Suppression effect of oxygen on the β to ω transformation in a β-type Ti alloy: insights from first-principles

Abstract

Oxygen has long been known to retard the formation of the hexagonal ω phase in body-centered-cubic β-Ti based alloys, but a quantum mechanical evaluation of the thermodynamics associated with the β to ω transformation remains unexplored. Our first-principles density functional theory calculations on a model Ti3Nb alloy containing 2 at% O reveal that O prefers an octahedral interstitial in both the β and ω phases and increases the ω energy relative to β (0.19 eV/O), thus making the ω phase thermodynamically less favorable. Interestingly, we find that O atoms in β can be categorized into two sets which have a strikingly distinct effect. The O atoms in collapsing {1 1 1} planes increase the energy barrier (29 meV/atom) in the β to ω transformation remarkably; whereas the O atoms in the uncollapsing {1 1 1} planes exert a much smaller retarding effect (5 meV/atom). This is because the former set of O atoms has to pass through high-lying intermediate tetragonal sites. The suppression effect of O can be understood with electronic structure analyses.