Experimental Investigation and Thermodynamic Description of the Co–Mo–Nb System

38 equilibrated alloys and 36 as-cast alloys were studied in this work. The microstructures of the alloys were characterized by Scanning Electron Microscopy (SEM). The compositions of the alloys and phases were measured by Energy Dispersive Spectrometer (EDS). And the crystallographic structures of the phases were identified by X-ray Diffraction (XRD). According to the research results, the phase equilibrium relations at 1000 °C and 1200 °C and the liquidus surface projection of the Co–Mo–Nb ternary system were constructed. At 1000 °C, two three-phase regions λ 1 + Co 3 Mo + fcc(Co) and μ + λ 1 + Co 3 Mo were experimentally confirmed. And at 1200 °C, one three-phase region μ + λ 1 + fcc(Co) was experimentally confirmed. In addition, six and four two-phase equilibria were measured at 1000 °C and 1200 °C, respectively. Seven primary phase regions, bcc(Mo, Nb), σ , μ , λ 1 , λ 2 , λ 3 and fcc(Co), and four invariant reactions, liq. → λ 1 + λ 3 + fcc(Co), liq. + λ 1 + λ 2 → λ 3 , liq. → μ + λ 1 + fcc(Co) and liq. + σ → bcc(Mo, Nb) + μ , were obtained in the liquidus projection. According to the experimental results, thermodynamic optimization was conducted using the CALculation of PHAse Diagram (CALPHAD) approach. The thermodynamic database of the Co–Mo–Nb ternary system was established. And the calculated results well reproduced experimental data in this study.