Co-Based superalloy morphology evolution: A phase field study based on experimental thermodynamic and kinetic data

Abstract

Cobalt-based superalloys with two phase γ/γ′ microstructures offer great promise as candidates for next-generation high-temperature alloys for applications, such as turbine blades. It is essential to understand the thermodynamic and kinetic factors that influence the microstructural evolution of these alloys in order to optimize the alloy compositions and processing steps with a goal to improve their coarsening, creep and rafting behavior. We are using a continuum phase field approach to study the diffusion process and to predict the equilibrium shapes of Co-Al-W γ′ precipitates. In order to obtain quantitatively predictive capabilities, we extract chemical free energies for the γ/γ′ phases based on CALculation of PHAse Diagrams (CALPHAD) thermodynamic data and diffusion mobilities for Co alloys based on CALPHAD kinetic data. We also use experimental or first-principles data for other quantities, such as misfit strain and interface information, for the parameterization of our model. A particular focus of our study is to understand how different energy balances, misfit strain and kinetics affect the coarsening and rafting behavior of γ′ precipitates, and the sensitivity of the final precipitate shape to materials parameters. We find that the equilibrium shape of the precipitate results from a delicate competition between chemical, interfacial, and elastic energies, and it is very sensitive to changes in model parameters. We examine how modeling input parameters affect the equilibrium shape of precipitates and relate these parameters to experimentally available values.