Phase equilibria and thermodynamic calculation of the CoeTa binary system

Abstract

Experimental investigation and thermodynamic evaluation of the CoeTa binary phase diagram was carried out. Equilibrium compositions obtained in two-phase alloys and diffusion couples were measured by electron probe microanalyzer (EPMA). A very narrow l3(C36) þ l2(C15) two-phase region is confirmed to be present around 26.5 at.% Ta at temperatures between 950 C and 1448 C. Equilibrium relationships above 1500 C among the liquid, Laves (l1(C14), l2 and l3, whose stoichiometry is described by Co2Ta), m(D8b) and CoTa2(C16) phases were investigated by microstructural examination in as-cast Co-(24-60 at.%)Ta alloys. The solvus temperature of the g0 Co3Ta (L12) phase precipitated in the 5.8 at.%Ta g(Co) and the peritectoid temperature of the Co7Ta2 phase in an 8.5 at.%Ta alloy were determined to be 1013 C and 1033 C, respectively, by differential scanning calorimeter (DSC). Fine precipitates of the g0 phase precipitated in the g (A1) matrix were observed by transmission electron microscope (TEM). Analyzing the present experimental results synthetically, the g0 Co3Ta phase was identified to be a metastable phase, of which the g/g0 transition temperature of the stoichiometric Co3Ta alloy was estimated to be 2000 C. Thermodynamic assessment of the CoeTa binary system was carried out based on the present results as well as on experimental data in the literature. Calculated results of not only stable but also metastable equilibria were found to be in good agreement with the revised phase diagram. The evaluated stability of the metastable g0 Co3Ta coincides with the enthalpy of formation (DH(g’,Co3Ta) = 23.44 kJ/mol) calculated by the ab initio method.