The metastable tetragonal γ″ phase (Ni$_{3}$Nb) is the principal strengthening phase in Ni-base superalloys such as alloy 718. Accurate modelling of the temporal evolution of the precipitate sizes during ageing at elevated temperatures is of great importance in the context of “Integrated Computational Materials Engineering”. The γ″ precipitates are embedded in an fcc solid-solution matrix and appear plate-shaped due to anisotropic misfit strains at the coherent cubic-to-tetragonal phase boundary. The interplay between interfacial energy and bulk elastic energy causes the precipitate shape to be strongly size-dependent. We investigate the intricate effects that influence the γ″ ripening kinetics using 3D phasefield simulations and ageing experiments.

We investigate microstructure coarsening during isothermal ageing between 700°C and 760°C using a 3D sharp phasefield model. The model considers the anisotropic misfit strains, phase-dependent elastic constants and diffusion of Nb. The sizes, shapes and orientations of the precipitates are measured using the method of central moments. The simulation results are compared to the ripening behavior of spherical precipitates in conventional ripening models and in phasefield simulations.

The results of the simulations elucidate the influence of the elastically induced precipitate shape on the phase transformation kinetics during Ostwald-ripening of the precipitates. The increased curvature of the plate-shaped precipitates leads to deviation from conventional ripening models. The size-dependency of the precipitate shape lowers the ripening exponent. Considering this effect yields temperature-dependent ripening kinetics and precipitate shapes consistent with the experimental data. The results of this work allow for the adaption of classical ripening models to γ′′ precipitates.