Multiscale Crystal Plasticity Analysis for Production of Ultrafine-Grained Metals Based on Self-Organization of Subgrain

超细晶粒金属(UFGM)作为高强度材料引起了人们的兴趣。在材料织构控制领域，数值预测UFGM生产过程的微观机理是可望的。然而，这样的研究从未被报道过。在本研究中，针对UFGM产生过程的预测，严格推导了描述位错晶胞结构和亚晶自组织的反应扩散方程。提出了反应扩散方程的速率系数的应力效应模型，以反映晶体中剪切应力分解对位错图案化的影响。为了将该自组织模型推广到多晶模型，改进了位错反应项，使其能够表示几何必要位错在晶界上的积累和晶界附近位错湮灭率的降低。T-…阶段转换时间的不同在应力效应系数模型和位错平均自由程模型中引入了由于晶体初始取向不同而引起的更多的三阶段硬化。在此基础上，将反应扩散方程中计算的静止位错密度代入晶体的硬化模数，建立了位错图案化与多晶变形耦合的多尺度晶体塑性模型，并将该模型应用于面心立方多晶板在大应变条件下的压缩问题，进行了位错图案化的有限差分方法和晶体变形的有限元模拟。数值模拟预测，在第二阶段，晶内形成微米尺度的晶胞结构，晶界附近GN位错大量聚集；第三阶段，沿剪切带产生大量微观剪切带，并形成具有累积GN位错的亚晶壁。晶胞向亚晶转变的时间随各晶胞向亚晶转变的时间不同而不同。此外，对超细晶的产生过程进行了适当的可视化，并阐明了当局部施加大于2的强应变时，在晶内诱导出GN晶界，晶界沿GN晶界被分离成具有大角度晶界的亚微米量级的多个细晶。这项研究为多尺度模拟奠定了基础，使之能够根据位错行为对UFGM的生产过程进行计算预测。较少

Ultrafine-grained metals (UFGM) have attracted interest as high-strength materials. It is expected in the field of material textures control that a microscopic mechanism of production process for the UFGM is numerically predicted. However, such a study has never been reported. In this study, aiming at a prediction of production process of UFGM, reaction-diffusion equations are rigorously derived, that describe the self-organization of dislocation cell structure and subgrain. A stress effect model for rate coefficients of the reaction-diffusion equations is proposed so as to reflect the information of resolved shear stress of a crystal on dislocation patterning. In order to extend this self-organization model to a model for polycrystal, a dislocation reaction term is improved so that it can express an accumulation of geometrically necessary (GN) dislocation on grain boundary and a decrease of dislocation annihilation ratio near the boundary. A difference of stage transition timing for t … More hree-stage hardening of a crystal due to difference between the initial orientations of grain is introduced into the stress effect coefficient model and a dislocation mean free path model. Furthermore, substituting the immobile dislocation density calculated from the reaction-diffusion equations into a hardening modulus of a crystal, a multiscale crystal plasticity model is developed, that couples the dislocation patterning and polycrystal deformation.A multiscale crystal plasticity simulation using a FDM for dislocation patterning and a FEM for crystal deformation is carried out applying this model to a compression problem of an FCC polycrystal plate under severe strain condition. It is numerically predicted that the cell structure in the micrometer size is formed in a grain and the GN dislocations are accumulated around grain boundary in stage II. It is also reproduced that a lot of micro shear bands generate and subgrain walls with accumulated GN dislocations are formed along the shear bands in stage III. The timing of transition from cell to subgrain is different depending on the resolved shear stress condition of each grain. Moreover, the generation process of ultrafine grain is appropriately visualized and it is clarified that GN boundaries are induced in a grain by applying severe strain locally over 2 and a grain is separated along the GN boundaries into plural fine grains in the sub-micron order with large angle boundary. This study lays a foundation of multiscale simulation that enables computationally to predict the production process of UFGM on the basis of dislocation behaviors. Less