Thermodynamic assessment of Co–Al–W system and solidification of Co-enriched ternary alloys

Based on the assessed three constituent binary systems and reported phase diagram data, the Co–Al–W ternary system was thermodynamically optimized by using CALculation PHAse Diagram (CALPHAD) method. The newly reported ternary phase γ′ was described with a two sublattice model, (Co,Al,W)0.75:(Co,Al,W)0.25, while liquid Fcc, Hcp(Co), and Bcc(W) were treated as substitutional solution phases. In order to describe the ternary solubility in the binary phases Co7W6 and CoAl, the models (Al,Co,W)7W2(Al,Co,W)4 and (Al,Co,W)0.5(Al,Co,W)0.5(Va)3 were, respectively, adopted. The rest 11 phases in the ternary systems were simply treated as stoichiometric compounds because their homogeneous ranges are small and can be neglected. A set of self-consistent parameters were obtained, which reasonably reproduced the phase relationship in Co-enriched corner. Finally, to confirm the reliability of the present assessment,the so-called Scheil–Gulliver model was used to simulate the solidification processes of three typical Co-based alloys. It was shown that the as-cast microstructures of the three ingots were well explained or predicted.