An Atomistic Study on Shape-Memory Effect by Shear Deformation and Phase Transformation

An atomistic study on the shape-memory effect is carried out by molecular dynamics simulations with the EAM potential for Ni-Al alloy. As preliminary simulations, the stable structures for several Ni contents at various temperatures are investigated, and reveal that the martensite structure is obtained at low temperature for models with 62% or higher Ni content, while bcc is stable at high temperatures. A 68% Ni model with martensite structure, which consists of several variants in two opposite orientations without macroscopic deformation, is applied for MD simulation under a series of thermo-mechanical conditions of shear loading, unloading, heating and cooling. When shear load is applied, some of the variants with unstable orientation against the load change orientation, and a permanent macroscopic deformation is obtained. When this deformed martensite is heated up, a phase transformation to bcc occurs and the deformation is diminished. The original martensite is regained by cooling. Since the orientation of the variants are random, the macroscopic deformation is not observed, which means that the deformation imposed by the external load recovers the original shape by the heat treatment.