Simulation of structural phase transitions in NiTi

By means of molecular-dynamics simulations, temperature-driven diffusionless structural phase transitions in equiatomic and nearly equiatomic ordered nickel-titanium alloys were investigated. For this purpose, a model potential from the literature was adopted [W. S. Lai and B. X. Liu, J. Phys.: Condens. Matter 12, L53 (2000)], which is based on the tight-binding model in second moment approximation. The model predicts a stable B19' phase at low temperatures and a nearly cubic B2 phase at high temperatures. After an analysis of crystallography and energetics of the emerging structures, the experimentally known strong dependence of transition temperatures on composition is confirmed and related to lattice instability. Free-energy calculations finally give insight into the driving forces of the phase transitions and reveal free energy barriers inhibiting them below the transition temperatures.