Transformation pathway of NiTi shape memory alloy and its modulation based on grain size gradient: A molecular dynamics study

The reduction of grain size to nanoscale is a common approach to enhance material properties, such as fatigue resistance and overall strength of Shape Memory Alloys (SMAs), but it normally decreases the volume fraction of the material undergoing reversible martensitic transformation (MT), leading to a reduction in the reversible strain amplitude. This paper explores the atomic relationship between specific transformation pathways and grain size/gradient of NiTi polycrystal based on molecular dynamics simulation. It is found that during a tensile loading -unloading cycle, four different transformation pathways can be observed, with their volume fractions being dependent on grain size. Importantly, these transformation pathways serve as the medium of the grain size effects on the global mechanical properties of NiTi. The gradient nanograined (GNG) NiTi with large gradients can maintain a high stress level while reducing the residual strain by adjusting the fraction of different transformation pathways, thus achieving both advantages of small residual strain and high strength during cyclic loading with large strain amplitude. The results provide atomic insights into the grain size effects on the transformation pathways of SMAs and the development of high-performance SMAs based on the grain size gradient.