Phase stability and thermodynamic modeling of the Re-Ti system supplemented by first-principles calculations

A new thermodynamic description of the Re-Ti binary system has been developed by complimenting the CALPHAD modeling technique with first-principles calculations based on density functional theory. Finite temperature thermodynamic properties of the confirmed compound in this system, Re ₂₄Ti ₅, are calculated using the Debye-Grüneisen model for the sake of simplicity and efficiency. The hcp solid solution phase was predicted from first-principles calculations of special quasirandom structures to have a positive enthalpy of mixing, indicating the existence of a miscibility gap. The bcc solid solution phase was also predicted from the use of special quasirandom structures to have a negative enthalpy of mixing indicating a strong tendency for Re solubility in bcc-Ti. Phonon calculations based on the supercell approach are used to investigate finite temperature properties of the reported ReTi compound. By combining the first-principles results and existing experimental phase equilibria data, the Gibbs energy functions of individual phases in the Re-Ti system have been evaluated with and without the ReTi compound. The calculated phase diagram without the ReTi compound shows good agreement with the available experimental data from the literature. Results from the modeling version with the ReTi compound show poorer agreement with the bcc solidus experimental data. The modeling also indicates that ReTi is stable, but could be metastable when the uncertainty of the first-principles calculations is taken in to account.