Effect of withdrawal rate on the microstructure and static recrystallization behavior of Ni-based single crystal superalloy

The deformation characteristics after directional solidification at different withdrawal rates and static recrystallization (SRX) during solution heat treatment of Ni-based single-crystal superalloy castings were systematically investigated. The primary dendrite arm spacing was slightly affected by changes in the withdrawal rate, whereas the size of the γ′ precipitates in the dendrite and interdendritic regions was considerably influenced by the withdrawal rate. Additionally, a stress-simulation analysis based on thermoelastic plasticity suggested a slight decrease in the size of the deformation zone and maximum equivalent plastic strain with an increase in the withdrawal rate. Conversely, the SRX area notably increased with higher withdrawal rates. Such abnormal SRX phenomenon was attributed to the significantly different dislocation densities caused by the different responses of the γ′ sizes to deformation. Moreover, the dislocation density was directly related to the γ′ surface area per unit volume. The morphological changes of γ′ precipitates in the deformation zone were ascribed to the combined effect of compressive stress-induced P-type rafting and elastic anisotropy-induced octodendritic growth caused by lattice mismatch. Finally, other potential factors contributing to this abnormal SRX phenomenon were discussed.