GIBBS ENERGY COUPLING OF PHASE-STABILITY AND THERMOCHEMISTRY IN THE HF-NB SYSTEM

The thermodynamic properties of the Hf-Nb system are little known from experimental data. There have been conflicting reports about the phas e diagram, and no measurements have been presented of the thermochemic al properties. As a consequence, there is a lack of information on the Gibbs energy (G(m)) of its various stable phases, which is necessary in order to establish systematically the thermal properties of transit ion metal alloys, and to model ternary and higher order phase diagrams based on Hf-Nb. This problem is studied in the present paper by apply ing a theoretical method which relies on the coupling between thermoch emistry and phase diagrams. The approach involves the analysis and syn thesis of selected phase equilibrium data using models for G(m) of bcc , hcp and liquid, and the evaluation of a set of optimum model paramet ers through a computer optimisation technique. The G(m) functions arri ved at in this way are used in constructing, by calculation, a phase d iagram for the Hf-Nb system, and in analysing the composition and stru cture dependence of the enthalpy of formation. Detailed comparisons ar e reported with estimates from the Miedema method and other predictive approaches, and with the behaviour of the related systems Hf-Ta, Zr-N b and Zr-Ta, which have recently been analysed.