Experimental investigation and thermodynamic description of the Co–Mo–Ta system

38 annealed and 30 as-cast alloys were investigated to construct the isothermal sections at 1273 and 1473 K and the liquidus surface projection of the Co–Mo–Ta system by SEM/EDS and XRD methods. The three-phase regions Co3Mo+λ3+μ and fcc(Co) + Co3Mo+λ3 at 1273 K, and the three-phase region fcc(Co)+λ3+μ at 1473 K, were determined. In the liquidus surface projection, eight primary solidification phase regions, fcc(Co), bcc(Mo, Ta), σ, μ, λ1, λ2, λ3 and CoTa2, and five invariant reactions, liq.→fcc(Co)+λ3+μ, liq.→λ1+λ3+μ, liq.+λ2→λ1+λ3, liq.+CoTa2→μ+bcc(Mo, Ta) and liq.+bcc(Mo, Ta)+σ→μ, were obtained. On the basis of the experimental results, the Co–Mo–Ta system was evaluated to obtain a set of self-consistent thermodynamic parameters by CALPHAD method.