Multiphase-field simulation of the solution heat treatment process in a Ni-based superalloy

Abstract

New generation of nickel based single crystal superalloy requires complex solution heat treatment (SHT) procedures to improve its high temperature performance. However, the designation of SHT is difficult due to severe segregation, significant fraction of eutectic, and undesired incipient melting. The multiphase-field simulation coupled with thermodynamic calculation was used to investigate microstructure evolution and element distribution from solidification to SHT process in a superalloy. The simulated solidified microstructure was used as input for subsequent SHT simulation. And the simulation results were verified by experimental measurements in terms of the γ/γ' eutectic fraction and microsegregation pattern in the as-cast state and during each step of SHT. Based on the simulated microstructure and concentration, the incipient melting temperature was obtained by thermodynamic calculation. The results show that the region with lowest melting temperature is located at the boundary between γ dendrite and interdendritic bulk γ' phase, which is last solidification region. And incipient melting temperature gradually increases during SHT. Besides, the variation of γ' phase fraction under different holding temperatures was simulated, and calculated γ' reduction rate increases significantly with the temperature. The current work has demonstrated the ability of multiphase-field simulation and thermodynamic calculation in designing SHT procedures for complex superalloys.