Role of hydrostatic pressure on the phase stability, the ground state, and the transformation pathways of NiTi alloy

Equiatomic NiTi shape memory alloys exhibit multiple martensitic transformations from a parent B2 phase. However, there is an apparent discrepancy between theoretical predictions and experimental observations of the transformation pathways and final products. We studied the phase stabilities of NiTi martensites with respect to the effect of hydrostatic pressure using a generalised solid-state nudge elastic band method implemented in density functional theory to simulate the minimum energy pathways of transformations. The analysis demonstrates that base-centred orthorhombic (BCO) phase is unstable, thus will undergo the BCO → B19′ phase transformation at above 9 GPa pressure, as confirmed by phonon calculations.