A thermodynamic description of the TiAl system

Abstract

A thermodynamic description of the TiAl system was developed in this study. Nine phases were considered and can be classified as three types — disordered solution phases: liquid, (αTi, hcp), (βTi, bcc), (Al, fcc); ordered intermetallic phases: α2-Ti3Al (DO19), γ-TiAl (L10), TiAl3 (DO22); and stoichiometric phases: TiAl2 and Ti2Al5. While the Redlich-Kister equation was used to describe the excess Gibbs energy of the disordered solution phases, a generalized bond-energy model recently developed by us at the University of Wisconsin at Madison was used to describe the Gibbs energy of the ordered intermetallic phases. The model parameters were optimized using the experimental phase equilibrium and thermodynamic data available in the literature. The calculated phase diagram as well as the thermodynamic functions are in good agreement with experimental data. The intrinsic defect concentrations calculated from the model parameters for γ-TiAl were found to be in accord with those obtained from a semiempirical relationship in terms of its enthalpy of formation and the available experimental data. The generalized bond-energy model parameters for the ordered intermetallic phases were converted to those of the compound energy model for the convenience of the users of Thermo-Calc.