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