Stability of pressure-dependent, thermally-induced displacive transformations in bi-atomic crystals

An important property of some metallic alloys, such as NiTi, for technological applications is their coupled thermomechanical shape memory behavior. This is due to temperature-dependent first-order displacive (martensitic) transformations in which their crystal structures transform between a higher symmetry cubic phase and lower symmetry phases (rhombohedral, tetragonal, orthorhombic, or monoclinic). In a recent paper, Elliott et al. (J. Mech. Phys. Solids, in press) proposed a nano-mechanical model based on temperature-dependent atomic potentials to explicitly construct an energy density W(F;O) to find all the different equilibrium paths and their stability of a stress-free bi-atomic perfect crystal as a function of temperature. In this work we investigate the influence of hydrostatic pressure. In general, hydrostatic compression increases the critical temperatures on the principal branches. For the same absolute value, hydrostatic tension is found to have a more pronounced effect on the equilibrium paths than hydrostatic compression.