Grain size-dependent energy partition in phase transition of NiTi shape memory alloys studied by molecular dynamics simulation

Abstract

Nanocrystalline NiTi shape memory alloys show fundamental changes in phase transition behavior when the grain size is reduced down to nano-scale (grain size <30 nm). However, due to the extreme difficulties of in-situ experimental observation, universally acknowledged physical mechanisms and detailed microstructures at atomic level are still not clear. In this paper, detailed microstructure images and quantified energy partition at atomic level during stress-induced phase transition are obtained by molecular dynamics simulation. Phase transition is gradually suppressed and incomplete phase transition occurs as grain size decreases. The potential energy landscape of the nanocrystalline system changes significantly from nonconvex to convex, which leads to corresponding changes in stress–strain response and microstructural evolution. With decreasing grain size, the interface (grain boundary and phase boundary) energy makes much more contributions in variation of the system potential energy than the crystallite (austenite and martensite) energy. The mechanism of energy dissipation changes from atomic interface friction in phase transition to plastic deformation caused by grain boundary sliding. It is the gradual dominance of interfacial energy terms in total potential energy that leads to the observed fundamental changes of phase transition behavior in nanocrystalline NiTi.