Orientation dependence of mechanical behavior and phase transformation of NiTi shape memory alloy with multilayer structures by molecular dynamics simulation

Molecular dynamics simulation is used to investigate orientation dependence of mechanical behavior and phase transformation of NiTi shape memory alloy with multilayer structures, including AMA (amorphous-monocrystal-amorphous) model, AMN (amorphous-monocrystal-nanocrystalline) model, and NMN (nanocrystalline-monocrystalnanocrystalline) model. The middle monocrystal layer has two different orientations. The [100]-oriented monocrystal is in non-preferential orientation, which does not contribute to the stress-induced martensitic transformation. The 1/2110]-oriented monocrystal is in preferential orientation and thus it can undergo martensitic transformation under a relatively low tensile stress. Furthermore, it is beneficial to inducing the formation of multiple martensite variants inside the adjacent grains. For the [100]-oriented AMN model, a small amount of residual martensite appears in the grains with non-preferential orientation or strong mechanical constraints, whereas there is almost no residual martensite in the other five models. A great number of shear transformation regions are observed in the amorphous phase, which causes the degradation of superelasticity in AMA and AMN models. A small number of dislocations are observed at the grain boundaries in nanocrystalline NiTi structure, but their contribution to plastic deformation is very small. Surface nanocrystallization can effectively improve the superelasticity of NiTi samples. For NMN structure without any amorphous phase, the irrecoverable strain can be reduced to about 0.3%. (c) 2022 The Authors. Published by Elsevier B.V.