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

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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