掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

DOI： 10.1088/1361-651X/ac13cd

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

DOI： 10.1016/j.commatsci.2020.109992

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

DOI： 10.2355/isijinternational.isijint-2019-305

CiNii Article

掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

DOI： 10.1088/1361-651X/ab1d28

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

DOI： 10.1088/1361-651X/ab1e8b

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

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.

DOI： 10.1007/s10853-018-2680-y

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

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.

DOI： 10.1016/j.commatsci.2018.05.046

掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

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. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2018.04.060

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

Numerical studies of the effects of anisotropic (misorientation-dependent) grain-boundary energy and mobility on polycrystalline grain growth have been carried out for decades. However, conclusive knowledge has yet to be obtained even for the simplest two-dimensional case, which is mainly due to limitations in the computational accuracy of the grain-growth models and computer resources that have been employed to date. Our study attempts to address these problems by utilizing a higher-order multi-phase-field (MPF) model, which was developed to accurately simulate grain growth with anisotropic grain-boundary properties. In addition, we also employ general-purpose computing on graphics processing units to accelerate MPF grain-growth simulations. Through a series of simulations of anisotropic grain growth, we succeeded in confirming that both the anisotropies in grain-boundary energy and mobility affect the morphology formed during grain growth. On the other hand, we found the grain growth kinetics in anisotropic systems to follow parabolic law similar to isotropic growth, but only after an initial transient period.

DOI： 10.1016/j.jcrysgro.2016.11.097

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

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.

DOI： 10.1038/s41524-017-0029-8

掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

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.

DOI： 10.1038/s41467-017-00017-5

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

Based on the multi-phase-field (MPF) model reported by Steinbach et al., we constructed a higher-order MPF model in a previous study that contains a higher-order term and an additional kinetic parameter to represent the properties of triple junctions (TJs); this model was observed to be suitable for the simulation of 2D grain growth with anisotropic grain-boundary (GB) energy and mobility, which are strongly dependent on the misorientation angle (Delta theta). In the current study, we attempt to improve the accuracy of 3D MPF simulations of anisotropic grain growth by extending this higher-order MPF model such that it accounts for the properties of quadruple junctions as well as those of TJs. In addition, using the extended higher-order MPF model, a series of grain-growth simulations are performed for a 3D columnar structure while considering the anisotropic GB properties, through which the accuracy of the model is examined in detail. The results confirm that the extended higher-order MPF model enables the anisotropic GB properties to be handled accurately for wider-ranging Delta theta than in previous models. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2016.04.014

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   国際・国内誌：国際誌  

The multi-phase-field (MPF) model proposed by Steinbach et al. has several advantages when it comes to numerically simulating the grain growth, recrystallization, and multiple phase transitions. In this study, in order to improve the accuracy of MPF simulations using the anisotropic grain-boundary energy and mobility, which depend strongly on the misorientation angles, we account for the triple-junction properties in the MPF model. Further, two-dimensional simulations of grain-boundary migrations in three-grain systems as well as simulations of abnormal grain growth in a polycrystalline system are performed using the proposed model, in order to confirm its validity. The results show that the proposed model allows for the introduction of the anisotropic energy and mobility with high accuracy for a wider range of misorientations, in contrast to the conventional model. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2015.10.010

会議種別：口頭発表（一般）  

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会議種別：ポスター発表  

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会議種別：口頭発表（招待・特別）  

会議種別：口頭発表（基調）  

開催地：Jeju, Korea  

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会議種別：口頭発表（一般）  

開催地：Frankfurt am Main, Germany  

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開催地：Frankfurt am Main, Germany  

会議種別：口頭発表（招待・特別）  

開催地：Seoul, Korea  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）