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.