Intragranularly misoriented grain boundary evolution affected by local constraints and grain size in micro-scale deformation of ultra-thin metallic sheets

Multi-stage micro-scale deformation-based processing of ultra-thin metallic sheets has been widely used in the manufacturing of light-weight parts with complicated structures. Because only a few grains are distributed along the thickness direction of ultra-thin sheets, the grain size effect on the in-grain microstructure evolution must be understood. Moreover, the intragranular orientation mismatch induced by prior processing needs to be studied to accurately describe the additional deformation resistance and anisotropy in the step-by-step fabrication. In this study, 0.1-mm-thick SS 316L sheet samples with different mean grain sizes were deformed to two strain levels, with the electron backscatter diffraction (EBSD) used to characterize the microstructures of the deformed samples. Based on the EBSD data and crystal plasticity finite element simulations, the effects of local constraints and grain size on intragranularly misoriented grain boundary (IMGB) evolution was statistically analyzed. The IMGB patterns were organized into four categories, and the characteristics of local constraints near the grain boundary triple junctions were delineated. The formation of IMGBs was found to be induced by the concentration of high local strain, which was governed by local constraints, including the geometrical and deformation-related factors. Because of the higher intragranular strain and longer dislocation moving distance before annihilation, the IMGBs accumulate more easily inside large grains. The collected local constraint characteristics and the linear correlation between IMGB length and grain area were applied in a proposed three-step procedure for estimating the IMGB patterns and magnitudes of the micro-scale deformation of ultra-thin sheets. This comprehensive and quantitative description of IMGB evolution provides a fundamental basis for understanding the grain-level deformation mechanisms and for the constitutive modelling of the mechanical responses of ultra-thin sheets in complex deformation.