WEB Growth and ripening kinetics of γ′′ precipitates in Nb containing nickel-base superalloys: Phase-field modelling and simulation
The excellent mechanical properties of the Nb containing nickel-base superalloy IN718 mainly result from the particle-strengthening effect of the coherent γ′′ precipitates. Due to a strongly anisotropic lattice misfit between the matrix and the precipitate phase, the particles exhibit pronounced plate-shaped morphologies. We discuss the γ′′ precipitate microstructure evolution using a non-isothermal multi-component phase-field model, which further accounts for the elastic particle-particle interaction due to anisotropic misfit strains. The multi-component thermodynamic and kinetic input data is generated from CALPHAD databases. Required elastic input data is measured in-house. We consider γ/γ" microstructure evolution during isothermal growth and ripening conditions. The results are compared to experimental data, specifically generated within this project, as well as predictions from the classical LSW theory of Ostwald ripening.
Further, we investigate various influencing factors that determine the equilibrium shapes of γ′′ precipitates, minimizing the sum of the total elastic and interfacial energy. Upon increasing precipitate phase fractions, the model predicts increasingly stronger particle-particle interactions, leading to shapes with significantly increased aspect ratios. We consider the periodic arrangement of precipitates in different hexagonal and rectangular superstructures, which result from distinct choices of point-symmetric and periodic boundary conditions. The energetically most favorable superstructure is found to be a hexagonal precipitate arrangement.