Abstract：In the super-elastic deformation process of nano-polycrystalline NiTi shape memory alloys (SMAs), different grain orientations can lead to a complex stress field in the polycrystalline system, which may affect the martensitic transformation and the super-elasticity capability of such alloys. Therefore, in this work, based on the Ginzburg-Landau’s theory, a two-dimensional phase field model was proposed to investigate the grain-orientation dependent super-elasticity of nano-polycrystalline NiTi SMAs. The super-elastic deformation processes of four nano-polycrystalline NiTi SMA systems with different distributions of grain orientations were simulated by utilizing the proposed phase field model. From the simulated microstructure evolution and stress-strain responses of the polycrystalline systems, the microscopic mechanism of the dependence of super-elasticity on the grain orientation was discussed and revealed. It is illustrated that the super-elasticity of nano-polycrystalline NiTi SMAs strongly depends on the grain orientation: Within the range of parameters considered, the wider the distribution range of grain orientation is (i.e., no obvious texture), the lower the super-elastic capability is; the narrower the distribution range of grain orientation is (i.e., with obvious texture), the higher the super-elastic capability is. Such a phenomenon can be explained as follows: the mismatched deformation between adjacent grains occurred during the martensitic transformation varies if the grain orientations are different, that is, within the range of parameters considered, the larger the orientation difference between adjacent grains, the more serious the mismatched deformation at the grain boundary is; further, the local internal stress caused by such a mismatched deformation can hinder the expansion of martensite transformation, and then the super-elastic capability of nano-polycrystalline NiTi SMAs decreases. The microscopic mechanism of the grain-orientation-dependent super-elasticity of nano-polycrystalline NiTi SMAs revealed in this work can provide a valuable reference for designing the NiTi SMA devices with different super-elastic capabilities by adjusting the texture to a prescribed degree in the polycrystalline systems in terms of some specific methods such as electroplating, rolling, annealing, cold working, etc.
2021, Vol. 42 
