title: Calculations for displacive ω-phase transformations in Ti-Al alloys with Nb additions at finite temperature.
abstract: We examine by means of first-principles calculations the bcc-like (bcc: body centered cubic) to ω-like phase transformations in Ti-Al alloys with Nb additions at finite temperature. To simulate the alloy we use different discrete atomic configurations in a six atom unit cell of the stoichiometry Ti(3)Al(2)Nb. Calculated ground state energies show an instability in the ternary Ti(3)Al(2)Nb alloy against the ω structure type atomic displacement. To better understand the role of entropy in the stability/instability of these systems, the first-principles calculations are extended to finite temperature by including various contributions to the free energy. In particular, the vibrational free energy is calculated within a quasiharmonic approximation. It is shown that the bcc structure is stabilized by the contribution of the low energy modes to the lattice entropy against ω type atomic displacements. We find that configurational entropy plays a major role in the ω to B8(2) transformation. Calculated lattice parameters and transition temperatures are found to be in excellent agreement with experiment.
authors.affiliation: Physics Department, Texas Tech University, Lubbock, TX 79409, USA. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
authors.forename: M
authors.initials: M
authors.lastname: Sanati
authors.forename: D
authors.initials: D
authors.lastname: West
authors.forename: R C
authors.initials: RC
authors.lastname: Albers