Thermal Stabilization of the HCP Phase in Titanium

TL;DR

This study uses a tight-binding model to predict the temperature and pressure conditions under which titanium transitions between hcp and omega phases, revealing entropy-driven stabilization of the hcp phase at room temperature.

Contribution

It provides the first theoretical prediction of the hcp-omega phase transformation in titanium, aligning well with experimental data.

Findings

Omega structure is the true zero-temperature ground state.

Entropy from phonons stabilizes the hcp phase at room temperature.

Quasi-harmonic approximation fails for bcc phase due to unstable phonons.

Abstract

We have used a tight-binding model that is fit to first-principles electronic-structure calculations for titanium to calculate quasi-harmonic phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega crystal structures. We show that the true zero-temperature ground-state is the omega structure, although this has never been observed experimentally at normal pressure, and that it is the entropy from the thermal population of phonon states which stabilizes the hcp structure at room temperature. We present the first completely theoretical prediction of the temperature- and pressure-dependence of the hcp-omega phase transformation and show that it is in good agreement with experiment. The quasi-harmonic approximation fails to adequately treat the bcc phase because the zero-temperature phonons of this structure are not all stable.