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伊東 達矢; 小川 祐平*; Gong, W.; 川崎 卓郎; 岡田 和歩*; 柴田 曉伸*; Harjo, S.
no journal, ,
The mechanisms of hydrogen charging in a 310S-type austenitic stainless steel, which promote strengthening and twinning, were investigated, along with the evaluation of dislocation and stacking fault evolution using in situ neutron diffraction during deformation. The hydrogen-charged sample exhibited increased yield stress, flow stress, and uniform elongation, consistent with previous studies. However, the effect of solute hydrogen on dislocation and stacking fault evolution was found to be minimal, in contrast to earlier reports suggesting that hydrogen promotes their formation to facilitate twinning. Further details will be presented and discussed.
久保 淳; 河合 江美*; 梅野 宜崇*
no journal, ,
It is well known that dislocations in metal materials form several types of microstructures by self-organizing under cyclic deformation. Those micro-structuring processes are directly related to fatigue crack initiation and thus have been of both scientific and industrial importance. The formation of the dislocation microstructures can be phenomenologically reproduced by the reaction-diffusion equations, which are often introduced to describe the self-organized patterns in nature such as characteristic patterns on animals' skin (e.g., giraffe, zebra, etc.). While the reaction and diffusion parameters in the equations should be properly determined to reproduce the microstructure formation, it has not been established how to determine those parameters on the basis of the microscopic mechanisms of dislocation motion because the reaction-diffusion model itself is a phenomenological mathematical model. In this study, we formulated the diffusion behavior of dislocations under cyclic loading deformation, as a part of an attempt to establish a microscopic basis of the reaction-diffusion model for dislocation micro-structuring under fatigue. We assumed a stochastic elementary process of dislocation motion driven by cyclic deformation and derived an analytical solution of the diffusion coefficient of dislocations as a simple function of basic material characters (the Burgers vector length, dislocation density, etc.) and loading condition (strain amplitude). In addition, we conducted a series of molecular dynamics simulations of dislocation motion under cyclic deformation for the purpose of validating the derived analytical model. It was confirmed that the diffusion coefficients obtained by the analytical model and molecular dynamics simulation are in good agreement under various calculation conditions, which indicates the validity of the analytical model and its applicability for parametrization of the reaction-diffusion model.
Lobzenko, I.; 椎原 良典*; 岩下 拓哉*
no journal, ,
金属ガラスの機械的性質は、長い間、高い関心を集めている研究テーマである。過冷却液体としてのMGの研究において、モンテカルロシミュレーションは、構造秩序と緩和時間の増加の問題を克服するのに役立つ手法である。本研究では、原子間ポテンシャルを用いた古典的分子動力学シミュレーションにおいて、スワップモンテカルロアルゴリズムを適用し、Cu50%Zr50%系の機械的性質に対する安定性の影響を調べた。本研究では、無熱準静的せん断計算を実施するとともに、これらの構造の安定性が機械的性質に与える影響および巨視的応力ひずみ曲線と各原子の微視的特性を調べた結果について報告する。
Gong, W.; Gholizadeh, R.*; Harjo, S.; 川崎 卓郎; 相澤 一也; 辻 伸泰*
no journal, ,
The addition of lithium (Li) to magnesium (Mg) alloys enables the formation of a body-centered cubic (BCC) phase. These Mg alloys with a BCC phase exhibit superior ductility at room temperature, addressing a common limitation of Mg alloys: their typically low ductility. However, these alloys generally show poor work-hardening capability at room temperature, resulting in low ultimate tensile strength and limited uniform elongation. This lack of work-hardening is likely due to the ease of dislocation recovery in the BCC phase. It is well known that lowering the deformation temperature can suppress dislocation recovery, potentially enhancing work hardening. Nonetheless, studies on the cryogenic deformation behavior of dual-phase Mg-Li alloys remain limited. In this study, we investigated the deformation behavior of a dual-phase Mg-Li alloy at cryogenic temperatures using in-situ neutron diffraction. The stress-strain curves showed that the yield stress of the alloy increased consistently as the deformation temperature decreased. Moreover, work hardening was significantly enhanced at cryogenic temperatures, resulting in simultaneous improvements in ultimate tensile strength and uniform elongation. At 295 K, the dislocation densities in both the BCC and HCP phases remained low and nearly constant during straining. In contrast, at 200 K, 77 K, and 20 K, the dislocation densities in both phases increased steadily with strain, reaching higher values as temperature decreased. Strain rate jump tests revealed a strain rate sensitivity of approximately 0.03 at 295 K, while a negative strain rate sensitivity was observed below 200 K, likely due to the dynamic strain aging effect.
Lobzenko, I.; 都留 智仁
no journal, ,
人工ニューラルネットワーク(ANN)の利用は、ここ数十年で科学分野の大部分に広がっている。特に材料の機械的特性の原子レベルモデリングは、第一原理計算から得られた大規模データセットでトレーニングされたANNベースの機械学習ポテンシャル(MLP)が、新しい複雑な材料のポテンシャルエネルギー面を再現する上で重要な役割を果たしている。多成分合金(MCA)などの材料の場合、信頼性の高い原子レベルモデリングは、ANNのフィッティング能力の活用が期待されているが、ANNで構築された原子間ポテンシャルの高精度かつ堅牢性を実現するには、大規模データセットが必要であるだけでなく、ネットワークのアーキテクチャとトレーニングプロセスを材料ごとに最適化する必要がある。そこで本研究では、MoNbTaVWおよびZrNbTaTiHf合金に基づくMCAなどの一連の底心立方(BCC)材料の新しく構築されたMLPを用いて機械的特性をモデル化した。MoNbTaにおけるMLPを用いた転位運動のモデリングについて検討した結果、単相BCC金属に見られる通常の110滑り面とは異なる、特異な(112)滑り面が見出されることを明らかにした。