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 1300C for 20h, 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 900C for 500h 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 was metastable and the three phases of γ, CoAl and Co3W were thermodynamically in equilibrium at 900C in the Co–Al–W ternary system.

Determination of Phase Equilibria in the Co-Rich Co–Al–W Ternary System with a Diffusion-Couple Technique. S.Kobayashi, Y.Tsukamoto, T.Takasugi, H.Chinen, T.Omori, K.Ishida, S.Zaefferer: Intermetallics, 2009, 17[12], 1085-9