Evaluation of the tantalum-titanium phase diagram from ab-initio calculations

The thermodynamic properties of the Ta-Ti binary system below 900 °C are not well known. In particular, the location and shape of the solvus between the phase separation region at low temperatures and the solid solution at high temperatures are not well defined. In this study, we present a thermodynamic description for this system based on ab-initio calculations. The formation enthalpies of bcc and hcp solid solutions are estimated using the special quasi-random structures methodology and their vibrational free energy calculated by the quasi-harmonic Debye model. The excess energies of the solid solutions are fitted to a sub-subregular model and used to define the phase diagram of the binary system. It is shown that the current empirical assessment of the energies of the pure elements leads to a phase diagram that strongly departs from the known experimental features at low temperatures. An ab-initio guided correction of these energies is necessary to obtain correctly the low temperature phase separation and the high temperature solid solution. The predicted solvus of the phase diagram is qualitatively different from those previously reported for the Ta-Ti system. It exhibits a miscibility gap between two distinct bcc phases, similar to those that exist in the closely related binary systems Ta-Zr, Ta-Hf, Cr-Ti, Mo-Ti, V-Ti, and Ti-W. The calculated Ta-Ti phase diagram, based on ab-initio evaluation of the excess Gibbs energy, including corrections of the Gibbs energies of the pure elements. The dashed line represents the monotonic solvus of the experimental phase diagram. [Display omitted]