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

Experimental information on the thermochemical properties and the phas e equilibria involving the condensed phases of the Zr-C system has bee n analysed using phenomenological models for Gibbs energy. The solid s olutions of carbon in the h.c.p. and b.c.c. phases of Zr and the non-s toichiometric NaCl-type structure ZrCx carbide are treated by using th e compound energy model (GEM) for interstitial phases. The liquid phas e is treated by using a substitutional solution model and the Redlich- Kister formalism for excess Gibbs energies. The parameters in the mode ls are determined by computerized optimization of selected experimenta l 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 re spectively, are described using estimated values for the entropy and e nthalpy of formation. These estimates are based on the regular behavio ur of the vibrational entropy and other cohesive properties which have been established in recent studies on transition metal compounds. A t hermodynamic description is obtained which is used to construct, by ca lculation, the Zr-C phase diagram. Detailed comparisons are presented between calculations and experimental data of various kinds. The entha lpy of formation values predicted by Miedema and coworkers are also co nfronted with the present results.