Temperature dependence of kinetics pathway of γ′ precipitation in Co-Al-W superalloys: A phase-field study

The expeditious development of novel Co-based superalloys depends on achieving a compressive understanding of the kinetics path way of γ′ precipitates. This paper investigates the temperature dependence of evolution processing and mechanism of γ′ phase in a model Co-9.5Al-9W (at%) alloy using phase-field method. The simulated microstructures during aging at 700, 800 and 900 ℃ show that spherical γ′ quickly changes to cuboidal aged at 900 ℃ while transforms into irregular interconnected morphology at low temperature, which is consistent well with experimental observations. By characterizing the long-range parameters, this irregular γ′ is attributed to the coalescence of adjacent precipitates with the same domain, primarily driven by the decrease of interfacial energy. The temporal evolution of average size of the γ′ precipitates suggests classical Lifshitz-Slyozov-Wagner coarsening at the three temperatures, and the coarsening rate constants K are determined. The temperature dependence of K is clarified by analyzing the thermodynamic driving force and chemical mobility. Additionally, the effects of particle inter-distance, shape and size on γ′ evolution are discussed from the perspective of diffusion potential.