizes and time steps are used to solve the phase-field equation, the advection-diffusion equations of heat and solute, and the lattice Boltzmann equations for fluid flow. Furthermore, we implemented parallel computations using multiple graphics processing units (GPUs) to accelerate the large-scale simulation. Through the application of the multiple mesh and time step method, the computation was accelerated by approximately one hundred times compared to the case using a constant mesh and time step for all equations. Moreover, we confirmed that the developed parallel-GPU computation combined with the multiple mesh and time step method could achieve good acceleration and scaling through increasing the number of GPUs. We also confirmed that the developed method could simulate multiple dendrite growth with thermosolutal convection.
  • Eisuke Miyoshi, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Shinji Sakane, Takayuki Aoki
    Modelling and Simulation in Materials Science and Engineering 27 5 054003  2019年05月16日 [査読有り][通常論文]
     
    In this study, assuming an ideal system free from thermal grooving and anisotropy in grain boundary properties, we analyze thin-film grain growth via three-dimensional (3D) phase-field simulations with approximately one million initial grains. The large-scale simulations accelerated by multiple graphics processing units allow for the reliable statistical investigation of grain growth behaviors in films with various thickness. Over the transition from 3D to two-dimensional (2D) growth modes, variations in the averages and distributions of grain sizes are quantified and compared for different regions of the films. Furthermore, we propose a comprehensive scaling law of thin-film grain growth, by which the 3D-2D transition behaviors and grain growth kinetics can be described in a unified manner independent of film thickness.
  • Yasushi Shibuta, Shinji Sakane, Eisuke Miyoshi, Tomohiro Takaki, Munekazu Ohno
    Modelling and Simulation in Materials Science and Engineering 27 5 054002  2019年05月16日 [査読有り][通常論文]
     
    The micrometer-scale polycrystalline microstructure is directly obtained from a 10 billion atom molecular dynamics (MD) simulation of the nucleation and growth of crystals from an undercooled melt, which is performed on a graphics processing unit-rich supercomputer. The grain size distribution in the as-grown microstructure obtained from the MD simulation largely deviates from that resulting from steady-state growth in ideal grain growth, whereas the distribution of the disorientation angle between grains in contact with each other basically agrees with a random distribution. The atomistic configuration of the polycrystalline microstructure is then converted into a phase-field profile (diffuse interface description) of a phase-field model (PFM) and the subsequent grain growth is examined by multi-phase-field (MPF) simulation. A significant achievement in this study is direct mapping of the atomistic configuration into the phase-field profile used in the MPF simulation since only representative parameters for larger-scale model (e.g. interatomic potentials for MD and interfacial parameters for PFM) are extracted from a smaller-scale simulation in conventional multi-scale modeling. Our new achievement supported by high-performance supercomputing can be regarded as an evolution of multi-scale modeling, which we call inter-scale modeling to differentiate it from conventional multi-scale modeling.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta
    Modelling and Simulation in Materials Science and Engineering 27 5 054001  2019年05月07日 [査読有り][通常論文]
     
    Due to the recent acceleration of phase-field simulations using a graphics processing unit (GPU), the mechanism of competitive growth among columnar grains has been better understood. In this study, the effects of natural convection, caused by the gravity driven buoyancy force, on the competitive growth of columnar grains during directional solidification of bi-crystal and polycrystal binary alloys are investigated by performing two-dimensional large-scale phase-field simulations using a GPU supercomputer. As a result, the downward flow accelerates and the upward flow decelerates the selection speed in competitive growth. It is also confirmed that these phenomena are caused by a characteristic liquid flow pattern at the converging and diverging grain boundaries.
  • Geunwoo Kim, Tomohiro Takaki, Yasushi Shibuta, Shinji Sakane, Kiyotaka Matsuura, Munekazu Ohno
    Computational Materials Science 162 76 - 81 2019年05月 [査読有り][通常論文]
     
    Morphological change of equiaxed dendritic structure in fcc-based alloys associated with transition in preferred growth direction was investigated by means of three-dimensional quantitative phase-field simulations. The anisotropy parameters of interfacial energy ε 1 and ε 2 were systematically changed to investigate the growth direction and growth morphology. The growth morphologies can be classified into four types; 〈1 0 0〉 growth, 〈1 0 0〉-like hyperbranched growth, 〈1 1 0〉-like hyperbranched growth and 〈1 1 0〉 growth morphologies. An orientation selection map for these morphologies was constructed in a space spanned by ε 1 and ε 2 . Furthermore, dependence of the orientation selection map on solidification condition and alloy system was investigated. When initial supersaturation is large and/or partition coefficient of alloy is small, the region of 〈1 0 0〉 growth, typical growth direction of fcc alloy, diminishes in the orientation selection map.
  • Munekazu Ohno, Masayoshi Yamashita, Kiyotaka Matsuura
    International Journal of Heat and Mass Transfer 132 1004 - 1017 2019年04月 [査読有り][通常論文]
     
    Microsegregation in Al-Cu and Fe-Mn alloys is analyzed by conducting solidification experiments, two-dimensional (2D) and three-dimensional (3D) quantitative phase-field simulations for two-sided asymmetric diffusion, and using the one-dimensional (1D) finite difference method (FDM). Comparisons of these results substantiate that the 3D quantitative phase-field simulation can predict microsegregation behaviors with a high degree of accuracy. The disagreement between the results of the 1D-FDM simulation and the experiment, which is similar to that often observed in early works, can be ascribed to the fact that 1D-FDM simulation does not consider the details of microstructural evolution. The main factor causing disagreements in microsegregation among the results of 1D, 2D and 3D simulations is discussed in terms of the solid-liquid interfacial area density and its relationship with overlapping of the solute diffusion layer.
  • Tian Long Zhang, Toko Tokunaga, Munekazu Ohno, Mi Lin Zhang, Kiyotaka Matsuura
    Acta Metallurgica Sinica (English Letters) 32 2 169 - 177 2019年02月04日 [査読有り][通常論文]
     
    An Al-coated Mg-8 mass% Li alloy rectangular bar was fabricated by hot extrusion, and then, it was hot-rolled into a thin sheet. The Al coating was uniform in thickness and had good bonding with the substrate during all the processing. This Al-coated Mg–Li alloy exhibited a good corrosion resistance in a 0.5 mass% HCl aqueous solution. No intermetallic compound was observed at the Al/Mg–Li interface after the extrusion and the rolling. The Al-coated Mg–Li alloy sheet exhibited an elongation to fracture of 35% at room temperature at a strain rate of 0.001 s −1 without any debonding between the coating and the substrate. When tensile tested at 573 K at 0.001 s −1 in the air, the Al coating remained undamaged even until an elongation of about 150%. Further elongation generated cracks on the coating and the specimen fractured at an elongation of about 200%. In an Ar atmosphere, the specimen exhibited a fracture elongation of over 400% under the same conditions at 573 K at 0.001 s −1 , although a large number of cracks generated on the Al coating.
  • T. Takaki, S. Sakane, M. Ohno, Y. Shibuta, T Aoki
    Acta Materialia (IF:7.293) 164 237 - 249 2019年02月01日 [査読有り][通常論文]
     
    Computer simulation is the most promising approach for the systematical permeability prediction of liquid flow in the mushy zone for various solidification conditions. In this study, we propose a permeability prediction method by using large–scale simulation of phase–field and lattice Boltzmann methods. Using the proposed method, permeability predictions are performed for the flow normal to the columnar solidification structures with multiple dendrites/cells. In addition, the prediction is also performed for columnar solidification structures with periodically arranged dendrites/cells to evaluate the permeability in the full range of solidification fraction and investigate the effect of primary arm array on the permeability. As a result, it is concluded that the dimensionless permeability using a specific interface area of columnar solidification structures can be well approximated by that of a regular hexagonal array of cylinders. Moreover, it is confirmed that the columnar dendrite/cell structure with a periodic regular hexagonal array provides realistic permeability predictions for columnar solidification structures.
  • Genki Saito, Norihito Sakaguchi, Munekazu Ohno, Kiyotaka Matsuura, Masayoshi Takeuchi, Taichi Sano, Koki Minoguchi, Takuya Yamaoka
    ISIJ International 59 11 2091 - 2098 2019年 [査読有り][通常論文]
     
    The effects of fine precipitates on the austenite grain refinement of micro-alloyed steel during cyclic heat treatment were investigated under different solution treatments. After three rounds of cyclic heat treatment of Ac3 and Ar3 transformations of the as-received steel rod with rapid heating and cooling, the austenite grain size was 3–10 μm. On the other hand, three rounds of cyclic heat treatment after solution treatment at 1 300°C reduced the austenite grain size to 2–5 μm. The as-received sample included AlN–Ti(C,N)–MnS composite particles with a mean diameter of 30 nm and a number density of 11 μm−3, and the mean diameter did not change during cyclic heat treatment. Thus, it was considered that the reduction in austenite grain size without solution treatment was caused by the increase in the nucleation site of austenite phase with increasing number of cycles, due to the refinement of the prior austenite grains with martensitic structure during the cyclic heat treatment. When solution-treated at 1 300°C, the AlN–Ti(C,N)–MnS composite particles were solved, and they were precipitated during the cyclic heat treatment with a mean diameter of 12 nm and an increased number density of 85 μm−3. Thus, it was considered that the further reduction in austenite grain size with solution treatment was caused by the pinning effect of the fine precipitates, in addition to the increase in the number of austenite phase nucleation sites with increasing number of heat treatment cycles.
  • Munekazu Ohno, Yasushi Shibuta, Tomohiro Takaki
    Materials Transactions 60 2 170 - 179 2019年 [査読有り][通常論文]
     
    Production processes of structural materials generally involve a variety of microstructural evolutions, spatiotemporal scales of which are different by several orders of magnitude. In this study, a multi-phase-field model for simulating transformation phenomena at multiple scales is developed by considering mesoscopic kinetics of interest based on diffuse interface description without curvature effect. In particular, the present model is developed for simulations of microstructural evolutions during welding of carbon steels. In this model, the motion of dendrite envelope is described to simulate solidification not at a dendritic scale but at a scale of grain structure. Moreover, the pinning effect is described based on a mean-field approximation, which allows for simulations of grain growth with existence of very fine particles. The present model is applied to two-dimensional simulations for welding processes of carbon steel. The microstructural evolution involving the melting, solidification, austenite grain growth and pinning effect due to very fine particles is demonstrated. [doi:10.2320/matertrans.ME201711]
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno
    Metals 8 12 2018年12月 [査読有り][通常論文]
     
    The occurrence of abnormal grain growth (AGG) of austenite during annealing is a serious problem in steels with carbide and/or nitride particles, which should be avoided from a viewpoint of mechanical properties. The effects of cold deformation prior to annealing on the occurrence of AGG have been investigated. It was found that the temperature range of the occurrence of AGG is shifted toward a low temperature region by cold deformation, and that the shift increases with the increase of the reduction ratio. The lowered AGG occurrence temperature is attributed to the fine and near-equilibrium AlN particles that are precipitated in the cold-deformed steel, which is readily dissolved during annealing. In contrast, coarse and non-equilibrium AlN particles precipitated in the undeformed steel, which is resistant to dissolution during annealing.
  • Munekazu Ohno, Hayato Sato
    International Journal of Heat and Mass Transfer 127 561 - 570 2018年12月 [査読有り][通常論文]
     
    A macrosegregation simulation model is developed by coupling solute and energy conservation equations with Lattice-Boltzmann Method (LBM), newly developing technique of computational fluid dynamics. Effect of the solidification shrinkage is taken into account in the present LBM as well as effects of the Darcy's flow and thermos-solutal convection. The present LBM-coupled model is based on modified lattice Bhadnager-Gross-Krook method, the numerical stability of which is better than that of the standard LBM. Accordingly, the present LBM-coupled model can be applied to simulations of macrosegregation behaviors in metallic alloy systems that cannot be handled by the previous LBM-coupled model. The validity of the model was demonstrated by comparing the results for steady-state flows with those of analytical solutions and a conventional model based on the Navier-Stokes equation. In addition, the computational speed of the present model is compared with the one of conventional model in cases of lateral directional solidification of Sn-Bi alloy and continuous casting of a steel slab. It is shown that the present LBM-coupled model enables remarkably faster computation than the conventional model especially in the latter case.
  • Tianlong Zhang, Toko Tokunaga, Munekazu Ohno, Ruizhi Wu, Milin Zhang, Kiyotaka Matsuura
    Materials Science and Engineering A 737 61 - 68 2018年11月08日 [査読有り][通常論文]
     
    A dual-phase Mg-Li-Zn alloy was processed by a severe plastic deformation method which is a method of combination of extrusion and rolling processes and enables production of a very fine grain structure. After this processing, the Mg-Li-Zn alloy exhibited a significantly large fracture elongation of 1400% at 473 K at 0.001 s−1. Moreover, an elongation of more than 600% was observed at 473 K even at high strain rate of 0.01 s−1. Also, at a lower temperature of 423 K, the alloy exhibited a large fracture elongation of 720% at 0.001 s−1. The values of the strain rate sensitivity were approximated to 0.5, which suggested that the superplastic deformation is based on grain boundary sliding. Dislocation glide is identified to be an accommodation mechanism according to the texture evolution during the superplastic deformation. The different trends of the changes of the textures in the α and β phases indicated an inhomogeneity of grain boundary sliding between phases.
  • Eisuke Miyoshi, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Shinji Sakane, Takashi Shimokawabe, Takayuki Aoki
    Journal of Materials Science 53 21 15165 - 15180 2018年11月01日 [査読有り][通常論文]
     
    Grain growth is one of the most fundamental phenomena affecting the microstructure of polycrystalline materials. In experimental studies, three-dimensional (3D) grain growth is usually investigated by examining two-dimensional (2D) cross sections. However, the extent to which the 3D microstructural characteristics can be obtained from cross-sectional observations remains unclear. Additionally, there is some disagreement as to whether a cross-sectional view of 3D grain growth can be fully approximated by 2D growth. In this study, by employing the multi-phase-field method and parallel graphics processing unit computing on a supercomputer, we perform large-scale simulations of 3D and 2D ideal grain growth with approximately three million initial grains. This computational scale supports the detailed comparison of 3D, cross-sectional, and 2D grain structures with good statistical reliability. Our simulations reveal that grain growth behavior in a cross section is very different from those in 3D and fully 2D spaces, in terms of the average and distribution of the grain sizes, as well as the growth kinetics of individual grains. On the other hand, we find that the average grain size in 3D can be estimated as being around 1.2 times that observed in a cross section, which is in good agreement with classical theory in the stereology. Furthermore, based on the Saltykov–Schwartz method, we propose a predictive model that can estimate the 3D grain size distribution from the cross-sectional size distribution.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki, Charles André Gandin
    Materialia 1 104 - 113 2018年09月 [査読有り]
     
    Competitive grain growth during the directional solidification of a polycrystalline binary alloy is investigated by performing systematic three-dimensional large-scale phase-field simulations with the GPU supercomputer TSUBAME2.5 at the Tokyo Institute of Technology. Contrary to two-dimensional investigations, in which an unusual growth of unfavorably oriented (UO) grains has been observed frequently in preference to favorably oriented (FO) ones, the grain selection in the present three-dimensional simulations follows essentially the Walton and Chalmers model, in which FO grains are predominant. The UO dendritic grains persist for longer times than the UO cellular grains, and the FO dendritic grains remain smaller than the FO cellular ones. The change in the number of grains follows a power law, with an exponent that is much lower than that of the Kolmogorov's model describing a purely geometrical growth selection.
  • Yasushi Shibuta, Munekazu Ohno, Tomohiro Takaki
    Advanced Theory and Simulations 1 9 201800065  2018年09月 [査読有り][招待有り]
     
    The application range of computational metallurgy is rapidly expanding thanks to the recent progress in high-performance computing. In this Progress Report, state-of-the-art collections of large-scale simulations of solidification and grain growth, performed on the GPU supercomputer, are introduced. One of the notable achievements in this direction is a billion-atom molecular dynamics simulation for nucleation and solidification, which revealed the heterogeneity in homogeneous nucleation. Moreover, a series of large-scale phase-field simulations shed light on the topics at issue including competitive growth of dendrites during the directional solidification, the effect of forced and natural convections on the solidification, and so on. Based on simulation results bridging the gap between atomistic and continuum-based simulations, a new criterion of multi-scale modeling is proposed in the age to come. We are now standing at the new era of cross-scale modeling, in which the overlap between atomistic and continuum simulations creates new research concepts and fields.
  • Eisuke Miyoshi, Tomohiro Takaki, Yasushi Shibuta, Munekazu Ohno
    Computational Materials Science 152 118 - 124 2018年09月 [査読有り][通常論文]
     
    To achieve a highly accurate and efficient prediction of polycrystalline grain growth, we propose a method to bridge atomistic and continuum-based simulations by converting molecular dynamics-generated atomic configurations into interfacial profiles of the phase-field model. This method enables us to perform phase-field grain growth simulations in succession to molecular dynamics nucleation simulation. Using the present method, molecular dynamics and phase-field grain growth simulations from the same initial structure are carried out and directly compared. The results of each simulation exhibit a similar tendency in terms of grain morphology and grain growth kinetics, but only after an initial short duration.
  • Toko Tokunaga, Munekazu Ohno, Kiyotaka Matsuura
    Journal of Materials Science and Technology 34 7 1119 - 1126 2018年07月 [査読有り][招待有り]
     
    Poor corrosion resistance is a serious drawback of Mg alloys, restricting their practical applications. Coating is one of the effective techniques for improvement in the poor corrosion resistance. In this paper, the coating processes for Mg alloys so far developed are reviewed. Among several processes, the coating processes based on mechanical energy, including metal forming, are attractive because the corrosion resistance and formability of Mg alloys are simultaneously improved.
  • Shin Okita, Eisuke Miyoshi, Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta
    Acta Materialia 153 108 - 116 2018年07月 [査読有り][通常論文]
     
    Grain growth kinetics under the anisotropic grain boundary properties is investigated by large-scale and long-time molecular dynamics (MD) simulations of contentious processes of nucleation, solidification and grain growth in a submicrometer-scale system. Microstructures obtained via homogeneous nucleation from undercooled melt iron consists of approximately 1500 grains and the number of grains decreases to one tenth of the number via the grain growth process. The grain growth exponent obtained from the MD simulation deviates from the ideal value since anisotropic effects in the grain boundary properties are inherently included in MD simulations. It is confirmed that the decrease of the reduced mobility (i.e., the product of the intrinsic grain boundary mobility and the grain boundary energy) is a dominant factor for the deviation from the ideal grain growth. The deviation from the Mackenzie function for the distribution of the disorientation angle between neighboring grains implies that the preferential selection of grain boundaries with small grain boundary energies occurs during the grain growth. This enhances the anisotropy in grain boundary properties and therefore decreases the reduced mobility of the grain boundary. Moreover, a multi-phase-field simulation starting from a MD configuration results in an ideal grain growth when a constant value of the reduced mobility is employed, which validates the discussion on the reduced mobility. The new insight in this study is achieved for the first time owing to a multi-graphics processing unit (GPU) parallel computation over 50 days for one case using 128 GPUs on the GPU-rich supercomputer.
  • Tomohiro Takaki, Ryotaro Sato, Roberto Rojas, Munekazu Ohno, Yasushi Shibuta
    Computational Materials Science 147 124 - 131 2018年05月 [査読有り][通常論文]
     
    In the formation of a typical equiaxed structure during the solidification of metals and alloys, multiple equiaxed dendrites typically grow with motion, collision, and coalescence and subsequently grain growth occurs after the formation of grain boundaries. In this study, we develop a phase-field lattice Boltzmann model that can simulate these complex formation processes involving equiaxed structures. In this model, multiple dendrites are represented by employing multiple phase-field variables, and the formation of grain boundaries is modeled by simply introducing an interaction term between the phase-field variables. Liquid flow is computed using the lattice Boltzmann method, and the motion of a solid is described by solving the equations of motion. Collision-coalescence representation in the present model was validated by performing simulations of collisions between two circular objects. Furthermore, grain growth was validated through static and dynamic conditions in a simple three-grain system. Good agreements with theoretical solutions were obtained for both cases. Finally, using the developed model, a series of formation processes of multiple-dendrite growth with motion, collision, and coalescence and the subsequent grain growth are successfully performed for the first time.
  • Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Takashi Shimokawabe, Takayuki Aoki
    Journal of Crystal Growth 483 147 - 155 2018年02月01日 [査読有り][通常論文]
     
    Three-dimensional growth morphologies of equiaxed dendrites growing under forced convection, with their preferred growth direction inclined from the flow direction, were investigated by performing large-scale phase-field lattice Boltzmann simulations on a graphical-processing-unit supercomputer. The tip velocities of the dendrite arms with their preferred growth directions inclined toward the upstream and downstream directions increased and decreased, respectively, as a result of forced convection. In addition, the tip velocities decreased monotonically as the angle between the preferred growth direction and the upstream direction increased. Here, the degree of acceleration of the upstream tips was larger than the degree of deceleration of the downstream tips. The angles between the actual tip growth directions and the preferred growth direction of the dendrite arms exhibited a characteristic change with two local maxima and two local minima.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno
    Metals 8 11 2018年 [査読有り][通常論文]
     
    The annealing behavior of surface-locally cold-deformed 0.2 mass% carbon steel with a large strain gradient was investigated, with special attention paid to the change in grain size. The surface local deformation was introduced by a ball-dropping (BD) model experiment. The local plastic strain profile evaluated from pure iron was used to confirm the occurrence of surface local deformation. It was found that the BD test led to severe local deformation near the surface, with a large strain gradient. Both the ferrite and as-transformed austenite exhibited a gradual change in grain size along the depth direction after annealing. The increased nucleation density of austenite in the deformed surface layer is not attributed to the increase in the density of the recrystallized ferrite–ferrite grain boundaries, but rather to the broken and dispersed cementite particles introduced by the deformation. The gradual change in ferrite and austenite grain size should be attributed to be the gradual change in the deformation degree of ferrite and perlite.
  • Toko Tokunaga, Masayuki Koshiyama, Kazuya Kurokawa, Munekazu Ohno, Kiyotaka Matsuura
    Materials Transactions 59 6 969 - 975 2018年 [査読有り][通常論文]
     
    It has been required to improve the heat efficiency of thermal power generation system for the sake of mitigation of the global warming and resource depletion problems. For improving the heat efficiency, it is effective to increase the steam temperature, and as a result, appropriate heatresistant alloys are needed. Although SUS304 stainless steel and Ni-based superalloys have been proposed as promising heat-resistant alloys until now, there still remain some concerns such as high-temperature corrosion by flaming gas and erosion by combustion ash. Thus, the present authors propose SiAlON ceramic coating on SUS304 and INCONEL X-750 because SiAlON has excellent heat, wear and corrosion resistances. In the present study, brazed bonding between SiAlON and these heat-resistant alloys was attempted with the applications of Cu and Ag as a soft filler material to reduce the residual stress generated due to the difference in thermal expansion coefficient between SiAlON and the heat-resistant alloys. As for the bonding with the Cu filler, the SiAlON/Cu/SUS joint was successful when the brazing time was short. However, when the brazing time was long (for example, 60 minutes), Fe-based grains were formed in the Cu filler layer, and the cracks were formed in the SiAlON near the joint interface during cooling in the brazing process. It was considered that the Cu filler was hardened by the formation of the Fe-based grains and could not reduce the residual stress. As for the bonding with the Ag filler, on the other hand, the SiAlON/Ag/SUS joint was successful even for a long brazing time. The SiAlON/Ag/INCONEL joint was also successful. The bending strengths of these SiAlON/Cu/SUS, SiAlON/Ag/SUS and SiAlON/Ag/INCONEL joints were evaluated by a three point bending test, and the results were approximately 200, 270 and 350 MPa, respectively. In all cases fracture occurred in the SiAlON, which means that the SiAlON and the alloys were strongly bonded.
  • Toko Tokunaga, Kazushi Sotomoto, Munekazu Ohno, Kiyotaka Matsuura
    Materials Transactions 59 3 432 - 436 2018年 [査読有り][通常論文]
     
    The poor corrosion resistance of Mg alloys, which is well known as one of the lightest structural metal materials, limits a wide use of these alloys. In order to dramatically increase not only the corrosion resistance but also the surface hardness of Mg alloys (AZ80), coating with A2024 Al alloy by hot extrusion has been attempted in this study. The coating was successfully realized by hot extrusion of a Mg alloy (AZ80) billet together with an Al alloy plate placed between the billet and the extrusion die. The A2024 Al alloy-coated Mg alloy exhibited an excellent corrosion resistance and no weight loss was observed after immersion in a 5 mass% HCl aqueous solution for 10 min while a weight loss of about 1500 g・m−2 was observed in case of AZ80 Mg alloy without coating under the same condition. The surface hardness of the coated alloy was increased to about 85 HV by aging for 144 h at room temperature after solution treatment for 3 h at 673 K.
  • Munekazu Ohno, Hayato Sato
    ISIJ International 58 1 114 - 122 2018年 [査読有り][通常論文]
     
    Lattice Boltzmann Method (LBM), newly developing technique of computational fluid dynamics, is coupled with solute and energy conservation equations to develop a macrosegregation simulation model (LBM-coupled model) with high computational efficiency. LBM does not require time-consuming calculations for correction of velocity and pressure of fluid in contrast to methods directly solving Navier-Stokes (NS) equation and, therefore, LBM is computationally efficient. In this study, the accuracy of the LBM-coupled model is investigated by calculating the steady state flows and by comparing the results with those of analytical solutions and a conventional model based on the NS equation. The results between them are almost identical with each other and it indicates that the accuracy of the LBM-coupled model is sufficiently high. Moreover, a macrosegregation simulation is carried out for a simple case where the macrosegregation emerges only by natural convection, by means of the LBM-coupled and conventional models. The LBM-coupled model yields almost the same result with the one of NS-based model. Importantly, however, the simulation of LBM-coupled model is about five times faster than the one of NS-based model.
  • 紺谷博人, 徳永透子, 大野宗一, 佐々木克彦, 松浦清隆
    日本金属学会誌 82 5 125 - 129 2018年 [査読有り][通常論文]
     
    Unidirectional orientation of the carbon fiber is essential in the fabrication of Cucarbon fiber composites with high thermal conductivity, because the fiber exhibits its very high thermal conductivity only in the longitudinal direction. In this study, hot extrusion of a mixture of Cu powder and carbon fiber was applied to fabricate the unidirectionally orientated carbon fiber reinforced Cubased composite and the effects of the volume fraction of carbon fiber on the thermal conductivity have been investigated. It has been demonstrated that hot extrusion of the Cu powdercarbon fiber mixture successfully align the fiber to orient along the extrusion direction, and consequently, the thermal conductivity in this direction increases with the volume fraction of carbon fiber. For the composite with more than 30 vol% carbon fiber, the application of ultrasonic vibration treatment to the powderfiber mixture is effective for improving the homogeneity of the mixing and hence increasing the relative density and the thermal conductivity of the composite.
  • Yasushi Shibuta, Shinji Sakane, Eisuke Miyoshi, Shin Okita, Tomohiro Takaki, Munekazu Ohno
    Nature Communications 8 1 10 - 10 2017年12月01日 [査読有り][通常論文]
     
    Can completely homogeneous nucleation occur? Large scale molecular dynamics simulations performed on a graphics-processing-unit rich supercomputer can shed light on this long-standing issue. Here, a billion-atom molecular dynamics simulation of homogeneous nucleation from an undercooled iron melt reveals that some satellite-like small grains surrounding previously formed large grains exist in the middle of the nucleation process, which are not distributed uniformly. At the same time, grains with a twin boundary are formed by heterogeneous nucleation from the surface of the previously formed grains. The local heterogeneity in the distribution of grains is caused by the local accumulation of the icosahedral structure in the undercooled melt near the previously formed grains. This insight is mainly attributable to the multi-graphics processing unit parallel computation combined with the rapid progress in high-performance computational environments.
  • Eisuke Miyoshi, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Shinji Sakane, Takashi Shimokawabe, Takayuki Aoki
    npj Computational Materials 3 1 25  2017年12月01日 [査読有り][通常論文]
     
    Grain growth, a competitive growth of crystal grains accompanied by curvature-driven boundary migration, is one of the most fundamental phenomena in the context of metallurgy and other scientific disciplines. However, the true picture of grain growth is still controversial, even for the simplest (or 'ideal') case. This problem can be addressed only by large-scale numerical simulation. Here, we analyze ideal grain growth via ultra-large-scale phase-field simulations on a supercomputer for elucidating the corresponding authentic statistical behaviors. The performed simulations are more than ten times larger in time and space than the ones previously considered as the largest; this computational scale gives a strong indication of the achievement of true steady-state growth with statistically sufficient number of grains. Moreover, we provide a comprehensive theoretical description of ideal grain growth behaviors correctly quantified by the present simulations. Our findings provide conclusive knowledge on ideal grain growth, establishing a platform for studying more realistic growth processes.
  • Munekazu Ohno, Tomohiro Takaki, Yasushi Shibuta
    Physical Review E 96 3 033311 - 033311 2017年09月20日 [査読有り][通常論文]
     
    A variational formulation of a quantitative phase-field model is presented for nonisothermal solidification in a multicomponent alloy with two-sided asymmetric diffusion. The essential ingredient of this formulation is that the diffusion fluxes for conserved variables in both the liquid and solid are separately derived from functional derivatives of the total entropy and then these fluxes are related to each other on the basis of the local equilibrium conditions. In the present formulation, the cross-coupling terms between the phase-field and conserved variables naturally arise in the phase-field equation and diffusion equations, one of which corresponds to the antitrapping current, the phenomenological correction term in early nonvariational models. In addition, this formulation results in diffusivities of tensor form inside the interface. Asymptotic analysis demonstrates that this model can exactly reproduce the free-boundary problem in the thin-interface limit. The present model is widely applicable because approximations and simplifications are not formally introduced into the bulk's free energy densities and because off-diagonal elements of the diffusivity matrix are explicitly taken into account. Furthermore, we propose a nonvariational form of the present model to achieve high numerical performance. A numerical test of the nonvariational model is carried out for nonisothermal solidification in a binary alloy. It shows fast convergence of the results with decreasing interface thickness.
  • Tomohiro Takaki, Roberto Rojas, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takashi Shimokawabe, Takayuki Aoki
    Journal of Crystal Growth 474 146 - 153 2017年09月15日 [査読有り][通常論文]
     
    Simulating dendritic growth with natural convection is challenging because of the size of the computational domain required when compared to the dendrite scale. In this study, a phase-field-lattice Boltzmann model was used to simulate dendritic growth in the presence of natural convection due to a difference in solute concentration. To facilitate and accelerate the large-scale simulation, a parallel computing code with multiple graphics processing units was developed. The effects of the computational domain size as well as those of gravity on the dendritic morphologies were examined by performing two-dimensional free dendritic growth simulations with natural convection. The effects of the gravity direction on the dendrite spacing and morphology were also investigated by simulating unidirectional solidification from multiple seeds.
  • Shin Okita, Wolfgang Verestek, Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta
    Journal of Crystal Growth 474 140 - 145 2017年09月15日 [査読有り][通常論文]
     
    Continuous processes of homogeneous nucleation, solidification and grain growth are spontaneously achieved from an undercooled iron melt without any phenomenological parameter in the molecular dynamics (MD) simulation with 12 million atoms. The nucleation rate at the critical temperature is directly estimated from the atomistic configuration by cluster analysis to be of the order of 1034 m−3 s−1. Moreover, time evolution of grain size distribution during grain growth is obtained by the combination of Voronoi and cluster analyses. The grain growth exponent is estimated to be around 0.3 from the geometric average of the grain size distribution. Comprehensive understanding of kinetic properties during continuous processes is achieved in the large-scale MD simulation by utilizing the high parallel efficiency of a graphics processing unit (GPU), which is shedding light on the fundamental aspects of production processes of materials from the atomistic viewpoint.
  • Shinji Sakane, Tomohiro Takaki, Roberto Rojas, Munekazu Ohno, Yasushi Shibuta, Takashi Shimokawabe, Takayuki Aoki
    Journal of Crystal Growth 474 154 - 159 2017年09月15日 [査読有り][通常論文]
     
    Melt flow drastically changes dendrite morphology during the solidification of pure metals and alloys. Numerical simulation of dendrite growth in the presence of the melt flow is crucial for the accurate prediction and control of the solidification microstructure. However, accurate simulations are difficult because of the large computational costs required. In this study, we develop a parallel computational scheme using multiple graphics processing units (GPUs) for a very large-scale three-dimensional phase-field-lattice Boltzmann simulation. In the model, a quantitative phase field model, which can accurately simulate the dendrite growth of a dilute binary alloy, and a lattice Boltzmann model to simulate the melt flow are coupled to simulate the dendrite growth in the melt flow. By performing very large-scale simulations using the developed scheme, we demonstrate the applicability of multi-GPUs parallel computation to the systematical large-scale-simulations of dendrite growth with the melt flow.
  • Munekazu Ohno, Tomohiro Takaki, Yasushi Shibuta
    Journal of Computational Physics 335 621 - 636 2017年04月 [査読有り][通常論文]
     
    Quantitative phase-field models have been developed as feasible computational tools for solving the free-boundary problem in solidification processes. These models are constructed with some polynomials of the phase-field variable that describe variations of the physical quantities inside the diffuse interface. The accuracy of the simulation depends on the choice of the polynomials and such dependence is indispensable for high-performance computing and valuable for extending the range of applications of the model to several physical systems. However, little is known about the dependence of the accuracy on the choice of the polynomials. In this study, numerical testing is carried out for quantitative phase-field models with extensive sets of polynomials (24 different models) for isothermal solidification in binary alloys. It is demonstrated in two-dimensional simulations of dendritic growth that a specific set of polynomials must be employed to achieve high accuracy in the models with double-well and double-obstacle potentials. Both types of model with the best set of polynomials yield almost the same numerical accuracy.
  • Yukimi Oka, Munekazu Ohno, Kiyotaka Matsuura
    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 103 12 755 - 762 2017年 [査読有り][通常論文]
     
    Synopsis : Heat transfer analyses of casting processes offer an effective way of understanding solidification processes which is important for prevention of formation of casting defects and for control of segregations. The accuracy of the simulations depends on the accuracy of input parameters such as thermal conductivity and heat transfer coefficient. It is not straightforward to evaluate such parameters, especially thermal conductivities of bulks and heat transfer coefficient with high accuracy. It is very important to develop a reliable method for estimation of these parameters. In this study, particle filter, which is a method of data assimilation, is applied to estimation of thermal conductivity of melt and heat transfer coefficient during alloy solidification in mold casting and its applicability is systematically investigated on the basis of twin experiments. It is shown that a constant heat transfer coefficient can be accurately estimated in this method by utilizing a single cooling curve measured in the mold or in the melt near the mold, while the thermal conductivity can be accurately estimated from a single cooling curve measured in the melt. In addition, this method can be utilized for estimation of time-dependent heat transfer coefficient without any assumption and/or approximation on its time dependence. Importantly, the thermal conductivity and time-dependent heat transfer coefficient can be estimated simultaneously with high accuracy.
  • Munekazu Ohno, Daichi Kimura, Kiyotaka Matsuura
    TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 103 12 45 - 53 2017年 [査読有り][通常論文]
     
    A prediction method for microsegregation in Fe-based alloys was developed based on an approach of machine learning called Deep Learning. A set of model and algorithm of Deep Learning suitable for description of microsegregation was constructed by employing training data obtained by one-dimensional finite difference calculations for interdendritic microsegregation. It is shown that the developed method enables accurate prediction of the microsegregation behavior in Fe-based binary and ternary alloys with the solute atoms of C, Si, Mn, P and S. The present results demonstrate that Deep Learning offers a promising way of constructing an easy-to-use approach for prediction of microsegregation with high accuracy. Importantly, it is expected that the present method can be extended to describe effects of microstructural processes on microsegregation behavior.
  • Munekazu Ohno, Daichi Kimura, Kiyotaka Matsuura
    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 103 12 720 - 729 2017年 [査読有り][通常論文]
     
    Synopsis : An approach of machine learning called Deep Learning is utilized for construction of a prediction method of microsegregation behavior in Febased binary alloys with solute atoms of C, Si, Mn and P. Training data for the machine learning are obtained by quantitative phase-field simulations for directional solidification. Therefore, effects of microstructural evolutions on the microsegregation behavior are taken into account in the present method. Importantly, this method can be coupled with a macrosegregation model. The simulation result of the macrosegregation model is quite different from those obtained by a conventional macrosegregation model with the Scheil model and a model with a prediction method constructed from the training data of one-dimensional finite difference calculations for the microsegregation. This fact highlights the importance of accurate description of microsegregation behavior in prediction of macrosegregation.
  • Toko Tokunaga, Koichi Takahashi, Munekazu Ohno, Katsuhiko Sasaki, Terumitsu Imanishi, Kiyotaka Matsuura
    Materials Transactions 58 6 938 - 944 2017年 [査読有り][通常論文]
     
    New heatsink materials having higher thermal conductivities have recently been required due to the recent rapid improvements in performance of the central processing unit, CPU, with increasing heat generation from computer devices. As an alternative material to conventional heatsink materials such as Al and Cu, composites containing carbon fibers have recently been gaining much attentions because of their extremely high thermal conductivity. However, carbon fiber exhibits high thermal conductivity only in its longitudinal direction. Therefore, it is essential to control the orientation of the carbon fibers in the composite materials. In the present study, hot extrusion of a powder-fiber mixture is applied to realization of unidirectional array of carbon fibers in Al matrix, and the effects of volume fraction of the carbon fibers on the thermal conductivity of the carbon fiber oriented Al-based composite have been investigated. It has been demonstrated that the carbon fibers are unidirectionally oriented in the extrusion direction, and the thermal conductivity in this direction increases with the increase in volume fraction of the carbon fibers. For the composites with more than 30 vol% of carbon fibers, the addition of Al-Si alloy powder or the application of the spark plasma sintering, SPS, before the extrusion was found to be effective for improving the sinterability of the powder-fiber mixture.
  • T. Yamanouchi, S. Miura, M. Ohno, K. Ikeda
    MRS Advances 2 25 1335 - 1340 2017年 [査読有り][通常論文]
     
    For the improvement of oxidation resistance of Nb-based alloys, PdAl-B2 is expected as Al reservoir for Al2O3 surface layer because it is in equilibrium with Nbss (bcc) phase. However, PdAl forms cracks during casting caused by B2 -> β' transformation. To suppress B2 -> β' transformation, Rh, Ru and Ir were chosen as additive elements. It is found that bcc / B2 two-phase field in Nb-Pd-Ir-Al system is limited to be small fraction of Ir, while (Pd,Rh)Al-B2 is in equilibrium with bcc solid solution in a wide composition range and addition of Rh prevents occurrence of B2 -> β' transformation. Composition range of (Pd,Ru)Al-B2 in equilibrium with bcc phase is wider than (Pd,Ir)Al-B2 but narrower than (Pd,Rh)Al-B2. Rh and Ru addition are also beneficial for improving eutectic temperature of Nb-bcc / B2 aluminide two-phase alloys.
  • Sankar Kumar Deb Nath, Yasushi Shibuta, Munekazu Ohno, Tomohiro Takaki, Tetsuo Mohri
    ISIJ International 57 10 1774 - 1779 2017年 [査読有り][通常論文]
     
    The partitionless solidification and melting in Al-Cu alloy system are investigated by means of molecular dynamics simulations with an embedded atom method (EAM) potential. The solid-liquid interfacial velocity for solid-liquid biphasic systems of Al-rich alloys is examined with respect to temperature and Cu composition. The kinetic coefficient is then derived from the slope of the interfacial velocity with respect to temperature. Our results show that the kinetic coefficient is largely dependent on the Cu composition. It sharply decreases with addition of small amount of Cu. There is almost no partition at the solid-liquid interface within the time scale of the simulation since the solid-liquid interfacial velocity is very fast at temperatures away from the equilibrium temperature. Since it is not straightforward to measure the kinetic coefficient directly from experiments, it is significant in this study to derive the composition dependence of the kinetic coefficient for binary alloys directly from the MD simulation without any phenomenological parameters.
  • 德永透子, 外本和嗣, 大野宗一, 松浦清隆
    日本金属学会 81 8 389 - 393 2017年 [査読有り][通常論文]
     
    The poor corrosion resistance of Mg alloys, which is well known as one of the lightest structural metal materials, limits a wide use of these alloys. In order to dramatically increase not only the corrosion resistance but also the surface hardness of Mg alloys (AZ80), coating with A2024 Al alloy by hot extrusion has been attempted in this study. The coating was successfully realized by hot extrusion of a Mg alloy(AZ80) billet together with an Al alloy plate placed between the billet and the extrusion die. The A2024 Al alloy-coated Mg alloy exhibited an excellent corrosion resistance and no weight loss was observed after immersion in a 5 mass% HCl aqueous solution for 10 min while a weight loss of about 1500 g•m-2 was observed in case of AZ80 Mg alloy without coating under the same condition. The surface hardness of the coated alloy was increased to about 85 HV by aging for 144 h at room temperature after solution treatment for 3 h at 673 K.
  • 徳永透子, 越山将行, 黒川一哉, 大野宗一, 松浦清隆
    日本金属学会誌 81 3 143 - 149 2017年 [査読有り][通常論文]
     
    It has been required to improve the heat efficiency of thermal power generation system for the sake of mitigation of the global warming and resource depletion problems. For improving the heat efficiency, it is effective to increase the steam temperature, and as a result, appropriate heat-resistant alloys are needed. Although SUS304 stainless steel and Ni-based superalloys have been proposed as promising heat-resistant alloys until now, there still remain some concerns such as high-temperature corrosion by flaming gas and erosion by combustion ash. Thus, the present authors propose SiAlON ceramic coating on SUS304 and INCONEL X-750 because SiAlON has excellent heat, wear and corrosion resistances. In the present study, brazed bonding between SiAlON and these heat-resistant alloys was attempted with the applications of Cu and Ag as a soft filler material to reduce the residual stress generated due to the difference in thermal expansion coefficient between SiAlON and the heat-resistant alloys. As for the bonding with the Cu filler, the SiAlON/Cu/SUS joint was successful when the brazing time was short. However, when the brazing time was long (for example, 60 min), Fe-based grains were formed in the Cu filler layer, and the cracks were formed in the SiAlON near the joint interface during cooling in the brazing process. It was considered that the Cu filler was hardened by the formation of the Fe-based grains and could not reduce the residual stress. As for the bonding with the Ag filler, on the other hand, the SiAlON/Ag/SUS joint was successful even for a long brazing time. The SiAlON/Ag/INCONEL joint was also successful. The bending strengths of these SiAlON/Cu/SUS, SiAlON/Ag/SUS and SiAlON/Ag/INCONEL joints were evaluated by a three point bending test, and the results were approximately 200, 270 and 350 MPa, respectively. In all cases fracture occurred in the SiAlON, which means that the SiAlON and the alloys were strongly bonded.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takashi Shimokawabe, Takayuki Aoki
    Acta Materialia 118 230 - 243 2016年10月01日 [査読有り][通常論文]
     
    Primary arm arrays formed during the directional solidification of a single-crystal binary alloy were investigated by performing large-scale phase-field simulations using the GPU supercomputer TSUBAME2.5 at Tokyo Institute of Technology. The primary arm array and spacing were investigated by Voronoi decomposition and Delaunay triangulation, respectively. It was concluded that a hexagonal array was dominant for both the dendrite and cell structures and that penta–hepta defects, which are typical defects in hexagonal patterns, were formed. The primary arms continuously moved such that the number of hexagons increased, and the distribution of primary arm spacing became uniform over time even after the number of primary arms was constant. The order of array was highest in the growth condition of the dendrite close to the cell-to-dendrite transition region. In addition, we proposed a realistic and accurate evaluation method of primary arm array by removing small sides from the Voronoi polygons.
  • Hiroo Suzuki, Toshiro Kaneko, Yasushi Shibuta, Munekazu Ohno, Yuki Maekawa, Toshiaki Kato
    Nature Communications 7 11797 - 11797 2016年06月02日 [査読有り][通常論文]
     
    Adding a mechanical degree of freedom to the electrical and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelectrical physics and devices with mechanical motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom-up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined experiments, molecular dynamics simulations and theoretical calculations with a phase-diagram analysis. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelectrical physics and industrial applications.
  • Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta, Shinji Sakane, Takashi Shimokawabe, Takayuki Aoki
    Journal of Crystal Growth 442 14 - 24 2016年05月15日 [査読有り][通常論文]
     
    Selections of growing crystals during directional solidification of a polycrystalline binary alloy were numerically investigated using two-dimensional phase-field simulations. To accelerate the simulations, parallel graphics processing unit (GPU) simulations were performed using the GPU-rich supercomputer TSUBAME2.5 at the Tokyo Institute of Technology. Twenty simulations with a combination of five sets of different seed orientation distributions and four different temperature gradients covering dendritic and cellular growth regions were performed. The unusual grain selection phenomenon, in which the unfavorably oriented grains preferentially grow instead of the favorably oriented grains, was observed frequently. The unusual selection was more remarkable in the cellular structure than in the dendritic structure.
  • Yasushi Shibuta, Shinji Sakane, Tomohiro Takaki, Munekazu Ohno
    Acta Materialia 105 328 - 337 2016年02月15日 [査読有り][通常論文]
     
    Nucleation and solidification from the undercooled iron melt are investigated from the atomistic point of view by large-scale molecular dynamics (MD) simulations up to 12 million atoms in systems of the submicrometer-scale. There exist some amount of atoms with icosahedral configuration in the undercooled iron melt and these atoms increase with decreasing temperature. It is expected that accumulation of atoms with icosahedral configuration in the initial β-relaxation regime of nucleation is the key to initiate the formation of bcc phase. On the other hand, mobility of atoms in the undercooled melt decreases drastically with decreasing temperature. These two competing factors in the atomistic scale are considered to derive a critical temperature at which nucleation rate becomes maximum, which agrees with a classical theory for homogeneous nucleation. Moreover, the Avrami exponents during solidification are approximately estimated to be close to 3 and 4 in two- and three-dimensional grain growths, respectively, which also agrees with empirical interpretation. Our novel approach utilizing the high parallel efficiency of the GPU supercomputer successfully links the empirical interpretation in metallurgy with the atomistic behavior of nucleation and solidification, which enlarges the application range of MD simulations for the study of structural materials.
  • Al被覆Mg合金薄板材の作製およびその機械的性質の調査
    徳永透子, 大野宗一, 松浦清隆
    Journal of Functionally Graded Materials 30 1 - 4 2016年 [査読有り][通常論文]
  • Kaho Tomita, Toko Tokunaga, Munekazu Ohno, Kiyotaka Matsuura
    Materials Transactions 57 6 927 - 934 2016年 [査読有り][通常論文]
     
    It has been shown using an Al-4.0mass%Cu alloy as a demonstration material that hot extrusion with the use of a specially-designed die leads to grain refinement and improvement in ductility. The specially-designed die is called in this study the "heteromorphic die" and it consists of two plates and one spacer. One plate is called the "strain-giving die" and has six small round holes of 5 mm in diameter, while the other plate is called the "shape-giving die" and has one big rectangular hole of 9 mm × 15 mm. The spacer separates these two plates at a distance of 10 mm and provides a space between them. The Al-Cu alloy billet was hot-extruded through the six small round holes of the strain-giving die to produce six thin round rods, and after filling the space between the two dies, the six rods passed together through the shape-giving die to form a thick rectangular bar or plate. Tensile tests of the plate were conducted at room temperature, and the results were compared with those of the un-extruded billet and the extruded plate produced by conventional hot extrusion method, that uses only the shape-giving die. The elongation values of the three different tensile specimens produced from (1) the un-extruded billet, (2) the plate extruded by the conventional method and (3) the plate extruded with the heteromorphic die were 21, 35 and 57%, respectively. Most significant elongation was found in the sample extruded with the heteromorphic die, which is most probably due to the finest grain structure observed in this sample. It is suggested that the recrystallization is induced by a large strain generated in the heteromorphic die due to the friction on the large contact area between the extruded metal and the die orifice wall of the "strain-giving die."
  • Masayuki Koshiyama, Hazuki Sako, Munekazu Ohno, Kiyotaka Matsuura
    Materials Transactions 57 9 1593 - 1596 2016年 [査読有り][通常論文]
     
    Combustion-synthesized Y-α-SiAlON and Ca-α-SiAlON powders were consolidated by spark plasma sintering (SPS) at 1300-1450°C for 10 min, and the mechanical properties of the consolidated bulk samples were investigated. XRD analysis revealed that α-SiAlON partially transforms into β-SiAlON during the SPS and a bulk mixture of α/β-SiAlON was obtained. The fraction of β-SiAlON increased with the increase in sintering temperature and the α to β transformation ratio was higher for Y-α-SiAlON than for Ca-α-SiAlON. The hardness of the consolidated bulk increased with sintering temperature, and after reaching a maximum at 1350°C, the hardness gradually decreased with temperature in both the Y-α-SiAlON and Ca-α-SiAlON. The increase in hardness with temperature arises from the increased density of the sintered body, while the decrease in hardness results from grain growth due to an increase in temperature. The fracture toughness tended to increase with temperature and did not show a maximum for either α- or β-SiAlON, although the Y-α-SiAlON always exhibited a greater toughness than the Ca-α-SiAlON. The greater toughness of the Y-α-SiAlON is attributable to its higher fraction of transformed β-SiAlON, because the elongated shape of the β-SiAlON leads to the prevention of crack propagation.
  • 徳永透子, 高橋晃一, 大野宗一, 佐々木克彦, 今西輝光, 松浦清隆
    日本金属学会誌 80 10 640 - 645 2016年 [査読有り][通常論文]
     
    New heatsink materials having higher thermal conductivities have recently been required due to the recent rapid improvements in performance of the central processing unit, CPU, with increasing heat generation from computer devices. As an alternative material to conventional heatsink materials such as Al and Cu, composites containing carbon fibers have recently been gaining much attentions because of their extremely high thermal conductivity. However, the carbon fiber exhibits high thermal conductivity only in its longitudinal direction. Therefore, it is essential to control the orientation of the carbon fibers in the composite materials. In the present study, hot extrusion of a powder?fiber mixture is applied to realization of unidirectional array of carbon fibers in Al matrix, and the effects of volume fraction of carbon fiber on the thermal conductivity of the carbon fiber oriented Al?based composite have been investigated. It has been demonstrated that the carbon fibers are unidirectionally oriented in the extrusion direction, and consequently, the thermal conductivity in this direction increases with the increase in volume fraction of the carbon fiber. For the composites with more than 30 vol? carbon fiber, the addition of Al?Si alloy powder or the application of the spark plasma sintering, SPS, before extrusion was found to be effective for improving the sinterability of the powder-fiber mixture.
  • Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takashi Shimokawabe, Takayuki Aoki
    ISIJ International 56 8 1427 - 1435 2016年 [査読有り][通常論文]
     
    Large-scale phase-field studies of three-dimensional (3D) dendrite competitive growth at the converging grain boundary (GB) of a bicrystal binary alloy were carried out using the GPU-rich supercomputer TSUBAME 2.5 at Tokyo Institute of Technology. First, a series of thin-sample simulations were performed to investigate the effects of thin-sample thickness, unfavorably oriented (UO) grain inclination angle, and dendrite arrangement on an unusual overgrowth phenomenon whereby the favorably oriented (FO) grain is overgrown by the UO grain. It was concluded that the unusual overgrowth easily occurs as the thickness of the thin sample and the UO grain inclination angle decrease. It was also concluded that the interaction between FO and UO dendrites at the converging GB depends on the dendrite arrangement for relatively large dendrite spacing. Next, realistic large-scale simulations whereby multiple dendrites interact at the converging GB were performed. Unusual overgrowth was also observed in such large-scale simulations, and this phenomenon easily occurred at smaller UO dendrite inclination angles. Furthermore, it was also concluded that the FO and UO dendrites rearrange toward a space-to-face interaction. Because the interaction between FO and UO dendrites differs according to the location on the GB, a zigzag GB was formed, especially at small UO grain inclination angles.
  • T. Tokunaga, M. Ohno, K. Matsuura
    Journal of Functionally Graded Materials 30 1 - 4 傾斜機能材料研究会 2016年 [査読有り][通常論文]
     
    In order to achieve a light and corrosion-resistant structural material, Al-coated Mg alloy sheet has been fabricated by hot extrusion followed by hot rolling. The sheet was successfully rolled at 623 K without any breaking of the Al coating layer, and the sheet exhibited an excellent corrosion resistance. The mechanical properties have been investigated by tensile tests at various temperatures and strain rates. At room temperature, the stress value drops immediately after reaching the maximum, while the value decreases as the temperature increases and the strain rate decreases. It has been demonstrated that the sheet exhibits elongation of 707 % at 573 K at 1.0×10-3 s-1 when tensile-tested in the rolling direction. After the elongation, the Al coating layer remained on the Mg alloy substrate. No cracks or debonding were observed at interfaces of Mg alloy/Al3Mg2 and Al3Mg2/Al.
  • Munekazu Ohno, Tomohiro Takaki, Yasushi Shibuta
    PHYSICAL REVIEW E 93 1 012802 - 012802 2016年01月 [査読有り][通常論文]
     
    We present the variational formulation of a quantitative phase-field model for isothermal low-speed solidification in a binary dilute alloy with diffusion in the solid. In the present formulation, cross-coupling terms between the phase field and composition field, including the so-called antitrapping current, naturally arise in the time evolution equations. One of the essential ingredients in the present formulation is the utilization of tensor diffusivity instead of scalar diffusivity. In an asymptotic analysis, it is shown that the correct mapping between the present variational model and a free-boundary problem for alloy solidification with an arbitrary value of solid diffusivity is successfully achieved in the thin-interface limit due to the cross-coupling terms and tensor diffusivity. Furthermore, we investigate the numerical performance of the variational model and also its nonvariational versions by carrying out two-dimensional simulations of free dendritic growth. The nonvariational model with tensor diffusivity shows excellent convergence of results with respect to the interface thickness.
  • Munekazu Ohno, Masato Maruyama, Kiyotaka Matsuura
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 11 5240 - 5247 2015年11月01日 [査読有り][通常論文]
     
    Experimental verification of a critical condition for the formation of coarse columnar γ grain (CCG) structure in as-cast hyperperitectic carbon steels, which was put forward based on theories of grain growth and phase-field simulations in early studies, is carried out by means of a Bridgman-type directional solidification experiment. The occurrence of the discontinuous and continuous grain growth processes and the resulting formation of CCG and equiaxed γ grain structures, respectively, are demonstrated. Importantly, these changes of the as-cast microstructures and the grain growth modes are in excellent agreement with the previously proposed critical condition of the CCG formation.
  • Roberto Rojas, Tomohiro Takaki, Munekazu Ohno
    Journal of Computational Physics 298 29 - 40 2015年10月01日 [査読有り][通常論文]
     
    In this study, a combination of the lattice Boltzmann method (LBM) and the phase-field method (PFM) is used for modeling simultaneous growth and motion of a dendrite during solidification. PFM is used as a numerical tool to simulate the morphological changes of the solid phase, and the fluid flow of the liquid phase is described by using LBM. The no-slip boundary condition at the liquid-solid interface is satisfied by adding a diffusive-forcing term in the LBM formulation. The equations of motion are solved for tracking the translational and rotational motion of the solid phase. The proposed method is easily implemented on a single Cartesian grid and is suitable for parallel computation. Two-dimensional benchmark computations show that the no-slip boundary condition and the shape preservation condition are satisfied in this method. Then, the present method is applied to the calculation of dendritic growth of a binary alloy under melt convection. Initially, the solid is stationary, and then, the solid moves freely due to the influence of fluid flow. Simultaneous growth and motion are effectively simulated. As a result, it is found that motion and melt convection enhance dendritic growth along the flow direction.
  • Toko Tokunaga, Danuta Szeliga, Kiyotaka Matsuura, Munekazu Ohno, Maciej Pietrzyk
    International Journal of Advanced Manufacturing Technology 80 1-4 507 - 513 2015年09月19日 [査読有り][通常論文]
     
    Although Mg alloys possess many attractive properties, the use of Mg alloys has been limited due to their extremely poor corrosion resistance. In the authors’ previous study, they have developed a new process for Mg/Al cladding by hot extrusion with a convex die. In the extrusion process, it was found that the thickness uniformity of the Al coating layer can be controlled by changing the die angle. However, the optimal condition of the extrusion and the influences of extrusion parameters on the coating thickness uniformity remain unclear. In this study, therefore, the sensitivity analysis has been performed in order to evaluate the influences of extrusion parameters on the coating thickness uniformity. The sensitivities of the following five parameters were evaluated: (1) initial thickness of the coating material plate, (2) extrusion temperature, (3) ram speed, (4) die angle, and (5) ratio of simulated flow stress to the experimental one for pure Al. In order to perform the sensitivity analysis, the process for Mg/Al cladding was modeled. The results from the extrusion model fairly agree with the experimental results. Thus, it can be said that the present model of Mg/Al cladding process is reliable. From the sensitivity analysis, it was found that the initial thickness of the coating material plate and die angle influence on the uniformity most. The extrusion temperature, ram speed, and flow stress ratio do not have significant influences.
  • Daisuke Sato, Munekazu Ohno, Kiyotaka Matsuura
    Computational Materials Science 106 188 - 192 2015年09月01日 [査読有り][通常論文]
     
    Abstract It was reported in the authors' previous study that second phase particles in δ phase induce a retardation effect on migration of δ-γ interface in carbon steels as is similar to the pinning effect on the grain growth in polycrystalline materials and effects of volume fraction and size of the particle were clarified. However, several other factors are expected to affect the magnitude of the retardation effect. In this study, the effect of the different interfacial energies is systematically investigated by means of two-dimensional phase-field simulations, and it is found that the magnitude of the retardation effect can be described by the parameter, Δσ = σ - σ where σ) is the interfacial energy between the particle and γ (δ) phases. The large positive value of Δσ yields the strong pinning effect. In addition, it is shown that the δ-γ interface migration is accelerated when Δσ is large negative. These results originate from the increase in the total interfacial energy when Δσ > 0 and the decrease when Δσ < 0 during the interface passing through the particles. A degree of the retardation, Δa was summarized in the relation, Δa = m1(r/l2)(1 + m2Δσ), where m1 and m2 are constants and r is radius of the particle and l is particle spacing.
  • Yasushi Shibuta, Kanae Oguchi, Tomohiro Takaki, Munekazu Ohno
    Scientific Reports 5 13534 - 13534 2015年08月27日 [査読有り][通常論文]
     
    Homogeneous nucleation from an undercooled iron melt is investigated by the statistical sampling of million-atom molecular dynamics (MD) simulations performed on a graphics processing unit (GPU). Fifty independent instances of isothermal MD calculations with one million atoms in a quasi-two-dimensional cell over a nanosecond reveal that the nucleation rate and the incubation time of nucleation as functions of temperature have characteristic shapes with a nose at the critical temperature. This indicates that thermally activated homogeneous nucleation occurs spontaneously in MD simulations without any inducing factor, whereas most previous studies have employed factors such as pressure, surface effect, and continuous cooling to induce nucleation. Moreover, further calculations over ten nanoseconds capture the microstructure evolution on the order of tens of nanometers from the atomistic viewpoint and the grain growth exponent is directly estimated. Our novel approach based on the concept of "melting pots in a supercomputer" is opening a new phase in computational metallurgy with the aid of rapid advances in computational environments.
  • Yasushi Shibuta, Munekazu Ohno, Tomohiro Takaki
    JOM 67 8 1793 - 1804 2015年08月27日 [査読有り][通常論文]
     
    Thanks to the recent progress in high-performance computational environments, the range of applications of computational metallurgy is expanding rapidly. In this paper, cutting-edge simulations of solidification from atomic to microstructural levels performed on a graphics processing unit (GPU) architecture are introduced with a brief introduction to advances in computational studies on solidification. In particular, million-atom molecular dynamics simulations captured the spontaneous evolution of anisotropy in a solid nucleus in an undercooled melt and homogeneous nucleation without any inducing factor, which is followed by grain growth. At the microstructural level, the quantitative phase-field model has been gaining importance as a powerful tool for predicting solidification microstructures. In this paper, the convergence behavior of simulation results obtained with this model is discussed, in detail. Such convergence ensures the reliability of results of phase-field simulations. Using the quantitative phase-field model, the competitive growth of dendrite assemblages during the directional solidification of a binary alloy bicrystal at the millimeter scale is examined by performing two- and three-dimensional large-scale simulations by multi-GPU computation on the supercomputer, TSUBAME2.5. This cutting-edge approach using a GPU supercomputer is opening a new phase in computational metallurgy.
  • 越山将行, 酒向はづき, 大野宗一, 松浦清隆
    日本金属学会誌 79 4 191 - 194 2015年04月 [査読有り][通常論文]
     
    Combustion-synthesized Y-α-rSiAlON and Ca-α-SiAlON powders have been consolidated by Spark Plasma Sintering (SPS) at 1300-1450°C for 10 min, and the mechanical properties of the consolidated bulk samples were investigated. XRD analysis revealed that α-SiAlON partially transforms into β-SiA10N during the SPS and a bulk of α/β-SiA10N mixture was obtained. The fraction of β-SiA10N increases with the increase in sintering temperature and the α to β transformation ratio is high for Y-α-SiAlON compared with Ca-α-SiA10N. The hardness of the consolidated bulk increases with sintering temperature and after reaching the peak at 1350°C the hardness gradually decreases with temperature in both the cases of Y-α-SiAlON and Ca-α-SiAlON. The increase in hardness with temperature arises from increase in density of the sintered body, while the decrease in hardness results from grain growth due to high temperatures. The fracture toughness monotonically increases with temperature and does not show a peak for both SiAlONs, although Y-α-SiAlON always exhibits higher toughness than Ca-α-SiAlON. The higher toughness of the Y-α-SiAlON is attributable to its higher fraction of transformed β-SiA10N, because the elongated shape of β-SiA10N leads to prevention of crack propagation.
  • 富田花帆, 徳永透子, 大野宗一, 松浦清隆
    日本金属学会誌 79 4 183 - 190 2015年04月 [査読有り][通常論文]
     
    It has been shown using an Al-4.0 mass%Cu alloy as a demonstration material that hot extrusion with the use of a specially-designed die, which is called a heteromorphic die, leads to grain refinement and improvement in ductility. The heteromorphic die consists of two plates and one spacer. One plate is called a strain-giving die and has six holes of 5 mm in diameter, while the other plate is called a shape-giving die and has one rectangular hole of 9 mm x 15 mm in side length. The spacer separates these plates at a distance of 10 mm and gives a space between them. The Al-Cu alloy billet was hot-extruded from the strain-giving die into the space between the two dies to produce six thin bars, and all the six bars passed together through the shape-giving die to become a thick rectangular bar. The rectangular bar produced by the present extrusion was tensile-tested at room temperature and the results were compared with those of the un-extruded billet and the bar produced by a conventional hot extrusion using only the shape-giving die. The elongation of the specimens made of the billet, the bar extruded by the conventional method and the one extruded using the present heteromorphic die were 21, 35 and 57%, respectively. The largest elongation found in the sample extruded using the present heteromorphic die should be due to the fine recrystallized grain structure in that sample. It is suggested that the recrystallization is induced by a large strain generated in the present heteromorphic die due to the friction at the large contact area between the die hole wall and the alloy.
  • Daisuke Sato, Munekazu Ohno, Kiyotaka Matsuura
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 2 981 - 988 2015年02月 [査読有り][通常論文]
     
    Retardation effect of dispersed inert particles on delta–gamma interface migration in carbon steels during isothermal delta to gamma transformation is analyzed by two-dimensional phase-field simulations. The effect is systematically investigated for different values of particle radius, r, particle spacing, l, and initial carbon concentration of delta phase. The retardation effect becomes stronger when the pinning parameter described by r/l2 is larger and the carbon concentration of delta phase is higher, indicating that delta to gamma transformation kinetics can be retarded in a similar way to the pinning effect on grain growth kinetics.
  • Munekazu Ohno, Shingo Tsuchiya, Kiyotaka Matsuura
    ISIJ International 55 11 2374 - 2382 2015年 [査読有り][通常論文]
     
    Formation processes of as-cast austenite grain structures in hypoperitectic carbon steels have been investigated by means of a rapid directional solidification method, the cooling conditions of which are similar to those in the vicinity of slab surfaces in continuous casting processes. Coarse Columnar austenite Grain (CCG) structure was observed in all the hypoperitectic carbon steels employed in this study. It was demonstrated that its formation mechanism is ascribable to the discontinuous grain growth from Fine Columnar austenite Grain (FCG) formed in the delta phase, in which both the delta phase and residual liquid phase act as the pinning phase in the grain growth process. Then, a summary of the findings was provided regarding the microstructural features and the formation mechanisms of as-cast austenite grain structures formed in the rapid directional solidification in carbon steels with the carbon composition ranging from 0.05 to 0.45 mass%.
  • Lukasz Madej, Toko Tokunaga, Kiyotaka Matsuura, Munekazu Ohno, Maciej Pietrzyk
    CIRP Annals - Manufacturing Technology 64 1 253 - 256 2015年 [査読有り][通常論文]
     
    Although Mg alloys possess many attractive properties, their use has been limited due to their poor corrosion resistance. Therefore, in the present paper a process for Al coating on Mg alloys by hot extrusion was investigated. Physical simulations confirmed capabilities of this process and Al coating on a Mg alloy was obtained. Prediction of microstructure evolution and determination of optimal process parameters were the objectives of the present numerical simulations. Cellular automata were used to reach the former goal, while for the latter goal optimization analysis was applied to find the best technological parameters.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 1 32 - 36 2015年01月 [査読有り][通常論文]
     
    The grain coarsening temperature (Tc) is quite low in a ferrite/pearlite (F/P) banded steel as compared with a non-banded steel. Here it is shown that the low Tc in the F/P banded steel can be increased significantly by applying cold deformation prior to austenitizing. The kinetics of abnormal grain coarsening above Tc is also largely retarded by cold deformation. These observations should be attributed to the uniform distribution of fine AlN precipitates caused by cold deformation.
  • Tomohiro Takaki, Munekazu Ohno, Takashi Shimokawabe, Takayuki Aoki
    Acta Materialia 81 272 - 283 2014年12月 [査読有り][通常論文]
     
    We investigated the competitive growth of dendrites at the converging grain boundaries (GBs) of bicrystals during the directional solidification of an Al-Cu alloy by means of two-dimensional phase-field simulations. In particular, the focus was on the recently observed phenomenon of unusual overgrowth during the directional solidification of a Ni-based superalloy, where the favorably oriented (FO) dendrites are overgrown by the unfavorably oriented (UO) ones. The phase-field simulations were accelerated by parallel computations on graphics processing units. The simulation results showed that unusual overgrowth occurs in Al-Cu alloys, indicating that this phenomenon is a common one in metallic materials. It was also concluded that the differences in the diffusion layers in front of the FO and UO dendrites had a dominant effect on the competitive growth of dendrites at the converging GB as well as on the unusual overgrowth. In addition, unusual overgrowth was observed in all the FO dendrites with a spacing that allowed the dendrite array to grow stably without necessitating a change in the number of dendrites. The FO dendrite at the GB is overgrown by the UO dendrite when the spacing between the FO dendrite at the GB and the next FO dendrite is approximately equal to the critical minimum spacing. However, the unusual overgrowth was not observed for UO dendrites with a large inclination angle. In this case, all the UO dendrites are blocked by the FO dendrite at the GB, and the FO dendrites migrate toward the UO dendrites.
  • Yasushi Shibuta, Kanae Oguchi, Munekazu Ohno
    Scripta Materialia 86 20 - 23 2014年09月01日 [査読有り][通常論文]
     
    Spontaneous evolution of anisotropy in the solid nucleus in the undercooling melt of iron is achieved by million-atom molecular dynamics simulation without an anisotropy parameter, which is indispensable for most existing phenomenological simulations. The spherical nucleus grows preferentially in <1 0 0> directions, which is fully consistent with the estimated anisotropy of the kinetic coefficient. The discussion of the morphological dynamics of the crystal growth based on this cutting-edge approach should initiate a new era in next-generation computational metallurgy. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 10 4623 - 4634 2014年09月 [査読有り][通常論文]
     
    The grain growth behavior of austenite reversely transformed from ferrite/pearlite (F/P)-banded and non-banded steels has been studied. It was found that the grain-coarsening temperature [the temperature at which abnormal grain growth (AGG) occurs] of the initially banded F/P structure is quite low compared with that of the non-banded sample. In the F/P-banded sample, the abnormal grains always originate from the former ferrite region. The occurrence of AGG is essentially attributable not to the austenite nucleation process during heating but to the grain growth process after the completion of austenizing. It was proposed that the lowered grain-coarsening temperature in the banded structure is due to the non-uniform pinning-effect of AlN precipitates between former ferrite and pearlite regions. © 2014 The Minerals, Metals & Materials Society and ASM International.
  • Toko Tokunaga, Kiyotaka Matsuura, Munekazu Ohno
    Journal of Alloys and Compounds 601 179 - 185 2014年07月15日 [査読有り][通常論文]
     
    An Al-coated Mg alloy sheet is fabricated by hot forging an Al-coated Mg alloy plate produced by hot extrusion of a Mg alloy billet together with an Al plate. The sheet consists of recrystallized fine grains both in the Mg alloy substrate and Al coating layer. A thin intermetallic compound layer of Al 3Mg2 formed at the Al/Mg interface in the hot-extruded plate. The subsequent hot forging reduces the thickness of the intermetallic layer but induces cracks in the intermetallic compound. This sheet exhibits a large tensile elongation of 550% at 573 K and at a strain rate of 1.0 × 10-3 s-1. Notably, a tensile test of an Al specimen exhibited only 40% elongation, while that of a Mg alloy specimen exhibited 800% elongation. It is suggested that the large elongation of Al as the coating layer on the Mg alloy is attributable to the good bonding of the Al coating layer and the Mg alloy substrate. The cracks in the intermetallic layer disappear during the tensile tests at elevated temperatures and no cracks are observed in the elongated specimens, which indicates growth of the intermetallic and reparation of the cracks occurred during the low strain-rate hot deformation of the Al-coated Mg alloy sheet.© 2014 Elsevier B.V. All rights reserved.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno
    Micron 59 28 - 32 2014年04月 [査読有り][通常論文]
     
    Cold plastic deformation produces misorientations inside the crystal grains, and the distribution of the misorientation is quite crucial to understand the deformation behavior of the metals or alloys. The misorientation manifestations in chemical etching contrast are investigated in this study in the case of cold-deformed iron. The chemical etching is performed by using nital, while the crystal orientation is determined by electron backscatter diffraction (EBSD). The correlation between the chemical etching contrast and crystal orientation have been studied in both cold-deformed and undeformed iron. The results clearly show that the chemical etching contrast strongly reflects the crystallographic orientation. The gradual change in chemical etching contrast inside the individual deformed grains gives information of both the misorientation and local plastic strain within the grains. This method can provide an easy and alternative way to qualitatively understand the misorientation and local plastic strain distributions in the microstructures. © 2013 Elsevier Ltd.
  • Munekazu Ohno
    TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 100 11 758 - 764 2014年 [査読有り][通常論文]
  • Kaho Tomita, Kiyotaka Matsuura, Munekazu Ohno, Toko Tokunaga, Ruizhi Wu, Milin Zhang
    Key Engineering Materials 641 69 - 80 2014年 [査読有り][通常論文]
     
    Al-coated Mg-Li alloy rods and plates having a homogeneous coating layer of 180μm in thickness have been produced by hot extrusion of Mg-Li alloy billets together with Al disks. The Al-coated Mg-Li alloy exhibits an excellent corrosion resistance in a weight-loss test with a 0.5 mass% HCl aqueous solution, and it has been revealed by a potentiometric corrosion test with a 3 mass% NaCl aqueous solution that the Al-coated Mg-Li alloy has the same electric potential as that of pure Al. Al-coated Mg-Li alloy sheets produced by hot forging of the extruded plates exhibits good elongation of 22 %, 40 % and 130 % at room temperature, 200 °C and 300 °C, respectively.
  • Transient Liquid Phase bonding of γTiAl using Al Films and SPS method
    K. Szastak, K. Matsuura, D. Kata, M. Ohno
    Journal of Achievements in Materials and Manufacturing Engineering 67 5 - 13 2014年 [査読有り][通常論文]
  • Munekazu Ohno, Masato Maruyama, Kiyotaka Matsuura
    Acta Materialia 61 19 7334 - 7341 2013年11月 [査読有り][通常論文]
     
    The formation of a coarse columnar austenite grain (CCG) structure is a serious problem in continuous casting processes of peritectic solidified carbon steels. In this study, a guiding principle for the avoidance of CCG formation is developed. The critical condition for CCG formation recently put forward based on phase-field simulations, which is given by a balance between the cooling condition and the growth rate of the CCG, is first re-examined and modified by considering the effect of a liquid phase during the CCG formation. The validity of this critical condition is then investigated by three different casting experiments combined with heat conduction analyses. From a comparison between the cooling conditions and the resulting microstructural changes, the validity of the critical condition is successfully demonstrated. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno, Seiichi Suzuki
    Materials Science and Engineering A 564 169 - 175 2013年03月01日 [査読有り][通常論文]
     
    Effects of indentation depth and the strain rate on the local plastic strain distribution in the depth direction beneath the deformed iron surface have been studied by means of ball-dropping (high strain rate) and ball-pressing (low strain rate) tests. Evaluation of the local plastic strain was conducted by using the electron backscatter diffraction (EBSD) analysis. It was indicated that large macroscopic plastic strains with high strain rates lead to steep and narrow distribution profile of local plastic strain. The peak of the local plastic strain profile does not appear at the deformed surface but immediately beneath the surface. The deformation at high strain rates is favorable to localize the plastic strain in the vicinity of the surface. On the other hand, the deformation at lower strain rates makes plastically deformed zone wider. © 2012 Elsevier B.V.
  • Tomohiro Hayashi, Kiyotaka Matsuura, Munekazu Ohno
    Materials Transactions 54 11 2098 - 2101 2013年 [査読有り][通常論文]
     
    Hard TiC coating layer is formed on commercially pure titanium by heat treatment in a spark plasma sintering (SPS) mold filled with graphite powder. In case of heat treatment at 1243K for 3600 s, the obtained thickness of the coating layer is about 10ìm and almost uniform. This coating itself consists of TiC and graphite, and there is no titanium oxide detected by X-ray diffractometer (XRD). Vickers hardness tests have revealed that the hardness of titanium coating is 1600HV, which is much higher than that of the titanium substrate (130 HV). The hard TiC coating on titanium is considered to be advantageous when applied to load bearing parts of hard tissue replacements. The growth behavior of TiC is parabolic with an activation energy of 218.6 kJ/mol. This value is close to the activation energy of carbon diffusion in TiC. Therefore, the growth rate of TiC is rate-controlled by inward diffusion of carbon in the TiC phase. © 2013 The Japan Institute of Metals and Materials.
  • Tomohiro Takaki, Takashi Shimokawabe, Munekazu Ohno, Akinori Yamanaka, Takayuki Aoki
    Journal of Crystal Growth 382 21 - 25 2013年 [査読有り][通常論文]
     
    Dendrites are typical growth morphologies in alloy solidification. However, the ways in which dendrites grow and form solidification microstructures remain poorly understood. Here we show unexpected findings that reveal the survival of unfavorably oriented dendrites and highly complicated dendrite-dendrite interactions in three-dimensional space during the directional solidification of a binary alloy. These results are observed for the first time through very-large-scale phase-field computations performed by a graphics processing unit (GPU) supercomputer and a high-performance algorithm developed for parallel computing. © 2013 Elsevier B.V. All rights reserved.
  • チタンのTiC被覆およびTi(C,N)被覆
    松浦清隆, 林知宏, 大野宗一
    チタン 61 141  2013年 [査読有り][通常論文]
  • Munekazu Ohno, Shinpei Ohyama, Kiyotaka Matsuura
    Computational Materials Science 79 558 - 563 2013年 [査読有り][通常論文]
     
    Theoretical and numerical investigations are carried out into the inverse pinning effect of platelet particles on the grain boundary migration. The theoretical expression for the driving force of the inverse pinning proposed in an early work is first revisited and modified to remove the limitation in the volume fraction of the particles. In addition, it is shown that there exists the maximum velocity of the grain boundary due to the inverse pinning. The validity of the present considerations is demonstrated by phase-field simulations for the grain growth. © 2013 Elsevier B.V. All rights reserved.
  • Munekazu Ohno, Taichi Yamaguchi, Daisuke Sato, Kiyotaka Matsuura
    Computational Materials Science 69 7 - 13 2013年 [査読有り][通常論文]
     
    In order to clarify existence or nonexistence of the thermal pinning, viz., retardation of grain growth in polycrystalline materials due to steep temperature gradient, effects of the temperature gradient on the grain growth were systematically investigated by means of the phase-field simulations. It was found that the local grain growth is determined only by the local thermal history regardless of the temperature gradient, and hence, the temperature gradient does not directly influence the local grain growth. It was concluded that the thermal pinning does not exist when the grain boundary energy and mobility are isotropic, and the grain growth is driven only by the reduction of total grain boundary energy. © 2012 Elsevier B.V. All rights reserved.
  • Mostafa Mirjalili, Mansour Soltanieh, Kiyotaka Matsuura, Munekazu Ohno
    Intermetallics 32 297 - 302 2013年01月 [査読有り][通常論文]
     
    Titanium-aluminum diffusion couples were prepared using pure titanium and aluminum sheets in the form of a tri-layer sandwich. The Ti-Al interface was investigated after heating the specimens at 550, 575, 600, 625, 640 and 650 °C. EPMA results indicated that TiAl 3 intermetallic layer is the only phase that can form and grow at the Ti-Al interface. Evidences from the SEM observations showed that aluminum is the dominant diffusing component through the TiAl 3 layer which reacts with titanium at the Ti-TiAl 3 interface. According to the normal parabolic behavior of the layer thickening, activation energies of 33.1 kJ mol -1 and 296.2 kJ mol -1 were calculated for the low temperature grain boundary diffusion controlled growth and the high temperature bulk diffusion controlled growth, respectively. Overall activation energy of 76.8 kJ mol -1 is calculated for the diffusion-controlled growth of the TiAl 3 layer at all applied annealing temperatures. © 2012 Elsevier Ltd. All rights reserved.
  • Munekazu Ohno
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 86 5 051603 - 051603 2012年11月08日 [査読有り][通常論文]
     
    A quantitative phase-field model is developed for simulating microstructural pattern formation in nonisothermal solidification in dilute multicomponent alloys with arbitrary thermal and solutal diffusivities. By performing the matched asymptotic analysis, it is shown that the present model with antitrapping current terms reproduces the free-boundary problem of interest in the thin-interface limit. Convergence of the simulation outcome with decreasing the interface thickness is demonstrated for nonisothermal free dendritic growth in binary alloys and isothermal and nonisothermal free dendritic growth in a ternary alloy. © 2012 American Physical Society.
  • Munekazu Ohno, Shingo Tsuchiya, Kiyotaka Matsuura
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 6 2031 - 2042 2012年06月 [査読有り][通常論文]
     
    The formation of coarse columnar grains (CCGs) in the as-cast austenite structure of peritectic carbon steels is a serious problem in continuous casting processes. Recently, it was elucidated that the formation of CCGs is ascribed to a discontinuous grain growth. Furthermore, the critical condition for the discontinuous growth to occur was elicited on the basis of phase-field simulations and a theory of grain growth. In this study, by means of the phase-field simulations, the detailed investigation is carried out for the grain coarsening of the as-cast austenite structure. It is demonstrated in the two-dimensional simulations that the coarsest grain structure emerges by the discontinuous growth in the vicinity of the critical condition. In addition, a model for predicting the upper limit of grain size during the discontinuous growth is proposed. The model successfully describes the experimental result with reasonable accuracy. © The Minerals, Metals & Materials Society and ASM International 2012.
  • Y. Li, K. Matsuura, M. Ohno, Y. M. Gao, J. Yu
    Powder Metallurgy 55 2 162 - 167 2012年04月 [査読有り][通常論文]
     
    TiB 2 based hard material has been fabricated by combustion synthesis from elemental powders using Fe-Al intermetallics as the binder phase. Both the crystal orientation map and the pole figures by electron backscatter diffraction analysis showed that the TiB 2 particles have no preferential orientation. Preheating of the power compact is helpful to increase the relative density and to reduce the particle size of the synthesised product, which increases the hardness of the product. The highest hardness and relative density were achieved for a composition of TiB 2-20 vol.-%FeAl © 2012 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
  • S. Tsuchiya, M. Ohno, K. Matsuura
    Acta Materialia 60 6-7 2927 - 2938 2012年04月 [査読有り][通常論文]
     
    Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15-0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15-0.41 mass% carbon concentrations, coarse columnar γ grains (CCGs) with a minor axis diameter of 1-3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CGs) whose minor axis diameter is less than 500 μm form before the formation of CCGs and the grain structure changes discontinuously from CG to CCG. The fraction of the CG region increases with an increase in the carbon concentration. In the samples with a carbon concentration higher than 0.43 mass%, the as-cast structure consists of CGs over almost the entire ingots. Analyses of the relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CGs originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Mostafa Mirjalili, Mansour Soltanieh, Kiyotaka Matsuura, Munekazu Ohno
    Defect and Diffusion Forum 322 185 - 194 2012年03月15日 [査読有り][通常論文]
     
    Pure titanium and aluminum sheets were used to prepare titanium-aluminum diffusion couples. The diffusion couples were heated to temperatures 550, 575, 600, 625, 640, 650 and 700°C. SEM observations of the titanium-aluminum interface and EPMA results revealed that a TiAl3 intermetallic layer has formed and thickened between the layers. Grain boundaries of the TiAl 3 compound, which were revealed by back-scattered electron imaging, indicated a size distribution across the layer. Finer grains which were located close to the titanium showed that TiAl3 has nucleated at the Ti-TiAl3 interface. Thus, the former grains which had formed close to the aluminum-rich side have grown and coarsened during annealing at high temperatures. Grain coarsening of TiAl3 decreased the kinetics of the layer thickening. © 2012 Trans Tech Publications, Switzerland.
  • Marta Ziemnicka-Sylwester, Kiyotaka Matsuura, Munekazu Ohno
    ISIJ International 52 9 1698 - 1704 2012年 [査読有り][通常論文]
     
    TiB 2-based cermets with various Co contents were fabricated from elemental powders "in situ" by means of the Self-propagating High-temperature Synthesis, SHS, and Pseudo-Hot Isostatic Pressing, PHIP method. The sample pressed into a cylindrical compact was ignited in a steel can by an external heating element coiling the can. After SHS initiation, which was detected by rapid temperature increase, the samples were quickly pressed pseudo-isostatically under a pressure of 192 MPa and held for 5 min. Samples with predominant concentration of TiB 2, which varied from 70 to 85 vol.% with the addition of 5 vol% of Ti, were investigated in this study. Appreciable differences in terms of microstructure, density and hardness were observed depending on the composition. The average TiB 2 grain size increased while porosity decreased with rising concentration of TiB 2. The material synthesized with increased to 85% concentration of superhard TiB 2 grains and minimized concentration of Co exhibited greatest densification, highest hardness of about 2 400 HV, and the most homogenous microstructure. The reaction mechanism was reportedly proposed, based on temperature monitoring during combustion and previously reported references. © 2012 ISIJ.
  • Munekazu Ohno, Chihiro Murakami, Kiyotaka Matsuura, Kohichi Isobe
    ISIJ International 52 10 1832 - 1840 2012年 [査読有り][通常論文]
     
    Effects of Ti addition on grain growth in reversely-transformed austenite structure during reheating of the as-cast 0.2 mass% C steel have been investigated for a Ti concentration range between 0 and 0.2 mass% and heating rates from 0.014 to 2.5°C/s. The austenite grain growth during reheating is retarded by the Ti addition and such an effect becomes stronger with the addition of higher amount of Ti. This retarding effect is ascribable to the pinning effect of fine Ti(C,N) particles which should precipitate from the as-cast structure during the reheating process. The experimental results on the grain growth behavior are well explained by the grain growth model including the Zener force and counting reduction of the pinning effect due to the existence of the coarse Ti(C,N) particles crystallized during the solidification. © 2012 ISIJ.
  • Liang Chen, Kiyotaka Matsuura, Munekazu Ohno, Daisuke Sato
    ISIJ International 52 10 1841 - 1847 2012年 [査読有り][通常論文]
     
    The effects of insoluble particles on migration of ferrite (αr)/austenite (γ) interface during isothermal α to γ transformation at 1 133 K have been studied by means of a model experiment using diffusion couple method and a multi-phase field simulation. It was found that the insoluble particles, ZrO 2 or TiO 2 particles, could retard the migration of α/γ interface. And the tendency that the retarding effect becomes stronger with higher volume fraction of TiO 2 particle was observed. However, the retarding effect of ZrO 2 on α/γ interface is not as strong as that on δ-ferrite (δ)/austenite (γ) interface during peritectic transformation. The phase field simulation indicates that this reduction of retarding effect originates from the appearance of carbon pile-up in γ phase, which is also caused by the existence of particles. In addition, it is shown that the difference between σ α/p and σ γ/p, the interfacial energy between matrix (α or γ) and particles, strongly affects the migration velocity of a/y interface. © 2012 ISIJ.
  • Toko Tokunaga, Kiyotaka Matsuura, Munekazu Ohno
    Materials Transactions 53 6 1034 - 1041 2012年 [査読有り][通常論文]
     
    Although magnesium-based alloys have excellent mechanical properties, their very poor corrosion resistance limits their application. It has been considered that aluminum coating would solve this problem because aluminum has an excellent corrosion resistance. This study proposes a superior coating method based on the hot extrusion process. An aluminum plate set between the magnesium-based alloy billet and an extrusion die having an inversely angled face was successfully extruded together with the magnesium-based alloy billet and, as a result, the aluminum coated the extruded magnesium-based alloy with a uniform thickness. Corrosion tests based on dipping in HCl aqueous solution and potential measurement in NaCl aqueous solution showed that the extruded sample exhibited the same corrosion resistance as the aluminum. A three point bending test at room temperature resulted in fracture after plastic deformation, while at high temperatures between 100 and 300°C plastic deformation without fracture occurred. Notably, the intermetallic compound layer formed on the boundary between the magnesium-based alloy substrate and the aluminum coating layer plastically deformed without cracking at 300°C. A tensile test at room temperature resulted in a UTS of about 320 MPa and a plastic elongation of 18%. © 2012 The Japan Institute of Metals.
  • 林知弘, 松浦清隆, 大野宗一
    軽金属 62 6 233 - 236 2012年 [査読有り][通常論文]
     
    Hard TiC coating layer is formed on commercially pure titanium by heat treatment in a Spark Plasma Sintering (SPS)mold filled with graphite powder. The thickness of the TiC coating layer is about 10 μm in case of heat treatment for 3600 s at 1243 K and the thickness of this layer is almost uniform. This coating consists of TiC and graphite, and titanium oxide is not detected by X-ray diffractometer(XRD) Vickers hardness test revealed that the hardness of TiC coating was 1600 HV, which was much higher than that of titanium substrate(130 HV) The hard TiC coating on titanium is considered to have advantage in the applications to load-bearing parts of hard tissue replacements. The growth behavior of TiC is conformed to be parabolic, and the activation energy of growth of TiC is 218.6 kJ/mol. This value is close to the activation energy of carbon diffusion in TiC. Therefore, the rate of growth of TiC is controlled by carbon diffusion in TiC phase. © 2012 The Japan Institute of Light Metals.
  • Liang Chen, Kiyotaka Matsuura, Daisuke Sato, Munekazu Ohno
    ISIJ International 52 3 434 - 440 2012年 [査読有り][通常論文]
     
    Effects of dispersion of insolvable particles on peritectic transformation kinetics in Fe-C binary alloy system have been investigated by means of a model experiment based on a diffusion couple method. During isothermal holding of the diffusion couple of δ and liquid phase samples, the peritectic transformation proceeds by migrations of ferrite(δ)/austenite(γ) interface and liquid(L)/γinterface. It was observed that the existence of insolvable particle, ZrO 2, in δ phase retarded the migration of δ/γ interface and also the migration of L/γ interface. The retarding effect by the ZrO 2 particles becomes strong as volume fraction of particles increases and/or particle radius decreases, which is qualitatively coincident with the effect of particles on grain growth (Zener pinning). These findings were verified by multi-phase field simulations. © 2012 ISIJ.
  • Toko Tokunaga, Kiyotaka Matsuura, Munekazu Ohno
    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals 75 11 633 - 639 2011年11月 [査読有り][通常論文]
     
    Although magnesium-based alloys have excellent mechanical properties, their very poor corrosion resistance limits their application. It has been considered that aluminum coating would solve this problem because aluminum has an excellent corrosion resistance. This study proposes a superior coating method based on the hot extrusion process. An aluminum plate set between the magnesium alloy billet and an extrusion die having an inversely angled face was successfully extruded together with the magnesium alloy billet and, as a result, the aluminum coated the extruded magnesium alloy with a uniform thickness. Corrosion tests based on dipping in HCl aqueous solution and potential measurement in NaCl aqueous solution showed that the extruded sample exhibited the same corrosion resistance as the aluminum. A three point bending test at room temperature resulted in fracture after plastic deformation, while at high temperatures between 100 and 300°C plastic deformation without fracture occurred. Notably, the intermetallic layer formed on the boundary between the magnesium alloy substrate and the aluminum coating layer plastically deformed without cracking at 300°C. A tensile test at room temperature resulted in a UTS of about 320 MPa and a plastic elongation of 18%. © 2011 The Japan Institute of Metals.
  • Munekazu Ohno, Shingo Tsuchiya, Kiyotaka Matsuura
    Acta Materialia 59 14 5700 - 5709 2011年08月 [査読有り][通常論文]
     
    The discontinuous grain growth leading to a coarse columnar austenite grain (CCG) structure in as-cast peritectic carbon steels is analyzed by means of two- and three-dimensional phase field simulations. On the basis of the theory of grain growth, the conditions for discontinuous grain growth to occur are elucidated in terms of cooling conditions and material properties. The theoretical analysis explains the results of the phase field simulations well and should be useful in predicting the formation of CCG structure in continuously cast slabs. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • S. Tsuchiya, M. Ohno, K. Matsuura, K. Isobe
    Acta Materialia 59 9 3334 - 3342 2011年05月 [査読有り][通常論文]
     
    The formation mechanism of as-cast coarse columnar γ grain (CCG) structure in hyperperitectic carbon steels is investigated by means of a rapid unidirectional solidification method. This method achieves cooling conditions similar to those in the vicinity of a practically continuously cast slab surface. The microstructural observation of the quenched samples indicates that the CCG structure develops from the mold side along the direction of the temperature gradient. In the solidifying samples, fine columnar γ grains (FCG) always exist ahead of the CCG region. Instead of continuous growth into large grains, FCG always shrink and vanish as a result of the growth of CCG initially formed near the mold side. Therefore, the grain size at a fixed point in the ingot discontinuously changes from the FCG to the CCG. The validity of this process is supported by numerical analyses. This finding is in marked contrast to the assumption made in conventional grain growth analysis on the CCG structure. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • 徳永透子, 松浦清隆, 大野宗一
    日本金属学会誌 75 11 633 - 639 2011年 [査読有り][通常論文]
  • Jun Yu, Kiyotaka Matsuura, Munekazu Ohno
    Advances in Tribology 105258  2011年 [査読有り][通常論文]
     
    TiC-TiB 2-based cermets with Ni binder were fabricated using combustion synthesis assisted by pseudohot isostatic pressing by heating the compacted powder mixture to approximately 700 °C. The effects of composition on microstructure and hardness of the synthesized samples were investigated. The samples exhibited finer microstructure and higher hardness as TiC/TiB 2 volume ratio increased and as Ni increased up to 30vol%. A relatively high hardness value of 1950HV was obtained for TiC-TiB 2-Ni(52.5/17.5/30vol%). However, the transverse rupture strength and the modulus of elasticity values were not very high. This may be due to weak bonding strength of the interface between hard phases and Ni binder and/or insufficient densification of the samples. Copyright © 2011 Jun Yu et al.
  • Munekazu Ohno, Tetsuya Yamaguchi, Kiyotaka Matsuura, Kohichi Isobe
    ISIJ International 51 11 1831 - 1837 2011年 [査読有り][通常論文]
     
    Effects of Nb addition on as-cast ? -austenite grain structure in 0.2 mass% carbon steel are investigated by means of furnace cooling and permanent mold casting experiments. In the furnace-cooled samples with Nb addition, Nb(C,N) particles crystallize from the last-solidifying liquid in non-equilibrium solidification condition and they act as pinning particles for ? grain growth just after the solidification completion. The Nb addition produces a strong pinning effect on the as-cast ? grain structure. In the permanent mold casting experiment, Coarse Columnar Grains (CCG) structure develops from the mold wall in the sample without Nb. The increase in Nb concentration gradually decreases the fraction of CCG region and increases the fraction of Fine Columnar Grains (FCG), thus leading to the grain refinement. This refinement could be ascribed to the pinning effect of Nb(C,N) particles. © 2011 ISIJ.
  • Munekazu Ohno, Kiyotaka Matsuura
    Acta Materialia 58 18 6134 - 6141 2010年10月 [査読有り][通常論文]
     
    The peritectic reaction process in carbon steel, L + δ →γ, has been analyzed by a quantitative phase-field simulation. The calculated moving velocities of the γ-L and γ-δ planar interfaces in the isothermal peritectic transformation precisely agree with the corresponding experimental data, which strongly supports the accuracy of the present simulation. The diffusion-controlled peritectic reaction rate and the growth velocity of the γ phase along the δ-L interface obtained by the present simulation were fairly consistent with the experimentally measured values. This indicates that recent experimental findings can be explained by a diffusion-controlled mechanism. This is in marked contrast to the claims made on the basis of the experimental data and an analytical model that the peritectic reaction is not controlled by the diffusion of carbon. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Munekazu Ohno, Kiyotaka Matsuura
    Acta Materialia 58 17 5749 - 5758 2010年10月 [査読有り][通常論文]
     
    A quantitative phase-field model for two-phase solidification processes is developed based on the anti-trapping current approach with the free energy functional formulated to suppress the formation of an extra phase at the interface. This model appropriately recovers the free boundary problem for the motion of interface in the thin-interface limit and, importantly, it is applicable to the solidification process in binary alloy systems with arbitrary values of the solid diffusivities and interfacial energies. The performance of the present model is investigated for the peritectic reaction process in carbon steel. The present model exhibits excellent convergence behavior with respect to the interface thickness. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Kencana Surya, Ohno Munekazu, Matsuura Kiyotaka, Isobe Kohichi
    鋳造工学 全国講演大会講演概要集 157 87 - 87 公益社団法人 日本鋳造工学会 2010年
  • 外山和宏, 松浦清隆, 大野宗一, 佐藤大樹, 中山昌紀
    軽金属 60 1 7 - 11 2010年01月 [査読有り][通常論文]
     
    Effects of hot rolling on microstructure and mechanical properties of a cast Al-Si alloy have been investigated. The hot rolling was carried out at 330°C with rolling reductions ranging from 0 to 80%. With increase in the rolling reduction, the shrinkage cavity decreases in size and volume fraction, and also the eutectic Si particles decreased in size and distance between the particles. In the hot rolled samples, the Al matrix phase consisted of recrystallized grains of which size is comparable to the distance between the eutectic Si particles. Ultimate tensile strength of hot rolled sample increased nearly 33% compared to the one without hot rolling. Elongation of hot rolled sample was about 4.5 times higher than the one without hot rolling.
  • Surya Kencana, Munekazu Ohno, Kiyotaka Matsuura, Kohichi Isobe
    ISIJ International 50 2 231 - 238 2010年 [査読有り][通常論文]
     
    Effects of Al addition on as-cast y-austenite grain structure in 0.2wt%C-0.035wt%P steel with Al concentration ranging from 0.04 to 1.04 wt% were studied by means of furnace cooling and casting experiments. In the furnace cooling experiment with a cooling rate of 0.03°C/s, the as-cast γ grain structure consisted of equiaxed grains and the γ grain size was not affected by the increase in Al concentration up to 0.54 wt%. In the casting experiment of the sample with 0.04 wt% Al, on the other hand, the as-cast y grain structure consisted of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Equiaxed Grain (EG) regions, sequentially, from the mold side to the center of the ingot. The increase in Al concentration leads to increase in the fraction of FCG region at the expense of both CCG and EG regions. Even in the samples with high Al concentrations, AIN particles were rarely found and also Al segregation did not occur substantially. Instead, P segregated in interdendritic regions. The concentration of the segregated P increased from the CCG region to the FCG region. It was suggested based on a thermodynamic calculation that the segregation of P is enhanced by Al addition and the high P concentration stabilizes the high temperature phase such as liquid or δ-ferrite, depending on Al concentration at lower temperatures. This stabilized high temperature phase is considered to retard the γ grain boundary migration. Therefore, the increase of FCG region and the decreases of CCG and EG regions due to Al addition should be attributable to pinning effect of the stabilized high temperature phase. © 2010 ISIJ.
  • Munekazu Ohno, Ying Chen, Tetsuo Mohri
    Materials Science Forum 631-632 401 - 406 2010年 [査読有り][通常論文]
     
    Multi-scale simulation of ordering process from electronic, atomistic scales to microstructural scale was carried out by hybridizing Phase Field Method (PFM) and Cluster Variation Method (CVM). The hybrid model was applied to disorder-L10 ordering process in Fe-Pd system. Furthermore, computation of relaxation constants in the PFM was attempted based on Path Probability Method (PPM) which is the time evolution version of the CVM, within a linearized analysis of order-order relaxation process. © (2010) Trans Tech Publications.
  • Effects of strain-graded plastic deformation on mechanical properties of metals
    K. Matsuura, M. Ohno
    Advances in Science and Technology 63 348 - 351 2010年 [査読有り][通常論文]
  • Keiji Nakajima, Hongwei Zhang, Katsunari Oikawa, Munekazu Ohno, Pär G. Jönsson
    ISIJ International 50 12 1724 - 1734 2010年 [査読有り][通常論文]
     
    The dramatic progress made over the last 10 to 15 years in the field of "comuter simulation of solidification and casting" is greatly due to the supports by academic as well as industrial research. The driving force behind this undertaking was the promise of predictive capabilities that will allow process and material developments. Here, the recent works on modeling were summarized, for the macrosegregation in the macroscale simulation, and the Cellular Automaton, the solidification path combined with the microsegregation, the phase-field model in the meso-scale and micro-scale simulation. © 2010 ISIJ.
  • Surya Kencana, Munekazu Ohno, Kiyotaka Matsuura, Kohichi Isobe
    ISIJ International 50 12 1965 - 1971 2010年 [査読有り][通常論文]
     
    Effects of addition of P and simultaneous additions of Al and P on as-cast γ grain structures of 0.2 mass% C steel have been investigated by means of permanent mold casting. The as-cast γ grain structure consists of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Coarse Equiaxed Grain (CEG) regions from the mold side to center of the ingot. The single addition of P increases the FCG region in which short axis diameter of the columnar γ grain is comparable to the primary dendrite arm spacing. The simultaneous additions of Al and P also lead to refinement of the structure and, importantly, the complete refinement of the as-cast γ grain structure, viz., the structure without CCG and CEG regions was obtained even with a small amount of P addition when Al was added. The EPMA analysis showed that the refinement is associated with P segregation at interdendritic regions which is enhanced by Al addition. From thermodynamic calculation, it was demonstrated that high P concentrations stabilize δ and liquid phases at lower temperatures and produce the pinning effect on the growth of γ grains at interdendritic regions. © 2010 ISIJ.
  • マイクロTIG溶接法による鉄系材料のアルミナイド被覆
    大井川央, 大野宗一, 松浦清隆
    傾斜機能材料論文集 24 53 - 57 2010年 [査読有り][通常論文]
  • 塑性ひずみ傾斜加工を施したAl-Si鋳造材の機械的性質
    大野宗一, 松浦清隆
    傾斜機能材料論文集 24 58 - 63 2010年 [査読有り][通常論文]
  • Shingo Tsuchiya, Munekazu Ohno, Kiyotaka Matsuura, Kohichi Isobe
    ISIJ International 50 12 1959 - 1964 2010年 [査読有り][通常論文]
     
    Effects of Cr addition on as-cast Coarse Columnar austenite Grain (CCG) structure were investigated for 0.2 mass% carbon steel by means of rapid unidirectional solidification method which realizes cooling conditions similar to those in the vicinity of continuous cast slab surface. Although the as-cast structure of the unidirectionally solidified samples always consisted of the CCGs regardless of Cr addition, the aspect ratio of the CCGs was remarkably reduced by the Cr addition. During solidification. Fine Columnar austenite Grains (FCGs) existed at the growing front of the CCG region and the migration velocity of the FCG/CCG boundary was reduced by the Cr addition. EPMA analysis revealed that the Cr addition enhances P segregation at interdendritic positions, which lowers a temperature for completion of y transformation, T y. This lowered Ty reduces the migration velocity of the FCG/CCG boundary and, as a result, induces the formation of CCGs having shorter major axis diameters. © 2010 ISIJ.
  • Munekazu Ohno, Kiyotaka Matsuura
    ISIJ International 50 12 1879 - 1885 2010年 [査読有り][通常論文]
     
    Motion and morphology of triple junction during peritectic reaction process is analyzed for a model alloy system based on a quantitative-phase-field simulation for two-phase solidification involving diffusion in the solid. It is demonstrated that the dominative process controlling the motion of the reaction front gradually changes from the solid-solid transformation to the secondary solid solidification as the moving velocity of solid-solid interface decreases. On the other hand, the local shape of the triple junction is mainly determined by the balance between the interfacial energies regardless of the difference in the moving velocity of solid-solid interface.© 2010 ISIJ.
  • Synthesis of Functionally Graded Intermetallics
    Kiyotaka Matsuura, Naoki Mizuta, Munekazu Ohno
    Materials Science and Technology Vol. 4 46 - 151 2010年 [査読有り][通常論文]
  • Munekazu Ohno, Kanji Yoh
    Materials Transactions 50 5 1202 - 1207 2009年05月 [査読有り][通常論文]
     
    Thermodynamic calculation of phase equilibria in As-Fe-In ternary system is performed based on Calphad approach, directing a special attention to fabrication process of Fe/InAs hybrid structure for spin injection device. For this, the thermodynamic assessment of Fe-In binary system is first carried out utilizing reported experimental data. Then, the liquidus surface of the ternary system and invariant reactions are calculated. The isothermal sections in low temperature region are presented and discussed in the light of the optimization of the growth temperature of Fe film on InAs substrate during the fabrication process. © 2009 The Japan Institute of Metals.
  • Munekazu Ohno, Kiyotaka Matsuura
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 79 3 031603 - 031603 2009年03月03日 [査読有り][通常論文]
     
    An antitrapping current scheme for quantitative phase-field model is extended to solidification process in a dilute binary alloy system involving diffusion in the solid. It is demonstrated in an asymptotic analysis that in the case of an arbitrary value of the solid diffusivity, five types of constraints exist between interpolating functions used in the phase-field model, which need to be satisfied simultaneously to eliminate all anomalous interface effects. Then, the authors present an appropriate form of the antitrapping current term for the two-sided case to remove all the spurious effects. The convergence test of the output with respect to the interface thickness was carried out for the isothermal dendrite growth process, which demonstrates an excellent performance of the present model. © 2009 The American Physical Society.
  • Masayoshi Sasaki, Kiyotaka Matsuura, Kenichi Ohsasa, Munekazu Ohno
    ISIJ International 49 9 1367 - 1371 2009年 [査読無し][通常論文]
     
    Effects of titanium and boron addition on columnar austenite grain structure of S45C steel have been investigated in casting experiments with different cooling rates of 4.50 and 16.67°C/s. Without addition of these elements, the columnar austenite grains develop over the whole sample under the present casting conditions. The addition of titanium and boron induces formation of equiaxed austenite grains and, importantly, fully equiaxed austenite structure was observed in the sample with 0.2 mol% Ti and 0.4 mol% B. The microstructural observations indicated that this behavior of austenite structure stems from the columnar-to-equiaxed transition of ferrite dendrite structure. The addition of these elements, furthermore, leads to refinement of the columnar austenite grains, which is ascribable to pinning effect of boron nitride and titanium carbonitride.
  • 小嶋桂介, 松浦清隆, 大笹憲一, 大野宗一
    軽金属 59 1 2 - 6 2009年01月 [査読有り][通常論文]
     
    In the present study, it is shown that TiC-20vol%Fe composites can be successfully fabricated by combustion synthesis from elemental powder compacts of titanium, carbon and iron. Particular concern is directed to reduction of the porosity in the product by preheating procedure of the powder compact. There exists 36% of porosity in the fabricated composites without the preheating treatment. However, the preheating treatment leads to the reduction of the volume fraction of porosity to about 10% and, furthermore, a two-step preheating treatment reduces the volume fraction of porosity down to about 5%. Vickers hardness of the TiC-Fe composites fabricated in this study was measured to be around 1300HV regardless of the preheating condition.
  • Munekazu Ohno, Kiyotaka Matsuura
    ISIJ International 49 10 1568 - 1574 2009年 [査読有り][通常論文]
     
    Dendrite growth simulations have been performed to analyze Columnar-to-Equiaxed Transition (CET) of δ- ferrite dendrite structure triggered by fine particles of a primary Ti(C, N) crystal. The CET position estimated by Hunt's criterion and the present simulation indicated that the existence of a large number of the Ti(C, N) particles gives rise to the CET of the dendrite structure even in the vicinity of the mold wall under the present casting condition. Furthermore, the capability of the Ti(C, N) leading to the CET was discussed in the light of the different number of Ti(C, N) particles and the different thickness of cast ingot. It was shown that a sufficient number of Ti(C, N) leads to the formation of fully equiaxed dendrite structure irrespective of the ingot thickness. © 2009 ISIJ.
  • Masayoshi Sasaki, Kiyotaka Matsuura, Kenichi Ohsasa, Munekazu Ohno
    ISIJ International 49 9 1362 - 1366 2009年 [査読有り][通常論文]
     
    Effects of Ti addition on as-cast austenite structure of S45C steel have been investigated by means of furnace cooling experiment at a cooling rate of 0.03°C/s, focusing on the Ti addition ranging from 0 to 0.5mol%. The Ti addition reduces average austenite grain diameter down to a size comparable to secondary dendrite arm spacing. In samples with the Ti addition, the austenite grain boundary is located at inter-dendritic position where Ti(C,N) particles exist and the refinement of austenite grain structure is ascribable to pinning effect of the Ti(C,N) particle formed in L+γ-Fe+Ti(C,N) phase field. The increment of Ti addition does not substantially change the size of Ti(C,N) particle but increases the number of the Ti(C,N) particles, leading to further refinement of the austenite grains.
  • Shingo Tsuchiya, Munekazu Ohno, Kiyotaka Matsuura, Kohichi Isobe
    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 95 9 629 - 635 2009年 [査読有り][通常論文]
     
    Synopsis : Effects of titanium addition on as-cast austenite grain size in 0.2 mass% carbon steel cooled at 0.03°/s has been studied. The average austenite grain size was reduced from 4.3 to 1.9 mm by increasing the titanium concentration from 0 to 0.13 mass%. Two different shapes of Ti(C, N) particles were observed in the solidified samples; one was a facet shape which were dispersed over the whole observation area and the other was a filmy shape, most of which were located on the austenite grain boundary. Thermodynamic calculations and EPMA analyses showed that the facet shaped Ti(C, N) particles crystallize at relatively high temperatures, while the filmy shaped ones form from the last solidifying liquid in the inter-dendritic regions at lower temperatures. The latter Ti(C, N) is considered to provide the grain refinement effect of austenite grain structure. The grain refinement effects of titanium are further discussed by comparing the results of 0.2 mass% carbon steel with those of 0.45 mass % one, which supports the substantial effect of the filmy shaped Ti(C,N) on the as-cast austenite grain structure.
  • 水田直気, 松浦清隆, 大野宗一, 宮本鉄生, 桐原聡秀
    軽金属 58 12 656 - 661 2008年12月 [査読有り][通常論文]
     
    A new titanium aluminide coating method on titanium surface has been proposed, which is characterized by arc surface melting of an aluminum plated titanium surface. The arc surface melting was performed using a computer aided 3-dimensional micro welder (3DMW) designed by the present authors. The arc heat melted the aluminum plated titanium surface and produced a titanium aluminide layer of a ∼300 μm thickness on the surface of titanium substrate. The titanium aluminide coated sample exhibited 500-700 in Vickers hardness number and excellent wear resistance.
  • Masayoshi Sasaki, Kiyotaka Matsuura, Kenichi Ohsasa, Munekazu Ohno
    TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 94 11 491 - 495 2008年11月 [査読有り][通常論文]
     
    Effects of Ti addition on as-cast austenite grain size in S45C carbon steel have been investigated in range of Ti addition between 0 and 0.5 mol%. The cooling rate was set to be 0.03 degrees C/s. It is shown that the average austenite grain diameter decreases from 1900 to 250 mu m as the Ti addition increases from 0 to 0.5 mol%. The microstructural observation revealed that the motion of austenite grain boundary is inhibited at inter-dendritic position, which is ascribable to pinning effect of Ti(C,N) particle. The CALPHAD analysis showed that this particle crystallizes mainly in L+gamma-Fe+Ti(C,N) phase field, As increasing Ti addition, the size of Ti(C,N) particle does not Substantially change, however, the number of the Ti(C,N) particles increases, leading to further refinement of the austenite grains.
  • Munekazu Ohno, Kanji Yoh
    Journal of Crystal Growth 310 11 2751 - 2759 2008年05月15日 [査読有り][通常論文]
     
    A thermodynamic model has been developed for the system As-Fe by combining the Calphad approach and the first-principles total energy calculations. Our first-principles calculations are based on the projector augmented wave method within the generalized gradient approximation. We performed these calculations because the experimental values for the enthalpy of formation of the compounds As2Fe, AsFe and AsFe2 may have a large uncertainty. Our results indicate significantly more negative values for the enthalpy of formation of these compounds relative to the experimentally established values. We demonstrate that applying our first-principles results in a thermodynamic analysis based on the Calphad approach leads to a calculated phase diagram and thermodynamic properties which are not significantly different from experimental data. © 2008 Elsevier B.V. All rights reserved.
  • Munekazu Ohno, Kanji Yoh
    Physica E: Low-Dimensional Systems and Nanostructures 40 5 1539 - 1541 2008年03月 [査読有り][通常論文]
     
    We performed semi-classical Monte-Carlo (MC) analysis on D'yakonov-Perel' spin relaxation process in two-dimensional channel of asymmetric (0 0 1) quantum-well. Our particular attention is directed to the persistent spin helix (PSH) state which is observable not in the time-resolved analysis but in the space-resolved analysis. It is demonstrated that the cubic term in the Dresselhaus model fairly leads to the decay of PSH state. © 2007 Elsevier B.V. All rights reserved.
  • A. Kozlov, M. Ohno, T. Abu Leil, N. Hort, K. U. Kainer, R. Schmid-Fetzer
    Intermetallics 16 2 316 - 321 2008年02月 [査読有り][通常論文]
     
    The basis of this work is a detailed investigation of phase equilibria of the Mg-Sn-Ca system for wide ranges of composition and temperature by means of a combined approach of our own key experiments, first-principles calculations of finite-temperature properties of the compounds and Calphad-type modeling. In this report, the generated thermodynamic description is applied to predict the phase formation and discuss the solidification behavior of practically important Mg-rich alloys. Solidification calculations, based on computational thermochemistry and the present dataset, are successfully applied to the analysis of experimentally observed as-cast microstructures and thermal analysis data for the Mg-rich alloy samples. © 2007 Elsevier Ltd. All rights reserved.
  • A. Kozlov, M. Ohno, R. Arroyave, Z. K. Liu, R. Schmid-Fetzer
    Intermetallics 16 2 299 - 315 2008年02月 [査読有り][通常論文]
     
    The phase equilibria of the Mg-Sn-Ca system for the entire composition and temperature ranges have been clarified based on the Calphad method. To obtain a reliable thermodynamic description, we performed key experiments for the phase boundary data and also utilized the first-principle results of the finite-temperature properties for the binary and ternary compounds. Experimental works for the phase equilibria, which consist of thermal, crystallographic and microstructural analyses, and the thermodynamic modeling combined with finite-temperature first-principle calculations are reported. The satisfying agreements between the experimental and calculated results support the reliability of the proposed thermodynamic description. The phase diagram for overall composition and temperature ranges of the ternary system based on the thermodynamic calculations is presented. In a second study this result is applied to obtain details of the phase formation during solidification for practically important Mg-rich as-cast alloys. © 2007 Elsevier Ltd. All rights reserved.
  • Munekazu Ohno, Kanji Yoh
    Physical Review B - Condensed Matter and Materials Physics 77 4 045323  2008年01月22日 [査読有り][通常論文]
     
    We analyzed the applicability of original Datta-Das proposal for spin-field-effect transistor (spin-FET) to nonballistic regime based on semiempirical Monte Carlo simulation for spin transport. It is demonstrated that the spin helix state in two-dimensional electron gas system is sufficiently robust against D'yakonov-Perel' spin relaxation to allow an operation of Datta-Das-type spin-FET in the nonballistic transport regime. It is also shown that the spin diffusion length of the spin helix state can be increased with an in-plane electrical field along the [1 1̄ 0] direction. In marked contrast to early proposals for nonballistic spin-FETs, the "on" and "off" states are characterized by a 180° phase difference in the spin precession motions, which is highly advantageous in terms of device flexibility. © 2008 The American Physical Society.
  • 外山 和宏, 大野 宗一, 松浦 清隆
    鋳造工学 全国講演大会講演概要集 153 100 - 100 公益社団法人 日本鋳造工学会 2008年
  • Munekazu Ohno, Kiyotaka Matsuura
    ISIJ International 48 10 1373 - 1379 2008年 [査読有り][通常論文]
     
    The effect of Ti addition on as-cast austenite (γ) structure of S45C steel has been investigated. The ascast γ structure without the Ti addition consists of coarse columnar grains. The Ti addition leads to formation of equiaxed γ-grains and also grain refinement of the γ structure. Fully equiaxed and very fine γ-grain structure forms in a limited range of Ti addition of 0.13 to 0.17 wt%. The thermodynamic calculation of phase diagram showed that Ti carbonitride crystallizes as a primary phase In this composition range. The formation of the very fine equiaxed γ-grain structure originates from the columnar-to-equlaxed transition (CET) in 5-dendrite solidification Induced by the primary Ti carbonitride particles. These particles act as the nucleation sites of the equiaxed 5-dendrite. The experimental results are suggestive of existence of finer Ti carbonitride particles retarding the grain growth of γ-phase after the peritectic transformation, which leads to the refinement of the as-cast γ structure. © 2008 ISIJ.
  • Masayoshi Sasaki, Kiyotaka Matsuura, Kenichi Ohsasa, Munekazu Ohno
    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 94 9 331 - 335 2008年 [査読有り][通常論文]
     
    We performed casting experiment of S45C steel, focusing on the effect of addition of Ti and B on as-cast austenite grain structure. Without the addition of these elements, the austenite structure consists of columnar grains over the whole sample in the present casting conditions. The addition of Ti and B leads to the formation of equiaxed austenite grains and, importantly, fully equiaxed austenite structure was observed in the sample with 0.2mol% Ti and 0.4 mol % B. The microstructural observations indicated that this behavior of austenite structure stems from the columnar-to-equiaxed transition of ferrite dendrite structure. The addition of these elements, furthermore, leads to refinement of the columnar austenite grains.
  • Yong Du, Jiong Wang, Jingrui Zhao, Julius Clemens Schuster, Franz Weitzer, Rainer Schmid-Fetzer, Munekazu Ohno, Honghui Xu, Zi Kui Liu, Shunli Shang, Wenqing Zhang
    International Journal of Materials Research 98 9 855 - 871 2007年09月 [査読有り][通常論文]
     
    A thermodynamic optimization for the Al-Mn system is performed by considering reliable literature data and newly measured phase equilibria on the Al-rich side. Using X-ray diffraction, differential thermal analysis, and scanning electron microscopy with energy dispersive X-ray spectroscopy methods, the melting behavior of λ-Al4Mn was correctly elucidated, and two invariant reactions associated with λ-Al4Mn (L + μ-Al4Mn ↔ λ-Al4Mn at 721 ± 2°C and L + λ-Al4Mn ↔ Al6Mn at 704 ± 2°C) are observed. The model Al12Mn4(Al, Mn) 10 previously used for Al8Mn5 was modified to be Al12Mn5(Al, Mn)9 based on crystal structure data. In addition, the high-temperature form of Al11 (Mn4 is included in the assessment. Employing fewer adjustable parameters than previous assessments, the present description of the Al-Mn system yields a better overall agreement with the experimental phase diagram and thermodynamic data. The obtained thermodynamic description for the Al-Mn system is then combined with those in the Al-Mg and Mg-Mn systems to form a basis for a ternary assessment. The thermodynamic parameters for ternary liquid and ternary compound Mn2Mg3Al18 (τ) are evaluated on the basis of critically assessed experimental data. The enthalpy of formation for τ resulting from CALPHAD (CALculation of PHAse Diagrams) approach agrees reasonably with that via first-principles methodology. Comparisons between the calculated and measured phase equilibria in the Al-Mg-Mn system show that the accurate experimental information is satisfactorily accounted for by the present description. A reaction scheme for the whole ternary system is presented for practical applications. © Carl Hanser Verlag GmbH & Co. KG.
  • Munekazu Ohno, Kanji Yoh
    Physical Review B - Condensed Matter and Materials Physics 75 24 241308  2007年06月26日 [査読有り][通常論文]
     
    The D'yakonov-Perel' spin relaxation process in the (001) InAs quantum well system is studied based on Monte Carlo (MC) simulation. The present space-resolved MC analysis demonstrates that the relaxation of spins oriented in any axes is totally suppressed with equal strength of Rashba and Dresselhaus effects, which is in marked contrast with the spin relaxation anisotropy reported previously in time-resolved analyses. Our calculation also shows a substantial contribution of the cubic term of the wave number vector in the Dresselhaus model onto the spatial spin distribution. © 2007 The American Physical Society.
  • Patrice E.A. Turchi, Igor A. Abrikosov, Benjamin Burton, Suzana G. Fries, Göran Grimvall, Larry Kaufman, Pavel Korzhavyi, V. Rao Manga, Munekazu Ohno, Alexander Pisch, Andrew Scott, Wenqing Zhang
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 31 1 4 - 27 2007年03月 [査読有り][通常論文]
     
    The increased application of quantum-mechanical-based methodologies to the study of alloy stability has required a re-assessment of the field. The focus is mainly on inorganic materials in the solid state. In a first part, after a brief overview of the so-called ab initio methods with their approximations, constraints, and limitations, recommendations are made for a good usage of first-principles codes with a set of qualifiers. Examples are given to illustrate the power and the limitations of ab initio codes. However, despite the "success" of these methodologies, thermodynamics of complex multi-component alloys, as used in engineering applications, requires a more versatile approach presently afforded within CALPHAD. Hence, in a second part, the links that presently exist between ab initio methodologies, experiments, and the CALPHAD approach are examined with illustrations. Finally, the issues of dynamical instability and of the role of lattice vibrations that still constitute the subject of ample discussions within the CALPHAD community are revisited in the light of our current knowledge with a set of recommendations. © 2006 Elsevier Ltd. All rights reserved.
  • Munekazu Ohno, Kanji Yoh
    Journal of Applied Physics 102 12 123908  2007年 [査読有り][通常論文]
     
    The magnetization reversal process of Fe thin film wire is studied based on two-dimensional micromagnetic simulation. It is demonstrated that the external field parallel to the width direction results in the formation of a 180° Ńel wall, whereas the field applied to the thickness direction yields the Bloch-like walls, which turn into C -type walls in the residual state. These behaviors are explained by the anisotropic dependence of wall energy in the direction of the external field. The stray field during this process is analyzed in detail. © 2007 American Institute of Physics.
  • T. Mohri, M. Ohno, Y. Chen
    Defect and Diffusion Forum 263 21 - 30 2007年 [査読有り][通常論文]
     
    First principles calculations have been applied in various fields in Materials Science. The authors have been attempting to reproduce a binary phase diagram by combining FLAPW electronic structure total energy calculations with Cluster Variation Method of statistical mechanics. Such a first principles calculation for static equilibrium has been quite successful for a series of Fe-based alloy systems. Recently, main attention is directed towards the extension of the first principles calculation to phase transformation dynamics by incorporating Phase Field Method. A series of preliminary calculations on disorder-L10 ordering in Fe-Pd and -Pt are satisfactory and the evolution process of Anti-Phase Boundaries was reproduced. In the present report, first-principles calculations of phase equilibria and phase transformation are briefly reviewed. Particular focuses are placed on coarse graining operation which authors developed and and time scaling as a remaining problem.
  • Munekazu Ohno, Tetsuo Mohri
    Materials Transactions 47 11 2718 - 2724 2006年11月 [査読有り][通常論文]
     
    A hybrid scheme of Cluster Variation Method (CVM) combined with Phase Field Method (PFM) is applied to multiscale analysis of disorder-B2 transition. By comparing the relaxation curves of Long-Range-Order (LRO) parameter obtained by the present hybrid model and by Path Probability Method (PPM), a critical estimation of the relaxation constant which determines the time scale of the temporal evolution process of microstructure is attempted for B2 ordering process. It is found that both the LRO relaxation curves are well described within the autocatalytic reaction model of chemical species. That both curves coincide satisfactorily assures the existence of a scaling property between PFM and PPM. Finally, microstructural evolution process is simulated within Time Dependent Ginzburg Landau equation. © 2006 The Japan Institute of Metals.
  • M. Ohno, A. Kozlov, R. Arroyave, Z. K. Liu, R. Schmid-Fetzer
    Acta Materialia 54 18 4939 - 4951 2006年10月 [査読有り][通常論文]
     
    The thermodynamic model of the Ca-Sn system was obtained, utilizing the first-principles total energies and heat capacities calculated from 0 K to the melting points of the major phases. Since the first-principles result for the formation energy of the dominating Ca2Sn intermetallic phase is drastically different from the reported experimental data, we performed two types of thermodynamic modeling: one based on the first-principles output and the other based on the experimental data. In the former modeling, the Gibbs energies of the intermetallic compounds were fully quantified from the first-principles finite temperature properties and the superiority of the former thermodynamic description is demonstrated. It is shown that it is the combination of finite temperature first-principle calculations and the Calphad modeling tool that provides a sound basis for identifying and deciding on conflicting key thermodynamic data in the Ca-Sn system. © 2006 Acta Materialia Inc.
  • M. Ohno, D. Mirkovic, R. Schmid-Fetzer
    Acta Materialia 54 15 3883 - 3891 2006年09月 [査読有り][通常論文]
     
    The phase equilibria and solidification process for Mg-rich Mg-Al-Mn-Zn alloys are analyzed based on a combination of computational thermochemistry and differential thermal analysis and differential scanning calorimetry measurements. Our main concern is the proper interpretation of the experimental results of thermal analysis. For a wide range of Mg-rich alloys it is demonstrated that: (i) the high-temperature signal does not represent the actual liquidus temperature and this signal is related to the phase boundary, L + Al8Mn5/L + Al8Mn5 + (Mg); and (ii) the low-temperature signal in thermal analysis is associated not with the end of the solidification process but with the precipitation of γ-Mg17Al12 phase under the Scheil condition. In addition, we demonstrate that the actual solidification temperature, either at a slow cooling rate (1 K/min) or in casting, is virtually identical to the incipient melting temperature of as-cast alloys during subsequent heating. © 2006 Acta Materialia Inc.
  • M. Ohno, R. Schmid-Fetzer
    International Journal of Materials Research 97 5 526 - 532 2006年05月 [査読有り][通常論文]
     
    The phase equilibria of Mg-rich Mg-Mn-Zn alloys are scrutinized on the basis of computational thermochemistry. The experimentally well-established facts are very well reproduced by the present calculation. The important invariant reaction involving Mg-solid solution is calculated to be a transition type reaction L + (Mg) ↔ αMn + Mg51Mn20 at 340.18°C. Some aspects that were incorrectly interpreted in early experimental works are pointed out and are explained by the present calculations also involving non-equilibrium effects.
  • M. Ohno, R. Schmid-Fetzer
    Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques 97 5 526 - 532 2006年05月 
    The phase equilibria of Mg-rich Mg-Mn-Zn alloys are scrutinized on the basis of computational thermochemistry. The experimentally well-established facts are very well reproduced by the present calculation. The important invariant reaction involving Mg-solid solution is calculated to be a transition type reaction L + (Mg) ↔ αMn + Mg51Mn20 at 340.18 °C. Some aspects that were incorrectly interpreted in early experimental works are pointed out and are explained by the present calculations also involving non-equilibrium effects.
  • Bernd Böttger, Janin Eiken, Munekazu Ohno, Gerald Klaus, Martin Fehlbier, Rainer Schmid-Fetzer, Ingo Steinbach, Andreas Bührig-Polaczek
    Advanced Engineering Materials 8 4 241 - 247 2006年04月 [査読有り][通常論文]
     
    A research approach that combined thermodynamics chemistry, casting experiments, and phase field simulations was presented with the aim to derive microstructure quality criteria to assist the computational design of new optimized magnesium-based cast alloys. A precise Calphad analysis was performed for the Mg-Al-Mn-zn system as an important basis for simulation and experiments. An existing phase-field model was extended to perform spatially resolved microstructure simulations directly coupled to the new assessed Mg-Al-Mn-Zn databases, for simulation of microstructure formation during solidification. The results show that the calculated phase equilibria are in excellent agreement with the experimental data and the solidification behavior of Mg-alloys.
  • M. Ohno, D. Mirkovic, R. Schmid-Fetzer
    Materials Science and Engineering A 421 1-2 328 - 337 2006年04月 [査読有り][通常論文]
     
    Phase equilibria in Mg-rich corner of Mg-Al-Zn system are analyzed in detail. Thermodynamic calculations are compared with literature data and own key experimental results by means of DSC and DTA measurements. The detailed comparison strongly supports the reliability of the selected thermodynamic description. Furthermore, our focus is placed on proper interpretation of experimental results obtained by thermal analysis. Based on thermodynamic calculation, it is clarified that a signal observed in thermal analysis, which was interpreted as end of solidification in the literature, is related to the start of the monovariant eutectic reaction L + (Mg) + γ-Mg17Al12 under non-equilibrium condition and the solidification process ends at lower temperature. This fact is supported by our microstructural observation. © 2006 Elsevier B.V. All rights reserved.
  • Tetsuo Mohri, Munekazu Ohno, Ying Chen
    Journal of Phase Equilibria and Diffusion 27 1 47 - 53 2006年02月 [査読有り][通常論文]
     
    Theoretical procedures of first-principles calculations of phase stability and phase equilibria are summarized. The present scheme is shown to be able to reproduce the transition temperatures with surprisingly high accuracy for Fe-Pd and Fe-Pt systems. The main emphasis of the present report is placed on the extension of the first-principles calculation to transition dynamics calculations. This is performed by combining the cluster variation method with the phase-field method via a coarse graining operation. The time evolution process of antiphase boundaries associated with L10 ordering for Fe-Pd system is demonstrated. © ASM International.
  • Rainer Schmid-Fetzer, Andreas Janz, Joachim Gröbner, Munekazu Ohno
    Advanced Engineering Materials 7 12 1142 - 1149 2005年12月 [査読有り][通常論文]
     
    The different dimensions of quality aspects in a thermodynamic Mg alloy database include correctness, reasonability, accuracy and safety. It is recommended to scrutinize the database by applying a set of standard procedures to check the dimensions of the quality aspects and thus detecting and eliminating errors. The quality assurance should be documented by the database developer as the typical end user will not have enough time, resources and platform to perform such a quality check. It is pointed out that computational thermodynamics can be used a powerful tool for complex multicomponent materials and processes.
  • M. Ohno
    Physical Review B - Condensed Matter and Materials Physics 72 10 104109  2005年09月01日 [査読有り][通常論文]
     
    We present natural iteration method for solving a set of simultaneous equations in the path probability method with spin flipping kinetics. The formulations for the pair and the tetrahedron approximations are demonstrated. It is proved that the path probability function always increases as iteration proceeds, which assures that iteration always converges to a maximum of the path probability function. Also, in the steady state, the superposition expression for path variables in the present method can be reduced to the one for the cluster probabilities in the cluster variation method. The present method is applied to the calculation for the disorder-L10 transition at 1:1 stoichiometry and the PPM calculation demonstrates a transient formation of L12 ordered phase during L10 ordering process. © 2005 The American Physical Society.
  • Munekazu Ohno, Rainer Schmid-Fetzer
    Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques 96 8 856 - 869 2005年08月 [査読有り][通常論文]
     
    A thermodynamic description of the Mg-Al-Mn system on the basis of critically assessed experimental data is presented. Particular attention is placed on the solubility of manganese in Mg-rich liquid alloys. The overall consistency between calculated and experimental phase equilibria is shown and the Calphad assessment enables a clear identification of consistency of various groups of data. In manganese-saturated liquid alloys the primary crystallizing phases are β-Mn and Al8Mn5 up to 23 wt.% Al and below 815°C. Two invariant reactions, ε(hcp) ↔ L + β-Mn + Al 8Mn5 (815°C) and L + α-Mn ↔ Mg(hcp) + Al8Mn5 (637°C) occur in Mg-rich liquid alloys. In addition, the solidus temperature and solid phase equilibria are given by the present thermodynamic calculation. © Carl Hanser Verlag, München.
  • J. Gröbner, D. Mirkovic, M. Ohno, R. Schmid-Fetzer
    Journal of Phase Equilibria and Diffusion 26 3 234 - 239 2005年06月 [査読有り][通常論文]
     
    The monotectic reaction L″ = L′ + δMn in the binary system Mg-Mn was measured at about 1200°C using differential thermal analysis. This was possible by sealing the samples in arc welded Ta capsules, thus solving the problem of high vapor pressure of Mg at such high temperature and also suppressing any reaction of Mg with the environment. Problems associated with the reaction of Mn with the Ta crucible are discussed in detail. In addition, a thermodynamic assessment and calculation of the entire Mg-Mn phase diagram was performed, incorporating published experimental data on the Mn solubility in the Mg-rich corner. ©ASM International.
  • Munekazu Ohno, Tetsuo Mohri
    Philosophical Magazine 83 3 315 - 328 2003年01月21日 [査読有り][通常論文]
     
    The order-order relaxation process of the long-range order parameter in the non-uniform L10 ordered phase is investigated by the hybridized calculation using the phase field method (PFM) and the cluster variation method (CVM). The resultant kinetics are composed of three processes sufficiently differing in the relaxation rates. The first and second processes correspond to antisite ordering-disordering relaxation within the ordered domain and relaxation due to a wetting-antiwetting of the antiphase boundary respectively. The third process is due to a coarsening of the ordered domain and its relaxation rate is fairly slow compared with the relaxation rates of the first and second processes; this is consistent with experimental observation. It is noted that the present result of relaxation kinetics involving the three processes is obtained only by the hybridized calculation using the PFM and CVM.
  • Munekazu Ohno, Tetsuo Mohri
    Materials Transactions 43 9 2189 - 2192 2002年09月 [査読有り][通常論文]
     
    A hybridized calculation of Phase Field Method and Cluster Variation Method is applied to investigate the relaxation process of Long-Range-Order parameter (LRO) originating from competitive growth of ordered domains. It is shown that the coarsening process proceeds by both the curvature-driven growth and coalescence among in-phase ordered domains. In the early annealing period, an average radius of ordered domains increases proportional to t1/2 as predicted by the curvature-driven growth. In the later period, however, a deviation from this relation is manifested by coalescence process. The LRO relaxation kinetics largely depends on the frequency of coalescence event.
  • M. Ohno, T. Mohri
    Materials Transactions 42 10 2033 - 2041 2001年10月 [査読有り][通常論文]
     
    The ordering processes of nucleation-growth type and spinodal ordering type are examined using the Phase Field method (PFM) with the free energy formulated based on the Cluster Variation Method. The relaxation curves of Long Range Order parameter, ξ, predicted by this approach are compared with those obtained by the Path Probability Method (PPM) in the homogeneous limit. Furthermore, it is shown that microstructural evolution process, which is described by time evolution of spatial distribution of ξ12, of spinodal ordering type is indistinguishable from the one of nucleation-growth type.
  • Munekazu Ohno, Tetsuo Mohri
    Materials Science and Engineering A 312 1-2 50 - 56 2001年08月15日 [査読有り][通常論文]
     
    The free energy of the B2 phase within the Cluster Variation Method is incorporated in the Phase Field method and the resultant time evolutions of Long Range Order and Short Range Order parameters are compared with the ones obtained by the Path Probability Method in the homogeneous limit. It is implied that there exist closed correlations between the Phase Field and Path Probability Methods. Furthermore, heterogeneous microstructural evolution process is investigated for a disorder-B2 transition. The results qualitatively reproduce the experimental results of Fe-A1 system. © 2001 Elsevier Science B.V. All rights reserved.
  • Tetsuya Shoji, Munekazu Ohno, Seiji Miura, Tetsuo Mohri
    Materials Transactions, JIM 40 9 875 - 878 1999年 [査読有り][通常論文]
     
    The transient behavior of stress-strain curves in view of the Cottrell-Stokes law was examined by employing stress-strain constitutive relationships within Gilman-Johnston and Alexander-Haasen models. Focusing on the two traditional theories, the constitutive equation of stress-displacement relation was formulated. In both models, two kinds of expression for the internal stress were assumed such that temperature dependencies of dislocation mobility and multiplication process were revealed. The interaction between mobile and immobile dislocations to play an essential role to observe the Cottrell-Stokes law.

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  • 高木知弘, 坂根慎治, 光山容正, 大野宗一, 澁田靖, 青木尊之 材料とプロセス(CD-ROM) 34 (1) 2021年
  • 大野宗一, 李在薫, 澁田靖, 高木知弘 材料とプロセス(CD-ROM) 33 (1) 2020年
  • 齊藤元貴, 坂口紀史, 大野宗一, 松浦清隆, 蓑口光樹, 山岡拓也 材料とプロセス(CD-ROM) 33 (1) 2020年
  • 辻莉緒, 齊藤元貴, 大野宗一, 松浦清隆, 竹内正芳, 蓑口光樹, 山岡拓也 日本鉄鋼協会日本金属学会両支部合同冬季講演大会概要集 2019 2020年
  • 大野宗一 日本鉄鋼協会日本金属学会両支部合同冬季講演大会概要集 2019 2020年
  • 李在薫, 大野宗一 日本鉄鋼協会日本金属学会両支部合同冬季講演大会概要集 2019 2020年
  • 堀口智弘, 棗千修, 大野宗一 日本金属学会東北支部研究発表大会予稿集 19th (CD-ROM) 2020年
  • 及川利彦, 佐藤拓実, 棗千修, 岡ゆきみ, 大野宗一 日本金属学会東北支部研究発表大会予稿集 19th (CD-ROM) 2020年
  • 佐藤拓実, 及川利彦, 棗千修, 岡ゆきみ, 大野宗一 日本金属学会東北支部研究発表大会予稿集 19th (CD-ROM) 2020年
  • 坂下修梧, 小山敏幸, 塚田祐貴, 大野宗一, 山中晃徳 日本機械学会計算力学講演会論文集(CD-ROM) 32nd 2019年
  • 坂根慎治, 高木知弘, 大野宗一, 澁田靖, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 32nd 2019年
  • 三好英輔, 高木知弘, 大野宗一, 澁田靖, 坂根慎治, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 32nd 2019年
  • 三好英輔, 高木知弘, 大野宗一, 澁田靖, 坂根慎治, 青木尊之 日本材料学会学術講演会講演論文集 68th 2019年
  • 純Feにおける固液界面物性値の温度依存性の推定
    大野宗一, 岡ゆきみ, 高木知宏, 坂根慎治, 澁田靖 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料 19委-12972 凝固プロセス-VIII-53 2019年 [査読無し][通常論文]
  • 高木知弘, 坂根慎治, 佐藤遼太郎, 大野宗一, 澁田靖 日本機械学会計算力学講演会論文集(CD-ROM) 31st 2018年
  • 三好英輔, 高木知弘, 大野宗一, 澁田靖 日本機械学会計算力学講演会論文集(CD-ROM) 31st 2018年
  • 大野 宗一, 柳楽 知也, 棗 千修, 及川 勝成, 江阪 久雄, 大笹 憲一, 安田 秀幸 ふぇらむ = Bulletin of the Iron and Steel Institute of Japan : (一社)日本鉄鋼協会会報 23 (10) 545 -551 2018年
  • Microstructure evolution in superplastic Al/Mg/Al clad sheet
    T. Tokunaga, K. Matsuura, M. Ohno Proceedings of 16th Inter-national Conference on Aluminum Alloys (ICAA16) 2018年 [査読有り][通常論文]
  • データ同化に基づいた鋳造プロセス・シミュレーションのパラメータ推定と状態推定
    大野 宗一 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料 19委-12926 凝固プロセス-VIII-30 2018年 [査読無し][通常論文]
  • 三好 英輔, 高木 知弘, 大野 宗一, 澁田 靖, 坂根 慎治, 下川辺 隆史, 青木 尊之 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science 22 3p 2017年05月
  • 高木 知弘, 坂根 慎治, 大野 宗一, 澁田 靖, 下川辺 隆史, 青木 尊之 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science 22 4p 2017年05月
  • 鈴木弘朗, 金子俊郎, 澁田靖, 大野宗一, 前川侑毅, 加藤俊顕 応用物理学会春季学術講演会講演予稿集(CD-ROM) 64th ROMBUNNO.16p‐B6‐9 2017年03月01日 [査読無し][通常論文]
  • 三好英輔, 高木知弘, 澁田靖, 大野宗一 日本機械学会計算力学講演会論文集(CD-ROM) 30th 2017年
  • 大野宗一 日本機械学会計算力学講演会論文集(CD-ROM) 30th 2017年
  • 高木知弘, 大野宗一, 澁田靖, 坂根慎治, 三好英輔, 下川辺隆史, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 30th 2017年
  • Munekazu Ohno Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 103 (12) 667 -1 2017年 [査読無し][通常論文]
  • ミクロ偏析予測の高精度化とマクロ偏析シミュレーションへの拡張
    大野宗一, 山下正義, 木村大地, 松浦清隆 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料 19委-12833 凝固プロセス-VIII-8. 2017年 [査読無し][通常論文]
  • 鈴木弘朗, 金子俊郎, 澁田靖, 大野宗一, 前川侑毅, 加藤俊顕 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 77th ROMBUNNO.16a‐A33‐5 2016年09月01日 [査読無し][通常論文]
  • 坂根 慎治, 高木 知弘, 大野 宗一, 澁田 靖, 下川辺 隆史, 青木 尊之 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science 21 4p 2016年05月
  • 三好英輔, 高木知弘, 坂根慎治, 大野宗一, 澁田靖, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 29th 2016年
  • 大野宗一, 高木知弘, 澁田靖 日本機械学会計算力学講演会論文集(CD-ROM) 29th 2016年
  • 坂根慎治, 高木知弘, 大野宗一, 澁田靖, 下川辺隆史, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 29th 2016年
  • データ同化を利用した鋳造過程における熱物性値の算出
    大野宗一, 松浦清隆, 岡ゆきみ 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料、(2016), 19委-12808 凝固プロセス-VII-112 2016年 [査読無し][通常論文]
  • 高木 知弘, 坂根 慎治, ロハス ロベルト, 大野 宗一, 澁田 靖, 下川辺 隆史, 青木 尊之 計算力学講演会講演論文集 2015 (28) "241 -1"-"241-2" 2015年10月10日
  • 坂根 慎治, 高木 知弘, 大野 宗一 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science 20 5p 2015年06月
  • 高木知弘, 坂根慎治, ROJAS Roberto, 大野宗一, 澁田靖, 下川辺隆史, 青木尊之 日本機械学会計算力学講演会論文集(CD-ROM) 28th 2015年
  • Kiyotaka Matsuura, Toko Tokunaga, Munekazu Ohno, Kaho Tomita M2D2015: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON MECHANICS AND MATERIALS IN DESIGN 2063 -2064 2015年 [査読有り][通常論文]
     
    It has been shown that hot extrusion with the use of a specially-designed die, which is called a heteromorphic die, leads to improvement in ductility of Al alloys. The heteromorphic die consists of two plates and one spacer. One plate is called a strain-giving die and has six holes of 5 mm in diameter, while the other plate is called a shape-giving die and has one rectangular hole of 9 mmx15 mm in side length. The spacer separates these plates at a distance of 10 mm and gives a space between them. The Al alloy billet was hot-extruded from the strain-giving die into the space between the two dies to produce six thin bars, and all the six bars passed together through the shape-giving die to become a thick rectangular bar. The rectangular bar produced by the present extrusion was tensile-tested at room temperature and the results were compared with those of the un-extruded billet and the bar produced by a conventional hot extrusion using only the shape-giving die. The elongations of the specimens made of the billet, the bar extruded by the conventional method and the one extruded using the present heteromorphic die were 21, 35 and 57 %, respectively. The largest elongation found in the sample extruded using the present heteromorphic die should be due to the fine recrystallized grain structure in that sample. It is suggested that the recrystallization is induced by a large strain generated in the present heteromorphic die due to the friction at the large contact area between the die hole wall and the alloy.
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno, Goro Miyamoto, Tadashi Furuhara PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015 33 -34 2015年 [査読有り][通常論文]
  • 一方向凝固におけるデンドライト成長方向の凝固条件及び合金濃度依存性
    大野宗一, 松浦清隆, 芳賀圭嗣, 奥田洋平 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料,19委-12762 凝固プロセス-VII-97. 2015年 [査読無し][通常論文]
  • 結晶粒成長のフェーズフィールド・シミュレーション
    大野 宗一 金属 85 929 -934 2015年 [査読無し][通常論文]
  • 高木 知弘, 大野 宗一, 下川辺 隆史 計算工学講演会論文集 Proceedings of the Conference on Computational Engineering and Science 19 3p 2014年06月 [査読無し][通常論文]
  • 高木 知弘, 大野 宗一, 下川辺 隆史, 青木 尊之 計算力学講演会 2014 (0) 583 -584 2014年 [査読無し][通常論文]
  • 大野 宗一 まてりあ 53 (10) 458 -461 2014年 [査読有り][招待有り]
  • 大野 宗一 ふぇらむ = Bulletin of the Iron and Steel Institute of Japan : (一社)日本鉄鋼協会会報 19 (11) 758 -764 2014年 [査読有り][招待有り]
  • 過包晶鋼における粗大柱状γ粒組織の形成臨界条件
    大野宗一, 松浦清隆, 丸山正人 日本学術振興会 製鋼第19委員会 凝固プロセス研究会、凝固プロセス研究会提出資料、(2014), 19委-12674 凝固プロセス-VII-46 2014年 [査読無し][通常論文]
  • 一方向凝固におけるデンドライト成長方向のフェーズフィールド・シミュレーション
    大野宗一, 松浦清隆, 芳賀圭嗣 日本学術振興会 製鋼第19委員会 反応プロセス研究会、凝固プロセス研究会提出資料 19委-12697 凝固プロセス VII-56 2014年 [査読無し][通常論文]
  • Phase-field-lattice Boltzmann simulations of dendritic growth and motion in the isothermal solidification of binary alloys
    R. Rojas, T. Takaki, M. Ohno Proceedings of 9th Pacific Rim International Conference on Modeling of Casting and Solidification Processes (MSCP 2014) 2014年 [査読有り][通常論文]
  • GPU accelerated phase-field simulations during dendrite competitive growth of binary alloy polycrystal
    T. Takaki, M. Ohno, T. Shimokawabe, T. Aoki Proceedings of 9th Pacific Rim International Conference on Modeling of Casting and Solidification Processes (MSCP 2014) 2014年 [査読有り][通常論文]
  • Phase-field simulations of growth direction of directionally-solidified dendrites
    M. Ohno, K. Haga, T. Takaki, K. Matsuura Proceedings of 9th Pacific Rim International Conference on Modeling of Casting and Solidification Processes (MSCP 2014) 2014年 [査読有り][通常論文]
  • 堀井 麻有, 高木 知弘, 大野 宗一 計算力学講演会講演論文集 2013 (26) "1613 -1"-"1613-2" 2013年11月02日
  • Xianguang Zhang, Kiyotaka Matsuura, Munekazu Ohno Journal of Physics: Conference Series 419 (1) in print 2013年 [査読有り][通常論文]
     
    Plastic strain gradients along the depth direction of a locally deformed surface of pure iron have been studied under different levels of strain and strain rates. Deformation at high strain rates was carried out by using a ball-dropping test, while deformation at low strain rates was performed by a ball-pressing test on a computer controlled servohydraulic machine. Evaluation of the local plastic strain was conducted by using the electron backscatter diffraction (EBSD) analysis. Experimental evidences indicate that high strains with high strain rates lead to large gradients of the plastic strain distribution profile and also a localized severe deformation near the surface. Furthermore, deformation at high strain rates is favourable to impose plastic deformation at the selected area near the surface. © Published under licence by IOP Publishing Ltd.
  • 堀井麻有, 高木知弘, 大野宗一 日本機械学会計算力学講演会論文集(CD-ROM) 26th 2013年
  • 大山 晋平, 大野 宗一, 松浦 清隆 計算力学講演会講演論文集 2013 (26) _1601 -1_-_1601-2_ 2013年
  • 高木 知弘, 下川辺 隆史, 大野 宗一, 山中 晃徳, 青木 尊之 計算力学講演会 2013 (0) _1614 -1_-_1614-2_ 2013年 [査読無し][通常論文]
  • 凝固・結晶成長のためのフェーズフィールド法
    大野 宗一 日本マイクログラビティ応用学会誌 30 24 -29 2013年 [査読有り][招待有り]
  • 炭素鋼のδ/γ界面移動に対する分散粒子の効果
    佐藤大祐, 陳良, 大野宗一, 松浦清隆 日本学術振興会 製鋼第19委員会 反応プロセス研究会、凝固プロセス研究会 、製鋼計測化学研究会提出資料 19委-12658 凝固プロセス-VII-30 2013年 [査読無し][通常論文]
  • 凝固におけるフェーズフィールド法の基礎と応用
    大野 宗一 日本鉄鋼協会 計算工学による組織と特性予測技術II 研究会 最終報告書 37 -46 2013年 [査読無し][通常論文]
  • 大野宗一, 松浦清隆 日本機械学会計算力学講演会論文集(CD-ROM) 25th 2012年
  • A review on mechanism and kinetics of peritectic phase transition in Fe-C alloy
    L. Chen, K. Matsuura, M. Ohno, D. Sato Proceedings of Asia Steel International Conference 2012年 [査読無し][通常論文]
  • Kanji Yoh, Z. Cui, K. Konishi, M. Ohno, K. Blekker, W. Prost, F. J. Tegude, J. C. Harmand Device Research Conference - Conference Digest, DRC 79 -80 2012年 [査読有り][通常論文]
     
    We have fabricated a spin transistor based on InAs nanowire. The transistor operates based on Datta-Das type spin transistor mode [1][2][3]. The spin polarized electrons are injected from the ferromagnetic electrodes and synchronized spin precession is controlled by the gate voltage through spin-orbit interaction. We have clearly observed drain current oscillation versus gate voltage as expected from the Dyakonov-Pérel mechanism. By controlling the spin precession, on/off switching is expected to be achieved with the steep slope. The best of our obtained results has shown the steepest slope of 18mV/dec to 23mV/dec near the off-state ((2n+1)π-spin rotation) where vertical electric field meats the condition of persistent spin helix (PSH) motion [3]. The present result provide alternative method of steep slope device mechanism in addition to the conventional ideas such as Tunnel FETs or impact ionization FETs. © 2012 IEEE.
  • Structure control of austenite grains formed after peritectic and Ac3 transformation
    Kiyotaka Matsuura, Munekazu Ohno, Shingo Tsuchiya Liang Chen, Daisuke Sato International Iron & Steel Symposium April 2 – 4 2012年 [査読無し][通常論文]
  • Pinning effect on migration of delta/gamma interface during peritectic transformation in Fe-C system
    L. Chen, K. Matsuura, D. Sato, M. Ohno Proceedings of 11th Asian Foundry Congress 233 -235 2011年 [査読無し][通常論文]
  • 上田 幹人, 菊地 竜也, 大野 宗一 工学教育研究講演会講演論文集 2011 (59) 110 -111 2011年 [査読無し][通常論文]
  • 鈴木 亮輔, 大塚 俊明, 米澤 徹, 林 重成, 大野 宗一, 上田 幹人 工学教育研究講演会講演論文集 2011 (59) 526 -527 2011年 [査読無し][通常論文]
     
    工学教育研究講演会第59回年次大会(平成23年度). 平成23年9月8日~平成23年9月10日. 北海道大学大学院工学研究院、北海道. セッションID: 2-325
  • 連続鋳造における粗大柱状γ粒組織の発達挙動と添加元素の影響
    松浦清隆, 大野宗一, 磯部浩一 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 19委-12558 凝固プロセス-VI-26 2011年 [査読無し][通常論文]
  • 過包晶鋼における初晶相の遷移と鋳片γ粒組織
    土屋真悟, 大野宗一, 松浦清隆 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 19委-12575 凝固プロセス-VI-30 2011年 [査読無し][通常論文]
  • 松浦 清隆, 水田 直気, 大野 宗一 材料の科学と工学 = Materials science and technology 47 (4) 146 -151 2010年08月20日 [査読有り][通常論文]
  • 大野 宗一, 松浦 清隆 計算工学 15 (2) 2300 -2303 2010年04月30日 [査読有り][招待有り]
  • 大野 宗一, 松浦 清隆 金属 80 (2) 99 -104 2010年02月 [査読有り][招待有り]
  • Tomohiro Tanaka, Kiyotaka Matsuura, Keisuke Kojima, Munekazu Ohno Materials Science Forum 638-642 1860 -1865 2010年 [査読有り][通常論文]
     
    TiC-Fe system cermets and TiC-SUS310L (TiC-SS) system cermets with fully relative density have been produced from elemental powders via self-propagating high-temperature synthesis (SHS) reaction combined with pseudo-hot isostatic pressing. X-ray diffraction (XRD) analysis showed that the SHS products consist of TiC and the binder phase of Fe or SS. Metallographic analysis revealed TiC particles of several micrometer size were homogeneously dispersed in the binder phase. Regardless of differences in the binders, both cermets have similar tendencies: their relative densities increased and their hardness and compressive strengths decreased with increases in the volume fraction of either Fe or SS. In addition, the cermets used stainless steel as the binder had twice superior corrosion resistance to those used iron and also their hardness and compressive strength showed excellent values. © (2010) Trans Tech Publications.
  • フェーズフィールド法における界面移動の記述
    大野宗一, 松浦清隆 金属 Vol.80 (2) 11 -16 2010年 [査読有り][通常論文]
  • 組成傾斜金属間化合物の合成
    松浦清隆, 水田直気, 大野宗一 材料の科学と工学 Vol. 47 (No.4) 2 -7 2010年 [査読無し][通常論文]
  • 定量的マルチフェーズフィールド法による包晶反応・包晶変態の可視化
    大野宗一, 松浦清隆 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 19委-12489 凝固プロセス-VI3 2010年 [査読無し][通常論文]
  • 包晶凝固鋼鋳片における粗大柱状γ粒の形成機構
    土屋真悟, 大野宗一, 松浦清隆, 磯部浩一 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 19委-12492 凝固プロセス-VI6 2010年 [査読無し][通常論文]
  • 凝固のフェーズフィールドモデル
    大野 宗一 初心者のためのフェーズフィールド法講習会テキスト 35 -41 2010年 [査読無し][招待有り]
  • 材料組織の熱力学
    大野 宗一 初心者のためのフェーズフィールド法講習会テキスト 26 -34 2010年 [査読無し][招待有り]
  • 大野 宗一 まてりあ : 日本金属学会会報 48 (7) 375 -378 2009年07月01日
  • First-principles approach to microstructural formation process
    T. Mohri, M. Ohno, Y. Chen Proceedings of Asia Steel 2009, ed. Korean Institute of Metals and Materials S13-02 2009年 [査読無し][通常論文]
  • 毛利 哲雄, 藤橋 直, 大野 宗一, 陳 迎 計算力学講演会講演論文集 2009 (22) 310 -311 2009年 
    Phase Field Method is combined with the Cluster Variation Method of statistical mechanics to investigate the microstructural evolution process from atomistic scale. Electronic structure total energy calculation is further performed to derive atomic interaction energies for Fe-based alloy systems and the time evolution of Anti Phase Boundary associated with order-disorder transition is calculated from the first-principles.
  • Titanium Aluminide Coating on Titanium Surface Using 3-Dimensional Micro Welder
    Kiyotaka Matsuura, Munekazu Ohno Proc. 1st Collaborative and Cooperative Symposoium between Harbin Engineering University and Hokkaido University 17 -33 2009年 [査読無し][通常論文]
  • Fabrication of TiC-based cermets using SHS process
    Kiyotaka Matsuura, Munekazu Ohno, Tomohiro Tanaka Proc. 1st Collaborative and Cooperative Symposoium between Harbin Engineering University and Hokkaido University 34 -48 2009年 [査読無し][通常論文]
  • ピン止め相安定化によるγ粒成長抑制
    松浦清隆, 大野宗一, 磯部浩一 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 2009年 [査読無し][通常論文]
  • Combustion synthesis of TiC-based cemented carbide composites from elemental powders and their microstructure control
    K. Kojima, K. Matsuura, T. Tanaka, K. Ohsasa, M. Ohno Proceedins of The 5th International conference on advanced materials and processing # 2008年 [査読無し][通常論文]
  • フェライト凝固の柱状晶-等軸晶遷移の検証と固相拡散を考慮した定量的フェーズ・フィールド・モデリング
    大野宗一, 松浦清隆, 磯部浩一 日本学術振興会 製鋼第19委員会 凝固プロセス研究会 2008年 [査読無し][通常論文]
  • 毛利 哲雄, 大野 宗一, 陳 迎 日本応用磁気学会研究会資料 146 9 -16 2006年02月20日 [査読無し][通常論文]
  • Quantitative understanding and Modeling of Microstructural Formation by Solidification Technique of Magnesium Alloys
    G. Klaus, B. Boettger, J. Eiken, A. Buhring-Polaczek, I. Steinbach, M. Ohno, J. Groebner, R. Schmid-Fetzer Proceedings of the 7th Int. Conference on Magnesium Alloys and their Applications 2006 1021 -1026 2006年 [査読無し][通常論文]
  • Raymundo Arroyave, Munekazu Ohno, Rainer Schmid-Fetzer, Zi Kui Liu Magnesium Technology 2006 429 -434 2006年 [査読無し][通常論文]
     
    In this work, the thermodynamic properties of several intermetallic compounds in the Mg-Ca-Sn system are calculated via first-principles methods based on density functional theory. The harmonic approximation is used to take into account the contributions from vibrational degrees of freedom, DOF. Thermal lattice expansion on the total free energies of the compounds is taken into account via the quasi-harmonic approach. To illustrate the incorporation of the first-principles calculations into a CAL-PHAD model, the Mg-Ca system is reassessed and compared with previous models.
  • Munekazu Ohno, Djordie Mirkovic, Rainer Schmid-Fetzer Magnesium Technology 2006 129 -132 2006年 [査読無し][通常論文]
     
    The liquidus and solidus data reported for commercial AZ and AM alloys are generally based on thermal analysis. In the present study, the following two points have been clarified by performing own experiments and Calphad-type thermodynamic calculations: (i) The measured "liquidus" temperature generally does not represent the actual equilibrium one, in other words, the primary precipitate for these Mg-alloys cannot be detected in the thermal analysis. (ii).The measured "solidus" does not correspond to the equilibrium solidus and not even to the end of non-equilibrium solidification process. The measured "solidus" is often associated with the precipitation of Mg17Al12 phase and, importantly, the solidification process of these Mg-alloys ends at much lower temperature.
  • M Ohno, T Mohri COMPLEX INORGANIC SOLIDS: STRUCTURAL, STABILITY, AND MAGNETIC PROPERTIES OF ALLOYS 187 -202 2005年 [査読無し][通常論文]
  • Tetsuo Mohri, Munekazu Ohno, Ying Chen Proceedings of an International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2005 2 633 -650 2005年 [査読無し][通常論文]
     
    First principles calculations of phase stability and phase equilibria for three kinds of Fe-based binary alloys, Fe-Ni, Fe-Pd and Fe-Pt, are reviewed. The combination of FLAPW total energy calculation with the Cluster Variation Method (CVM) provides very accurate results for the phase stability and L1 0-disorder transition temperature for each system. The effects of magnetism and thermal vibration are examined and it is found that these play deterministic role in the phase equilibria. Then, CVM is hybridized with Phase Field Method and multi-scale calculation is performed for the time evolution process of Anti Phase Boundary associated with L10 ordering transition. Finally, the first principles calculation is attempted for L1 0 ordering transition in Fe-Pd system. The absolute length scale and crystallographic orientation are uniquely assigned in the calculated results.
  • T Mohri, M Ohno, Y Chen PRICM 5: THE FIFTH PACIFIC RIM INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS AND PROCESSING, PTS 1-5 475-479 3075 -3080 2005年 [査読有り][通常論文]
     
    Phase Field Method (PFM) is hybridized with Cluster Variation Method (CVM) to investigate the ordering dynamics of L1(0)-disorder transition at atomistic and microstructural scales simultaneously. For this, coarse graining operation is attempted on the inhomogeneous free energy functional of CVM. The resultant gradient energy coefficient is found out to be dependent on temperature and order parameters, which is in marked contrast to a conventional PFM formalism. Electronic structure total energy calculations for Fe-Pd system are incorporated to the hybridized scheme and the first principles calculation of microstructural evolution process is attempted.
  • マルチスケールシミュレーションに関する粗視化の問題
    毛利哲雄, 大野宗一, 陳迎 日本学術振興会 合金状態図第172委員会研究報告書 5 (2) 45 -49 2005年 [査読無し][通常論文]
  • First principle calculations of phase stability, equilibria and transformation dynamics
    T. Mohri, M. Ohno, Y. Chen Proceedings of the 12th National Symposium on Phase Diagram, Materials Design and their Applications 1 -4 2004年 [査読無し][通常論文]
  • Theoretical calculation of ordering process based on TDGL equation and CVM
    M. Ohno, T. Mohri Proceedings of the 15th International Symposium on Transport Phenomena 420 -425 2004年 [査読無し][通常論文]
  • M. Ohno, T. Mohri Materials Research Society Symposium - Proceedings 753 411 -416 2003年 [査読無し][通常論文]
     
    During ordering process, anti-site ordering proceeds in atomistic scale and anti-phase domain structure evolves in microstructural scale. In order to describe both the processes, a hybridized calculation of the Phase Field Method(PFM) and Cluster Variation Method(CVM) is attempted. The main objective of the present study is focused on the time evolution of atomic configuration during L10 ordering processes below and above the spinodal ordering temperature and their resultant microstructures. In order to investigate the interplay between atomistic and microstructural processes, we conducted two types of calculations, One is for a homogeneous system without an anti-phase boundary and the other is for an inhomogeneous system in which microstructure is formed by anti-phase domains. For the homogeneous system, the relaxation curve of Long-Range-Order parameter(LRO) indicates a transient appearance of an L12-like atomic configuration below the spinodal ordering temperature. Such an L12-like state corresponds to a saddle point configuration in the CVM free energy surface. When the composition of an alloy is located near L10 + L12 phase field in the phase diagram, the L12-like phase becomes highly ordered state. For the inhomogeneous system, it is implied that the appearance of the L1 2-like phase affects the resultant microstructure by providing the nucleation sites for the L10 ordered phase.
  • 大野 宗一 まてりあ : 日本金属学会会報 41 (10) 725 -725 2002年10月20日 [査読有り][招待有り]
  • 大野 宗一, 毛利 哲雄 ふぇらむ 6 (10) 766 -772 2001年 [査読有り][招待有り]
  • 原子レベルからの内部組織の計算
    大野宗一, 毛利哲雄 “相分解による組織形成過程の研究最前線”, 日本鉄鋼協会, 材料の組織と特性部会, 95 -106 2001年 [査読無し][通常論文]

特許

受賞

  • 2019年 日本学術振興会・製鋼第19委員会 優秀研究賞
     
    受賞者: 大野 宗一
  • 2019年 革新的ハイパフォーマンス・コンピューティング・インフラ(HPCI) 令和元年度HPCIシステム利用研究課題優秀成果賞
     
    受賞者: 髙木知弘,青木 尊之,大野宗一,澁田靖,下川辺隆史,坂根慎治,三好英輔,佐藤遼太郎,岡ゆきみ,金根佑
  • 2017年 10th Pacific Rim International Conference on Modeling of Casting and Solidification Processes BEST POSTER PRESENTATION AWARD
     
    受賞者: S. Sakane;T. Takaki;M. Ohno;Y. Shibuta;T. Shimokawabe;T. Aoki
  • 2016年 (一社)日本計算工学会 第21回計算工学講演会グラフィクスアワード特別賞(ビジュアルコンピューティング賞)
     
    受賞者: 坂根慎治,髙木知弘,大野宗一,澁田靖,下川辺隆史,青木尊之
  • 2016年 日本学術振興会 科研費審査員・表彰
     
    受賞者: 大野 宗一
  • 2015年 Modeling of Casting, Welding and Advanced Solidification Processes XIV Best Poster Award
     
    受賞者: S. Sakane;T. Takaki;M. Ohno;T. Shimokawabe;T. Aoki
  • 2015年 本多記念会 第36回 本多記念研究奨励賞
     
    受賞者: 大野 宗一
  • 2010年 FGM 研究奨励賞
  • 2010年 日本鉄鋼協会 研究奨励賞
  • 2010年 第22回 計算力学講演会 優秀講演表彰
  • 2008年 第18回 日本金属学会奨励賞(組織部門)
  • 2007年 第55回 日本金属学会論文賞(組織部門)受賞
  • 2002年 MRS Fall Meeting Symposium BB内・優秀ポスター賞
  • 2001年 日本金属学会・鉄鋼協会北海道支部会奨励賞受賞

共同研究・競争的資金等の研究課題

  • 凝固組織予測の先進的シミュレータ開発と計算・実験の一体化技術への展開
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2020年04月 -2023年03月 
    代表者 : 高木 知弘, 坂根 慎治, 大野 宗一, 安田 秀幸, 青木 尊之
  • 原子・組織アプローチのデータ同化に基づく非平衡固液界面物性の学理開拓
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2019年04月 -2022年03月 
    代表者 : 大野 宗一, 澁田 靖
     
    本年度は、双子実験をベースに界面物性値を高精度に算出するデータ同化の手法の高度化・高精度化を実施した。Fe及びAlの過冷凝固を対象にして、固液界面エネルギー、その異方性強度、界面移動度とその異方性強度の四つの因子を、それぞれ単独に推定することから着手し、さらにこれらを同時に推定することを試みた。まず、これらの解析に必要なフェーズフィールド法のGPU並列計算コードを作成した。続いて、データ同化の方法として、アンサンブル・カルマンフィルタと粒子フィルタのそれぞれをコーディングした。 そして、双子実験によって、固液界面物性値を高精度に算出可能な実測値の条件を調査した。まず、1辺が数百マイクロメートルの二次元系(薄板)において、固相が数百ps間等温凝固する現象を対象とした。このとき、二次元上で(100)、(110)及び(111)固液界面が成長するように固相の結晶方位を決定した。この固相の成長によって異なる異方性強度の影響が顕在化するためである。なお、この等温凝固は分子動力学法で直接シミュレート可能な現象であるため、これを対象とした。まずは、フェーズフィールド法を用い、予め定めた固液界面物性値を用いて、上述の固相の成長を計算し、それを観察データとして、データ同化の精度を検証した。フィルタリングの時間間隔、観察ノイズとシステムノイズの大きさ、初期パラメータの範囲など、様々なハイパーパラメータが推定精度に及ぼす影響を調査した。その結果、アンサンブル・カルマンフィルタ及び粒子フィルタのいずれの手法を用いても、最適なハイパーパラメータを用いれば、固液界面物性値をすべて同時に推定可能であることが示された。
  • 日本学術振興会:科学研究費助成事業 挑戦的研究(萌芽)
    研究期間 : 2017年06月 -2020年03月 
    代表者 : 大野 宗一
     
    本研究では、拡散方程式の時間反転プロセスを計算する手法(diffusive solution法)の開発を試みた。この手法は、組織形成シミュレーションの時間反転プロセスに応用することを目指したものであり、最終組織から、その発達途中の組織や初期状態に関する情報を得る手段の構築を目指したものである。その結果、diffusive solution法によって、ある条件下であれば、濃度プロファイルの短波長成分の増大を押さえながら、時間反転方向に計算をすることが可能になった。ただし、より高精度な解析のために、さらなる改善が求められる。
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2017年04月 -2020年03月 
    代表者 : 高木 知弘, 大野 宗一, 澁田 靖, 安田 秀幸, 青木 尊之
     
    材料開発では,材料組織の適切な予測と制御が極めて重要である.本研究では,フェーズフィールド(PF)計算,SPring-8によるその場観察,分子動力学(MD)計算の三つの最先端研究をデータサイエンスにより融合し,革新的メゾスケール材料組織予測アプローチの構築を目指した.MD計算結果を観察データとし,PFとEnKFによるデータ同化によって界面物性値を算出する手法を開発した.SPring-8その場観察結果を観測データとし,PFとEnKFによるデータ同化によって物性値と場の情報を得る基盤技術を構築した.大規模PF計算による高精度材料組織予測法を開発し,凝固と粒成長の組織予測とメカニズム解明を行った.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2016年04月 -2019年03月 
    代表者 : 澁田 靖, 大野 宗一, 高木 知弘
     
    本研究では合金組織のさらなる高精度制御に向けて,核生成・凝固,粒成長過程の複合的ダイナミクスを系統的に解析し,合金組織形成過程の支配因子を原子スケールの動力学解析により解明することを目的とし,具体的に10億原子分子動力学(MD)計算により均質核生成過程における不均質性など古典核生成理論の範囲を超えた核生成の原子論的描像を明らかにした.また結晶粒淘汰過程における幾何学因子,粒界モビリティ・粒界エネルギーの時間変化を解析し,理想粒成長の二乗則からのずれの原因を明らかにした.さらに合金系ポテンシャルの溶質分配特性を考察し,合金系大規模MD計算実現の足掛かりを作った.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2016年04月 -2019年03月 
    代表者 : 大野 宗一, 澁田 靖, 奥田 洋平, 南 洋乃, 金 根佑
     
    本研究では、金属材料の凝固におけるデンドライトの優先成長方位が合金濃度に依存するという新奇の現象が、多様な合金系において生じることを実験研究によって明らかにした。そして、そのメカニズム解明のための熱力学解析と原子シミュレーションを実施するとともに、組織形成シミュレーション法のフェーズフィールド・モデルを発展させ、そのシミュレーションによって、組織形態の多様性を総括する形態マップを構築した。
  • フェーズフィールド法を核とする凝固組織形成予測モジュールの開発
    独立行政法人科学技術振興機構(JST):SIP 受託研究
    研究期間 : 2014年10月 -2019年03月 
    代表者 : 大野 宗一
  • 格子ボルツマン法に基づく流動・凝固シミュレーション・モデルの開発
    日本鉄鋼協会:第25回 鉄鋼研究振興助成
    研究期間 : 2016年04月 -2018年03月 
    代表者 : 大野 宗一
  • 固液共存体の挙動制御によるマクロ偏析低減
    日本鉄鋼協会:研究会I
    研究期間 : 2014年04月 -2017年03月 
    代表者 : 大野 宗一
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2013年04月 -2016年03月 
    代表者 : 大野 宗一, 澁田 靖
     
    鋳造材の内部では、構成元素の濃度がマイクロメートルのスケールで不均一に分布しており、これをミクロ偏析と呼ぶ。鋳片・鋳塊の高品質化のためにこの偏析を高精度に制御することが望まれている。本研究では現在までに注意を払われてこなかった多成分系拡散のタイライン・シフト現象とミクロ偏析の関係を解明し、この現象を利用した新しい偏析制御技術の発展につながる知見を得た。特に、拡散の遅い原子の偏析は、タイライン・シフトを活用することで低減できることが示された点が重要と考えられる。
  • 包晶凝固鋼における優先成長相遷移条件の高精度解析に基づくγ粒組織制御法の開拓
    日本鉄鋼協会:第22回 鉄鋼研究振興助成
    研究期間 : 2013年 -2015年 
    代表者 : 大野 宗一
  • 鋼の包晶凝固、γ粒成長に関する研究
    日本学術振興会:製鋼第19委員会テーマ応募式研究助成
    研究期間 : 2011年 -2013年 
    代表者 : 大野 宗一
  • 包晶反応と共析逆変態反応による超微細粒鋼の創製
    日本学術振興会:科学研究補助金・若手研究(A)
    研究期間 : 2010年 -2013年 
    代表者 : 大野 宗一
  • スピンチャンネルから量子ドットへのスピンブロッケードとその量子ビットへの応用
    日本学術振興会:科学研究費補助金 基盤研究(A)
    研究期間 : 2008年04月 -2011年03月 
    代表者 : 陽 完治
  • 自己伝播合成反応プロセスにおける微細組織の予測シミュレーション法の考案
    日本学術振興会:科学研究補助金・挑戦的萌芽
    研究期間 : 2009年 -2011年 
    代表者 : 大野 宗一
  • 鋳造凝固における柱状オーステナイト粒組織の形成機構の解明
    日本鉄鋼協会:第18回 鉄鋼研究振興助成
    研究期間 : 2009年 -2011年 
    代表者 : 大野 宗一
  • スピントロニクスデバイス材料の高精度熱化学計算
    日本学術振興会:科学研究補助金・若手研究(B)
    研究期間 : 2007年 -2009年 
    代表者 : 大野 宗一
  • 半導体上のスピネル型ハーフメタルの成長と高偏極率スピン注入の実証
    日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2008年 -2008年 
    代表者 : 陽 完治, 大野 宗一
     
    インジウム砒素基板上に200Å成長させたマグネタイト薄膜の磁化特性を超伝導磁束量子干渉計(SQUID)を用いて測定した. マグネタイト薄膜の磁化容易軸方向に外部磁場を印加した場合, 0.4T付近で磁化が飽和し, その値は483emu/ccであった. この値はバルクのマグネタイトのそれと近い値である. 一方, マグネタイト薄膜の磁化容易軸方向と垂直な外部磁場を印加した場合, 飽和し始めたのは1T付近であった. またマグネタイトの抵抗の温度依存性を測定するために, シリコン酸化膜上にマグネタイト薄膜100Åを成長させた. 成長温度は300℃, 酸素分圧は7.5×10-7Torr, 成長レートは約100Å/hour成長したマグネタイト薄膜を原子間力顕微鏡(AFM)で観察した結果その表面粗さは3.92nm/μm^2であった. この試料抵抗の温度依存性を, 1/Tとしてプロットすると120Kにおいて抵抗変化曲線の傾きが変化するVerwey転移を示した. マグネタイトの最適成長温度は摂氏300度となったため、マグネタイト電極を用いたスピン注入デバイス作製のためのプロセスを開発する必要がある. レジストフリーの試料全面に高温成長されたマグネタイトを, 必要な部分のみ残してそれ以外をArプラズマによりエッチングし、レジストカバー直下, すなわち電極間に残っているマグネタイトをシリコン酸化膜とともにリフトオフをおこなうことで達成した. これらの結果については「電子情報通信学会、電子デバイス研究会2009年2月」で報告した. また光学的評価についてはスピン偏極率の確認は、InAs系のスピンダイオード構造およびGaAs/AlGaAs量子井戸耕造での近赤外円偏光測定を行うための測定系の構築を終えた。
  • 日本学術振興会:科学研究費助成事業 若手研究(B)
    研究期間 : 2007年 -2008年 
    代表者 : 大野 宗一
     
    スピン注入デバイスの実現化に有力な候補である強磁性Fe/III-V 半導体ハイブリッド構造の相平衡を,第一原理計算及びCALPHAD 法から計算した.Al-As-Fe-Ga-In 系を対象として,各二元系及び三元系のモデリングを行い,特にFe/InAs 構造とFe/GaAs 構造作製時に形成される化合物の解明や,結晶成長の最適温度に関しての知見を得た.
  • スピン制御トランジスタ実現のための基礎技術の開発
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2005年 -2007年 
    代表者 : 陽 完治, 大野 宗一, 末岡 和久
     
    高スピン偏極材料として期待されているマグネタイト(Fe_<3>O4)に着目し、高品質のスピネル型のマグネタイトが(100)InAs上に45度回転した形でMBE成長すること、酸素分圧の制御により結晶の成分調整ができること、またその磁化特性により面内方向に磁化容易軸を持つ強磁性体の特性を示すことがあきらかになった。一方、鉄薄膜電極/インジウム砒素チャンネルで構成されるいわゆるDatta-Das型スピントランジスタ構造を作製し、スピン軌道相互作用に基づきゲート電圧の制御によるチャンネル中電子のスピン歳差運動を制御することにより生じると考えられる電流振動を観測した。これらの観測された実験結果を理論的に裏付けるために一連のモンテカルロ計算を行った。いわゆるInAs系Datta-Das型スピントランジスタにおいて、2っのスピン軌道相互作用(Rashba項およびDresselhaus項)が揃った条件のときは、スピン緩和が消失することが示された。このことは室温においても非弾性領域でも成り立ち、スピン制御電流振動をすることが示された。また横方向電界(ドレイン電圧)増大とともにスピン振幅が増大する(スピン緩和長が伸びる)ことも示され実験結果ともよく一致していることが明らかとなった。その他、鉄薄膜細線の磁化反転特性はマイクロマグネティックス解析の結果、鉄薄膜細線界面から5nm程度と浅くかっごく近傍のエッジの狭い(0.1um幅)領域のみ最大400mT程度の漏れ磁場があることが得られた。また鉄/砒素系半導体界面に生成される可能性のあるAs_<2>F(anti-ferro),AsF(anti-ferro),AsF_<2>(normal meta1)のいずれの鉄砒素系化合物の生成エネルギーも強磁性相より低く安定していることが第一原理計算により明らかになった。熱的安定性の点からも鉄酸化物系の開発が望ましいことを裏付けている。
  • 原子の集団挙動から内部組織形成までの統一的理論計算
    財団法人服部報公会:工学研究奨励援助金
    研究期間 : 2002年 -2003年 
    代表者 : 大野 宗一

教育活動情報

主要な担当授業

  • 組織設計学特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 工学院
    キーワード : 凝固、固相変態、粒成長、析出
  • 組織設計学特論
    開講年度 : 2021年
    課程区分 : 博士後期課程
    開講学部 : 工学院
    キーワード : 凝固、固相変態、粒成長、析出
  • 弾塑性学
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 応力 歪 連続体力学 弾性変形 塑性変形
  • 材料組織学
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 状態図、相変態、材料組織、界面ダイナミックス 
  • 材料工学演習Ⅳ
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 弾性変形、塑性変形、応力、ひずみ、剛体球モデル、結晶構造、転位
  • 一般教育演習(フレッシュマンセミナー)
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : マテリアル工学、ナノマテリアル、省エネルギー、地球環境、持続的発展

大学運営

委員歴

  • 2021年04月 - 現在   日本金属学会   代議員
  • 2020年10月 - 現在   日本学術会議   連携会員
  • 2019年04月 - 現在   日本金属学会   講演大会委員及び調査研究委員
  • 2019年01月 - 現在   日本金属学会 北海道支部   理事
  • 2018年04月 - 現在   日本鉄鋼協会   代議員
  • 2015年04月 - 現在   日本金属学会北海道支部   支部代議員
  • 2009年 - 現在   日本学術振興会 製鋼19委員会   委員
  • 2009年 - 現在   日本鉄鋼協会   高温プロセス部会 凝固・組織形成フォーラム 運営委員   日本鉄鋼協会
  • 2017年04月 - 2021年03月   日本鉄鋼協会   論文誌編集委員会 専門委員
  • 2016年06月 - 2020年03月   ポスト「京」サブ課題「高信頼性構造材料」委員会 委員
  • 2016年04月 - 2020年03月   日本金属学会   会誌編集委員会・欧文誌編集委員会査読委員
  • 2014年04月 - 2016年03月   日本学術振興会   科学研究費委員会 専門委員
  • 2013年 - 2016年03月   HPCI戦略プログラム分野2 CMSI   第五部会 委員
  • 2012年 - 2016年   日本鉄鋼協会   ふぇらむ編集委員
  • 2013年 - 2015年   日本金属学会   会報編集委員
  • 2010年 - 2013年   次世代スーパーコンピュータ戦略プログラム 分野2 新物質・エネルギー創成   広報委員
  • 2009年 - 2013年   日本鉄鋼協会   高温プロセス部会 若手フォーラム 委員   日本鉄鋼協会
  • 2009年 - 2011年   日本金属学会   北海道支部 幹事   日本金属学会
  • 2009年 - 2010年   日本鉄鋼協会   高温プロセス部会 若手フォーラム 座長   日本鉄鋼協会


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