Molecular dynamics simulation of grain size effect on mechanism of twin martensite transformation of nanocrystalline NiTi shape memory alloys

In this work, a constitutive model considering grain size and model size is established to describe the stress-strain response of nanocrystalline NiTi Shape Memory Alloys (SMAs) with various grain sizes, and the stress-strain curves of nanocrystalline NiTi SMAs with various grain sizes are obtained by Molecular Dynamics (MD) simulation. The correctness of the simulation results is verified by the comparison with the constitutive model. Then, the dependence of the deformation mechanism of the twin martensite transformation on the grain size is studied by MD simulation. The results show that neither temperature nor stress induces twin martensite transformation for grain size less than 20 nm. While twin martensite occurs in the models with grain sizes of 20 and 30 nm during phase transformation. Only one twin plane occurs for the grain size of 20 nm, but multiple twin planes occur for the grain size of 30 nm. During twin martensitic phase transformation, the proportion of grain boundary remains basically unchanged, while the proportion of phase boundary increases. With the grain size increasing, the twin plane and the proportion of phase boundary increase, and the appearance of multiple twin planes is closely related to the increase of phase boundary.