Microstructure prediction of TMW-4M3 during heat treatment

The calculation model on the basis of multi-phase-field model, coupled with the calculation of phase diagram (CALPHAD) approach and classical nucleation theory (CNT) model, was studied for the precipitation behavior of γ′ precipitates in Ni-Co base superalloy TMW-4M3 (“TMW” is a trademark of National Institute for Materials Science registered in Japan) during the heat treatment process. This alloy is precipitation strengthened by γ′, and thus the control of size and distribution of γ′ precipitates is very important for practical applications. Precipitation behavior of γ′ in TMW-4M3 is strongly influenced by heat treatment conditions. Therefore, numerical simulation is useful for finding the most appropriate manufacturing condition prior to experimental trial and error approach. Firstly, we investigated the γ′ solvus temperature and improved several thermodynamic parameters of the conventional database to obtain reasonable phase boundaries between γ/η(Ni₃Ti) and γ/γ′. Based on the new thermodynamic description, the nucleation rates related to temperature histories were calculated by using the CNT with composition balance equation under subsolvus solution treatment. Finally, the multi-phase-field method coupled with the CALPHAD approach and CNT model was applied to the microstructure evolution of intragranular secondary γ′ particles. The experimental and simulated microstructures show good agreement with each other. This method might be useful for microstructure evolution prediction of γ/γ′ dual phase during the heat-treatment.