ANALYSIS OF THERMOCHEMICAL PROPERTIES AND PHASE-STABILITY IN THE ZIRCONIUM CARBON SYSTEM

Experimental information on the thermochemical properties and the phase equilibria involving the condensed phases of the Zr-C system has been analysed using phenomenological models for Gibbs energy. The solid solutions of carbon in the h.c.p. and b.c.c. phases of Zr and the non-stoichiometric NaCl-type structure ZrCx carbide are treated by using the compound energy model (GEM) for interstitial phases. The liquid phase is treated by using a substitutional solution model and the Redlich-Kister formalism for excess Gibbs energies. The parameters in the models are determined by computerized optimization of selected experimental data. The metastable carbides ZrC0.5 and ZrC3, that are involved in the CEM for the interstitial solutions of C in h.c.p. and b.c.c. Zr respectively, are described using estimated values for the entropy and enthalpy of formation. These estimates are based on the regular behaviour of the vibrational entropy and other cohesive properties which have been established in recent studies on transition metal compounds. A thermodynamic description is obtained which is used to construct, by calculation, the Zr-C phase diagram. Detailed comparisons are presented between calculations and experimental data of various kinds. The enthalpy of formation values predicted by Miedema and coworkers are also confronted with the present results.