First-principles study of ω-phase formation in the Ti3Al2V system

Abstract

Using first-principles methods, the phase stability of the underlying body-centered-cubic (bcc) structureof Ti3Al2V and slightly rearranged atomic structures are investigated. Thecalculated ground-state energies show an instability in the ternaryTi3Al2V alloy withrespect to the ω structure-type atomic displacement. A Mulliken population analysis shows strong bondingbetween the transition metals and Al. It is shown that Ti–Al is the strongest bond and thatω-type displacements increase the population overlap for this bond and reduce the energy ofthe system. The first-principles calculations are extended to finite temperature andvarious contributions to the free energy are calculated within the quasiharmonicapproximation. It is shown that, at high temperatures, the bcc structure is stabilized bythe contribution of the low-energy modes to lattice entropy. In agreement withexperiment and in contrast to the Ti–Al–Nb system, we find that the metastableB82 structure cannot form in this alloy.