Experimental determination and thermodynamic assessment of the phase diagram in the Co–Zr system

Abstract

The phase diagram in the Co–Zr system was determined using the electron probe microanalyzer (EPMA), differential thermal analysis (DTA) and X-ray diffraction (XRD) technique. The experimental results indicate that (1) the solubility region of the Co 2Zr phase is from 25 to 34 at.% Zr; (2) the CoZr 3 phase exists and the temperature of the peritectoid reaction (CoZr 2 + (βZr) ↔ CoZr 3) is about 981 °C; (3) the solubilities of Zr in the (αCo) phase and Co in the (βZr) phase are about 0.15 and 2.5 at.%, respectively. A thermodynamic assessment of the Co–Zr binary system was carried out by the CALPHAD (calculation of phase diagrams) method. The Gibbs free energies of the solution phases (liquid, fcc, bcc and hcp) were described by the subregular solution model, and those of the intermetallic compounds (Co 11Zr 2, Co 23Zr 6, Co 2Zr, CoZr, CoZr 2 and CoZr 3) were described by the sublattice model. A proper set of the thermodynamic parameters has been derived for describing the Gibbs free energies of each phase in the Co–Zr system. An agreement between the calculated results and experimental data is obtained.