Gibbs Energy Modelling of the Phase Diagram and Thermochemical Properties in the Hf-Ta System

Abstract

The thermochemical properties of the Hf-Ta system are not known from direct measurements and there is a lack of information on the Gibbs energy G m of its various stable phases, which is necessary in establishing systematically the thermal properties of transition metal alloys, and in the modelling of ternary and higher-order phase diagrams based on Hf-Ta. This kind of information has been obtained using a theoretical method which relies on the coupling between thermochemistry and phase diagrams. The approach involves the analysis and synthesis of selected pieces of phase equilibrium data using models for G m of bcc, hcp and liquid, and the evaluation of a set of optimum model parameters through a computer optimization technique. The G m functions arrived at in this way are used in constructing, by calculation, a phase diagram for the Hf-Ta system, and in analysing the composition and structure dependence of the enthalpy of formation. Detailed comparisons with estimates from the Miedema method and other predictive approaches are also reported.