纳米单晶NiTi合金单程形状记忆效应的分子动力学模拟

采用基于第二近邻修正型嵌入原子势的分子动力学方法研究了纳米单晶NiTi合金的单程形状记忆效应，详细阐明了温度诱发马氏体相变和应力诱发马氏体重定向过程中纳米单晶的变形行为和微结构演化，进一步分析了加/卸载速率对NiTi合金单程形状记忆效应的影响。结果表明，NiTi纳米单晶在应力加载过程中发生马氏体重定向，卸载后存在残余应变；当加热到奥氏体转变结束温度以上时，马氏体逆相变为奥氏体相，残余应变逐渐减小，但未完全回复；随着应力加载速率的增加，重定向临界应力和模量逐渐增加；再次降温过程中不同加载速率下的原子结构演化各不相同。

Molecular Dynamics Simulation on the One-way Shape Memory Effect of NiTi Shape Memory Alloy single crystal

The one-way shape memory effect of NiTi shape memory alloy nano-single crystal was investigated by the molecular dynamics simulations based on a second nearest neighbor modified embedded atom interaction potential. The deformation behavior and microstructural evolution of the NiTi nano-single crystal during temperature-induced martensitic transformation and stress-induced martensitic reorientation were elucidated in detail, and the effect of loading rate on the one-way shape memory effect in a nanoscale was discussed. Simulated results show that different martensite variants are coexisted when the nano-single crystal is cooled below the martensite finish temperature and then the martensitic reorientation occurs during the stress loading process. The residual strain after unloading gradually decreases with the increase of temperature, but cannot fully recover even if the temperature is much higher than the austenite finish temperature, since the irreversible plastic deformation occurs during the martensite reorientation. It is also found that the austenite transformation temperature of the single crystal during the subsequent heating after the stress-induced martensitic reorientation is much higher than that without undergoing the martensitic reorientation. On the one hand, the plastic deformation will disturb the stress field in the single crystal, resulting in a pinning effect on the martensitic reverse transformation; then, a larger driving force is needed to trigger the martensitic reverse transformation. On the other hand, since the interface morphology among various reoriented martensite variants is different from that among twinned martensite variants, the martensite interfacial energy will be changed by the martensitic reorientation, which affecting the critical temperatures of martensite transformation. In addition, it is found that the martensitic reorientation depends on the loading rate strongly. With the increase of the loading rate, the modulus and critical stress of martensitic reorientation increase gradually, which are caused by the viscosity of the movement of interfaces among different martensite variants. Finally, simulated results show that the evolutions of atomic structures are different at various loading rates in the subsequent cooling process. All the above results will provide a basis for the research on the molecular dynamics simulations on the shape memory effect of NiTi shape memory alloy polycrystal.