PHASE-DIAGRAM AND THERMOCHEMICAL PROPERTIES OF THE ZR-TA SYSTEM - AN ASSESSMENT BASED ON GIBBS ENERGY MODELING

The thermochemical properties of the Zr-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 phases, which is necessary in systematic ana lyses of cohesive and thermal properties of transition metal systems, and in thermodynamic modelling of ternary and higher-order phase diagr ams based on Zr-Ta. This kind of information has been obtained by usin g a theoretical method which relies on the coupling between thermochem istry and phase diagrams. The approach involves the analysis and synth esis 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 optimisation technique. The G(m) functi ons arrived at in this way are applied to construct, by calculation, a phase diagram for the Zr-Ta system, which is used in critical compari sons with the experimental data available. In addition, the enthalpy o f formation values generated by the present study are compared with es timates based on the Miedema method and other predictive approaches.