Experimental investigation of phase equilibria in the Co-Hf system

Phase equilibria in the Co-Hf binary system were investigated based on the experimental results from X-ray diffraction, transmission electron microscopy, scanning electron microscopy, electron probe microanalysis and differential scanning calorimetry on annealed and as-cast alloys. A new orthorhombic compound Co11Hf2 with a stability range from ∼650 to 1273 °C was observed instead of Co7Hf reported in literature. Another stoichiometric phase Co7Hf2 proposed in literature was not observed in the present work. The reaction type for the formation of CoHf2 was confirmed to be peritectic, but the eutectic composition for the reaction L ↔ CoHf2 + (βHf) was modified to be ∼71.5 at.% Hf. Homogeneity ranges of different intermetallic compounds and solution phases at 700 and 1100 °C as well as the invariant reaction temperatures were experimentally determined. The newly constructed phase diagram shall provide accurate information for further thermodynamic modeling of the Co-Hf binary system.