Ab initio study of pressure stabilised NiTi allotropes: pressure-induced transformations and hysteresis loops

TL;DR

This study uses density functional theory to analyze pressure-induced phase transitions in NiTi alloys, revealing the instability of the B19' phase and the atomic mechanisms behind shape memory effects and hysteresis.

Contribution

It demonstrates that the experimentally observed B19' phase is unstable under pressure and uncovers the atomic trajectories responsible for shape memory behavior.

Findings

B19' phase is unstable under hydrostatic pressure in bulk form.

Unique atomic trajectories during phase transition are linked to shape memory.

Hysteresis arises from complex energy surface features.

Abstract

Changes in stoichiometric NiTi allotropes induced by hydrostatic pressure have been studied employing density functional theory. By modelling the pressure-induced transitions in a way that imitates quasi-static pressure changes, we show that the experimentally observed B19' phase is (in its bulk form) unstable with respect to another monoclinic phase, B19". The lower symmetry of the B19" phase leads to unique atomic trajectories of Ti and Ni atoms (that do not share a single crystallographic plane) during the pressure-induced phase transition. This uniqueness of atomic trajectories is considered a necessary condition for the shape memory ability. The forward and reverse pressure-induced transition B19'{$\leftrightarrow$}B19" exhibits a hysteresis that is shown to originate from hitherto unexpected complexity of the Born-Oppenheimer energy surface.