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Phase equilibria and microstructural evolution in ternary Co-Al-W between 750 °C and 1100 °C



Eric Lass, Richard D. Grist, Maureen E. Williams


Phase equilibria between 750 °C and 1100 °C in the Co-rich portion of the Co-Al-W ternary system are investigated via isothermal annealing of five ternary alloy compositions. At temperatures of 900 °C and below, the phase diagram is dominated by a three-phase tie-triangle between gamma, D019, and B2. The 1000 °C section contains two three-phase tie-triangles, gamma+D019+mu and gamma+B2+mu; while only one, gamma+B2+mu, exists at 1100 °C. Results at 950 °C suggest that it is possible that gamma-prime is in equilibrium, with four out of five compositions exhibiting microstructures supporting its thermodynamic stability, and the fifth containing only a very small amount of a fourth phase. However, if true, the temperature range of gamma-prime stability would be quite small, and it is more likely that the kinetics of gamma-prime dissolution are the cause of its persistence at 950 °C. At 850 °C, gamma-prime is observed to dissolve more rapidly compared to 900 °C. A time-temperature-transformation diagram constructed from the combined results exhibits a classic nose-shape indicative of the tradeoff between thermodynamics and kinetics; further supporting a small driving force for the dissolution of gamma-prime as the reason for its persistence in the microstructures of Co-Al-W alloys at 900 °C and 950 °C. The measured equilibrium phase compositions and five corresponding isothermal sections of the phase diagram will provide essential data for constructing a new thermodynamic description of ternary Co-Al-W.
Journal of Phase Equilibria and Diffusion


Co-based alloy, superalloy, thermodynamics, phase equilibrium, phase diagram


Lass, E. , Grist, R. and Williams, M. (2016), Phase equilibria and microstructural evolution in ternary Co-Al-W between 750 °C and 1100 °C, Journal of Phase Equilibria and Diffusion, [online], (Accessed July 22, 2024)


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Created April 11, 2016, Updated November 10, 2018