The one-way shape memory effect (OWSME) of nanocrystalline NiTi shape memory alloy (SMA) and its cyclic degeneration under thermo-mechanically coupled cyclic loading conditions were investigated by molecular dynamics (MD) simulations. Firstly, the critical temperatures of martensite transformation were obtained by carrying out the MD simulations of temperature-induced martensite transformation; then, the microscopic mechanisms of martensite reorientation-induced plasticity, the deformation during the subsequent heating and cooling, and the cyclic degeneration of OWSME were investigated based on the correspondent MD simulations. The results show that the martensite reorientation of nanocrystalline NiTi SMA occurs via the migration of the interfaces between martensite variants, the growth of martensite variants with favorable orientations and their swallowing to the variants with unfavorable orientations under specific mechanical loading conditions; the cyclic degeneration of OWSME in nanocrystalline NiTi SMA is caused by the interaction among the martensite reorientation, martensite transformation, martensite reorientation-induced plasticity and martensite transformation-induced plasticity occurred in the martensite region and at the interfaces between martensite variants. The martensite reorientation in each OWSME cycle is reflected by the migration of martensite variant interfaces and the growth and swallowing of martensite variants; the martensite transformation in each OWSME cycle is the nucleation and growth of martensite phase; and the accumulation of plastic deformation is caused by the migration of martensite variant interfaces and the dislocation slipping at the martensite variant interfaces which leads to the cyclic degeneration of OWSME. The MD simulations are verified further by comparing them with the available experimental results of NiTi SMA.

通过分子动力学 (MD) 模拟研究了纳米晶 NiTi 形状记忆合金 (SMA) 的单向形状记忆效应 (OWSME) 及其在热机械耦合循环加载条件下的循环退化。首先通过温度诱发马氏体转变的MD模拟得到马氏体转变的临界温度；然后，基于相应的MD模拟研究了马氏体再取向诱导塑性、后续加热和冷却过程中的变形以及OWSME循环退化的微观机制。结果表明，纳米晶 NiTi SMA 的马氏体重新定向是通过马氏体变体之间界面的迁移发生的，在特定的机械载荷条件下，具有有利取向的马氏体变体的生长及其对不利取向的变体的吞噬；纳米晶 NiTi SMA 中 OWSME 的循环退化是由马氏体区域和马氏体变体之间的界面处发生的马氏体重新定向、马氏体转变、马氏体重新定向诱导塑性和马氏体转变诱导塑性之间的相互作用引起的。每个 OWSME 循环中的马氏体重新定向通过马氏体变体界面的迁移和马氏体变体的生长和吞噬来反映；每个OWSME循环中的马氏体转变是马氏体相的形核和长大；塑性变形的积累是由马氏体变体界面的迁移和马氏体变体界面处的位错滑移引起的，导致OWSME的循环退化。通过将它们与 NiTi SMA 的可用实验结果进行比较，进一步验证了 MD 模拟。