Based on the Ginzburg-Landau's theory, the free energy function of polycrystalline system was modified by introducing an extra grain boundary energy, and a new two-dimensional phase field model considering the continuous variation of temperature was proposed to investigate the grain size dependent super-elasticity (SE) and shape memory effect (SME) of nanocrystalline NiTi shape memory alloys (SMAs) and to reveal the microscopic mechanism of such a grain size dependence. The simulated results show that: in the SE process, the nucleation-expansion and reduction-disappearance mode of local martensite band observed in the nanocrystalline NiTi SMAs with relatively large grain size can be gradually converted to a uniform martensite transformation and its reverse in the ones with smaller grain size; in the process reflecting one-way shape memory effect (OWSME), with the reduction of grain size, the content of remained austenite phase in the martensitic polycrystalline system obtained by quenching an original austenite one increases, and the involved inelastic deformation mechanism during tension-unloading progressively changes from martensite reorientation to reversible martensite transformation; for the stress-assisted temperature-induced martensite transformation (SATIMT), the content of austenite phase in the polycrystalline system at the lowest applied temperature increases with decreasing the grain size, and the martensite transformation cannot occur when the grain size is below a certain critical one. Further analysis indicates that the dependence of the SE and SME of nanocrystalline NiTi alloys on the grain size can be attributed to the increased proportion of un-transformable grain boundary with decreasing the grain size, whose inhibition to the martensite transformation within grains becomes stronger and stronger, i.e., the energy barrier increases progressively.

基于金兹堡-朗道的理论，通过引入额外的晶界能对多晶体系的自由能函数进行了修正，并提出了一种考虑温度连续变化的二维二维相场模型来研究晶粒尺寸依赖性超晶格。纳米晶NiTi形状记忆合金（SMAs）的弹性（SE）和形状记忆效应（SME），并揭示了这种晶粒尺寸依赖性的微观机理。仿真结果表明：在SE过程中，晶粒尺寸较大的纳米晶NiTi SMA中观察到的局部马氏体带的形核-膨胀和还原消失模式可以逐步转变为均匀的马氏体相变，而在相反的情况下则相反。晶粒较小 在反映单向形状记忆效应（OWSME）的过程中，随着晶粒尺寸的减小，通过淬火原始奥氏体获得的马氏体多晶体系中残余奥氏体相的含量增加，并且在拉伸过程中所涉及的非弹性变形机制从马氏体重新定向到可逆马氏体相变逐渐卸载；对于应力辅助的温度诱发马氏体相变（SATIMT），在最低施加温度下，多晶体系中的奥氏体相含量随晶粒尺寸的减小而增加，并且当晶粒尺寸低于某个临界值时，不会发生马氏体相变一。