Coarsening kinetics of γ′ precipitates in cobalt-base alloys

The expeditious development of novel cobalt-base γ-γ′ alloys as possible next generation superalloys critically depends on achieving a comprehensive understanding of the coarsening kinetics of ordered γ′ precipitates. This paper discusses the coarsening of L1 ₂ ordered Co₃(W, Al) precipitates in a model ternary Co-10Al-10W (at.%) alloy during isothermal annealing at 800 and 900 C. The experimentally determined temporal evolution of average size of the γ′ precipitates suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening at both temperatures. The γ′ coarsening rate constants have been determined using a modified coarsening rate equation for non-dilute solutions. Furthermore, using the Cahn-Hilliard formulation for interfacial energy, the γ/γ′ interfacial energies at the respective annealing temperatures have been correlated to the concentration profile across the interface that has been experimentally determined using atom probe tomography. The calculated interfacial energies are in comparable range with those observed in nickel-base superalloys. Additionally, this analysis has permitted, for the first time, the determination of the gradient energy coefficient for γ/γ′ interfaces in Co-base alloys, a critical input for phase-field and other simulation models for microstructural evolution.