GIBBS ENERGY MODELING OF THE PHASE-DIAGRAM AND THERMOCHEMICAL PROPERTIES IN THE HF-TA SYSTEM

The thermochemical properties of the Hf-Ta system are not known from d irect measurements and there is a lack of information on the Gibbs ene rgy G(m) of its various stable phases, which is necessary in establish ing 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 theoretic al method which relies on the coupling between thermochemistry and pha se diagrams. The approach involves the analysis and synthesis of selec ted 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 t hrough a computer optimization technique. The G(m) functions arrived a t in this way are used in constructing, by calculation, a phase diagra m for the Hf-Ta system, and in analysing the composition and structure dependence of the enthalpy of formation. Detailed comparisons with es timates from the Miedema method and other predictive approaches are al so reported.