Molecular dynamics simulation of shape memory behaviour using a multi-grain model

Shape-memory behaviour in multi-grain material is simulated using a molecular dynamics method. An embedded-atom-method potential for NiAl alloy is applied, and a sequence of conditions including loading, unloading, heating and cooling is imposed. Two types of grain arrangement are used, and the deformation and shape recovery due to phase transformation are observed for both models. The stress–strain relation is revealed to draw a hysteresis loop, and the individual curves are smoother than those previously obtained from a single-crystal model. The deformation mechanism during loading is discussed using local structure analysis. Local deformation is initiated at the grain boundaries, and the deformed region propagates along the twin plane in the grain. The propagation is then obstructed by the grain boundaries, and a band pattern of the deformed area is formed. The influence of the grain shape and distribution, as well as the crystal orientation of each grain, on the deformation behaviour is also investigated. Qualitatively common features in the deformation mechanism and stress–strain relation are observed despite different grain distributions, while the critical values in stress vary, owing to the crystal orientations of the grains.