Phase Equilibria and Microstructural Evolution in Ternary Co-Al-W Between 750 and 1100 °C

Abstract

Phase equilibria between 750 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 γ, D019, and B2. The 1000 °C section contains two three-phase tie-triangles, γ + D019 + μ and γ + B2 + μ; while only one, γ + B2 + μ, exists at 1100 °C. Results at 950 °C suggest that it is possible that γ′ is in equilibrium, with four out of five compositions exhibiting microstructures supporting its thermodynamic stability, and the fifth composition containing only a very small amount of a fourth phase. However, if true, the temperature range of γ′ stability would be quite small, and it is more likely that the kinetics of γ′ dissolution are the cause of its persistence at 950 °C. At 850 °C, γ′ is observed to dissolve more rapidly than at 900 °C where dissolution exceeds 8000 h. A time-temperature-transformation diagram constructed from the combined results exhibits a classic nose-shape indicative of the tradeoff between thermodynamics and kinetics. This further supports a small driving force as the reason for the persistence of γ′ in the microstructures of Co-Al-W alloys at 900 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.