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.