Interactions of austenite-martensite interfaces with Ni4Ti3 precipitates in NiTi shape memory alloy: A molecular dynamics investigation

Abstract

Precipitation of secondary phases is a common strategy used to control both the structural and functional properties of shape memory alloys. It can be used to promote nucleation of the martensitic transformation as well as improve cyclic stability. Less is understood about how precipitates affect the progression of an ongoing transformation, i.e., motion of austenite-martensite interfaces. In this study, we performed molecular dynamics simulations of the interaction of austenite-martensite interfaces moving in the NiTi alloy with Ni₄Ti₃ precipitates. It was found that the nanoscale precipitates obstruct interface motion until a sufficient undercooling is reached. The simulation results can be quantitatively explained with thermoelastic effects – elastic deformation of the precipitates acts to oppose the thermodynamic driving force favoring the transformation. A simple model is proposed to predict a more difficult transformation in shape memory alloys with higher concentrations of and/or harder precipitates. Additionally, simulations of cyclic transformations implicate inelastic deformation at the precipitate-matrix interface as one mechanism responsible for the cyclic drift in transformation characteristics. Deformation originated in a thin, amorphous interfacial layer and expanded with increasing cycles.