Thermodynamic modeling of the Ta-O system

Abstract

CALPHAD (CALculation of PHAse Diagrams) thermodynamic models of the Ta-O system are developed using experimental data from the literature and are supplemented by first-principles calculations. The standard-pressure equilibrium polymorphs of α and β Ta2O5 are modeled as stoichiometric compounds with tabulated thermochemical data. The metallic phase, body-centered cubic (BCC) Ta, has significant O solubility at high temperatures. A sublattice model of the BCC phase is fit to electromotive force (EMF) and metallographic data to obtain thermodynamic values for O dissolution in Ta. First-principles calculations of the enthalpies of mixing for O in BCC Ta and enthalpies of formation for β-Ta2O5 agree with the modeled and experimental data. The liquid Ta-O phase lacks data, but preliminary associate solution and ionic liquid models are fit to the metallographic data from the literature. The calculated phase diagrams are consistent with existing experimental phase diagrams.