  By means of molecular-dynamics simulations, temperature driven diffusionless
structural phase transitions in equi- 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.
