Precipitation of γ′ in the γ binder phase of WC-Al-Co-Ni cemented carbide: A phase-field study

High-temperature properties of WC-Co cemented carbides can be greatly improved by introducing ordered γ′ precipitates (L12 structure) into disordered γ Co-binder matrix (fcc structure). The microstructure evolution of γ′ precipitates in γ matrix was investigated by using multi-phase-field (MPF) method. In order to avoid indistinguishable miscibility during MPF simulations, a phenomenon caused by mistaking the chemical free energies of γ and γ′ phase for each other, the thermodynamic driving force and diffusivities were taken from CALPHAD databases with the Gibbs energies of ordered and disordered phases separated according to a single Gibbs energy expression in a thermodynamic database. The interfacial energy between γ and γ′ phases was estimated based on a thermodynamic model applied to coherent interfaces. The elastic strain energy was introduced to reproduce the cuboidal shape of the ordered γ′ precipitates. Specially, the MPF approach was applied to study the morphological evolution of γ + γ′ microstructure as a function of concentration, temperature and aging time. It was shown that with increasing of Al concentration the cuboidal shape of precipitates appears at early stages of growth. After long term aging at 1373K the γ′/γ interface becomes semi-coherent and some γ-channels are formed. The simulation results are in good agreement with experimental data and have demonstrated the ability of the model to capture the major structural characteristics of microstructural evolution in Al–Co–Ni binder system during long-term heat treatment. Based on the knowledge of microstructure evolution, it is possible to design the optimal process parameters efficiently instead of expensive and time-consuming experiments.