Atomistic investigation on superelasticity of NiTi shape memory alloy with complex microstructures based on molecular dynamics simulation

Superelasticity of NiTi shape memory alloy (SMA) with complex microstructures is investigated at the atomistic scale based on molecular dynamics (MD) simulation. Six models representing different microstructures are established and they deal with BG (bicrystal grain) model, NG (nanocrystal grain) model, MG (mixed grain) model, which stands for the mixture of a coarse grain and NG, PS (polymorphic structure) model, which is composed of a coarse grain, NG and amorphous phase, {114} twin model and {112} twin model. Only the NG model presents a perfect stress plateau in the case of loading under the tensile strain of 8%. However, PS model, {114} twin model and {112} twin model show a monotonic stress rise during loading and they exhibit a monotonic stress decline during unloading. In particular, {112} twin model exhibits an extremely high stress level. Irrecoverable strain occurs in all the six models, where {114} twin model possesses the largest irrecoverable strain, whereas {112} twin model has the smallest irrecoverable strain. Dislocation slip, amorphous phase and grain boundary play an important role in the formation of the irrecoverable strain. Stress-induced martensitic transformation of NiTi SMA is influenced by grain size, grain orientation, phase composition, substructure and temperature, where the phase transformation behaviour of any given grain is closely related to its adjacent environment as well. As for the {112} twin model, martensitic transformation is not induced in the grain interior, but at the grain boundaries. It can be deduced that the exceptional superelasticity of {112} twin model is not completely attributed to stress-induced martensitic transformation.