纳米多晶NiTi形状记忆合金超弹性的晶粒取向依赖性相场研究

本文基于Ginzburg-Landau理论，建立了一个反映纳米多晶NiTi形状记忆合金取向依赖性的二维多晶相场模型，研究了晶粒取向对其超弹性性能的影响。结果表明，纳米多晶NiTi形状记忆合金的超弹性行为依赖于晶粒取向分布，即：多晶模型中在所研究的参数变化范围内，晶粒取向分布范围越广、晶粒间取向差越大（无明显织构），超弹性性能越差；而晶粒取向分布范围越窄、晶粒间取向差越小（织构越强），超弹性性能越好。该晶粒取向依赖性可解释为：由于晶粒取向的不同，马氏体相变过程中相邻晶粒间的变形不匹配程度不同，因此，多晶模型中在所研究的参数变化范围内，晶粒间取向差异越大，晶界处的变形失配越严重，由此而产生的局部内应力将阻碍其附近马氏体相变的扩展，进而导致纳米多晶NiTi形状记忆合金超弹性性能下降。

PHASE FIELD SIMULATION ON THE GRAIN ORIENTATION DEPENDENT SUPER-ELASTICITY OF NANOCRYSTALLINE NITI SHAPE MEMORY ALLOYS

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.