Determination of phase equilibria in the Co-rich Co-Al-W ternary system with a diffusion-couple technique

Phase equilibria in the Co-rich Co-Al-W ternary system were determined with a unique diffusion-couple technique in which Co-27Al and Co-15W binary alloys (at. %) were first coupled for interdiffusion and then heat-treated for precipitation. After a diffusion process at 1300 °C for 20 h, concentration gradients of Al and W were formed in the γ-Co(A1) matrix in the vicinity of the coupled interface. After a heat treatment at 900 °C for 500 h the γ′-Co₃(Al,W)(L1₂) phase was formed with a coarsened shape in contact with the γ, CoAl(B2) and Co₃W(D0₁₉) phases. Additionally, it appeared with a submicron cuboidal shape within the γ matrix. After 2000 h, however, the coarsened γ′ phase became infrequent and the three phases of γ, CoAl and Co₃W came into frequent contact with each other. These results clearly demonstrate that the γ′ phase is metastable and the three phases of γ, CoAl and Co₃W are thermodynamically in equilibrium at 900 °C in the Co-Al-W ternary system.