Phase equilibria and thermodynamic calculation of the Co–Ta binary system

• Phase diagram of the Co–Ta binary system was determined experimentally.
• Phase equilibria between Laves phases were determined precisely.
• Existence of metastable Co3Ta(L12) intermetallic compound was confirmed.
• Thermodynamic evaluation of the Co–Ta binary system was carried out.
• Evaluated stability of metastable Co3Ta(L12) agrees well with its ab initio enthalpy.

Experimental investigation and thermodynamic evaluation of the Co–Ta 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 λ3(C36) + λ2(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 (λ1(C14), λ2 and λ3, whose stoichiometry is described by Co2Ta), μ(D8b) and CoTa2(C16) phases were investigated by microstructural examination in as-cast Co-(24–60 at.%)Ta alloys. The solvus temperature of the γ′ Co3Ta (L12) phase precipitated in the 5.8 at.%Ta γ(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 γ′ phase precipitated in the γ (A1) matrix were observed by transmission electron microscope (TEM). Analyzing the present experimental results synthetically, the γ′ Co3Ta phase was identified to be a metastable phase, of which the γ/γ′ transition temperature of the stoichiometric Co3Ta alloy was estimated to be 2000 °C. Thermodynamic assessment of the Co–Ta 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 γ′ Co3Ta coincides with the enthalpy of formation (ΔH(γ'Co3Ta) = −23.44 kJ/mol) calculated by the ab initio method.