Predicting the morphologies of γʹ precipitates in cobalt-based superalloys

Cobalt-based alloys with γ/γ′ microstructures have the potential to become the next generation of superalloys, but alloy compositions and processing steps must be optimized to improve coarsening, creep, and rafting behavior. While these behaviors are different than in nickel-based superalloys, alloy development can be accelerated by understanding the thermodynamic factors influencing microstructure evolution. In this work, we develop a phase field model informed by first-principles density functional theory and experimental data to predict the equilibrium shapes of Co-Al-W γ′ precipitates. Three-dimensional simulations of single and multiple precipitates are performed to understand the effect of elastic and interfacial energy on coarsened and rafted microstructures; the elastic energy is dependent on the elastic stiffnesses, misfit strain, precipitate size, applied stress, and precipitate spatial distribution. We observe characteristic microstructures dependent on the type of applied stress that have the same γ′ morphology and orientation seen in experiments, indicating that the elastic stresses arising from coherent γ/γ′ interfaces are important for morphological evolution during creep. The results also indicate that the narrow γ channels between γ′ precipitates are energetically favored, and provide an explanation for the experimentally observed directional coarsening that occurs without any applied stress.

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