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

The phase constituents and compositions of annealed and as-cast alloys in the Co–Hf–Nb system were measured and analyzed with the help of X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) techniques. A liquidus surface projection was established, in which nine primary solidification regions were experimentally determined and one primary solidification region was deduced based on the liquidus of Co₁₁Hf₂ in the Co–Hf system. Two isothermal sections at 1200 °C and 1100 °C were established, in which four and six three-phase equilibria were experimentally determined, respectively. The measured solubility of Nb in CoHf₂, CoHf, Co₁₁Hf₂, and Co₂₃Hf₆ was ~ 12.6, ~ 22.0, ~ 3.0, and ~ 6.1 at. pct at 1200 °C, and ~ 11.4, ~ 18.6, ~ 3.5, and ~ 5.6 at. pct at 1100 °C, respectively. The measured solubility of Hf in μ and λ₃ was ~ 9.3 and ~ 7.6 at. pct at 1200 °C, and ~ 8.9 and ~ 10.9 at. pct at 1100 °C, respectively. By reproducing the experimental phase diagrams, a thermodynamic assessment of the Co–Hf–Nb system was carried out using the CALPHAD method, and reasonable thermodynamic parameters were obtained.