Titanium $\alpha - \omega$ phase transformation pathway and a predicted metastable structure

TL;DR

This study uses advanced computational methods to elucidate the phase transformation pathway of titanium from alpha to omega, discovering two transition states and a metastable structure with potential implications for material stability.

Contribution

It introduces a combined SS-NEB and DFT+U approach to accurately map the transformation pathway and predicts a new metastable orthorhombic structure in titanium.

Findings

Identified two transition states along the alpha-omega transformation pathway.

Discovered a metastable orthorhombic structure with stable phonons.

Found that the metastable phase decreases in stability with increasing pressure.

Abstract

As titanium is a highly utilized metal for structural light-weighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Impurities, pressure, and temperature control the phase stability and transition barriers in most industrial and geophysical materials - in Ti, interstitial O, N, or C retard while substitutional Al and V suppress the $\omega$ phase. Using a proper solid-state nudged elastic band (SS-NEB) method employing double-climbing images (C2-NEB) combined with density-function theory (DFT+U) methods for accurate energetics, we detail the pressure-induced $\alpha$ (ductile) to $\omega$ (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path (MEP) and discover a metastable body-centered orthorhombic (bco) structure, with stable phonons, a lower density than the endpoint phases, and decreasing stability with increasing pressure.