Effect of super-solidus heat treatments on microstructure and homogenization of CMSX-4 plus single crystal nickel-based superalloy

The development of advanced single crystal Ni-based superalloys with high refractory element content has enabled significant improvements in high-temperature mechanical properties but has also introduced challenges related to chemical segregation and microstructural heterogeneity in as-cast materials. Conventional solution annealing heat treatments for homogenization are lengthy and can increase porosity, limiting the practical application of these alloys. This study systematically investigates the effects of isothermal super-solidus and super-solidus hot isostatic pressing heat treatments on the microstructure and chemical homogenization of the third-generation single crystal Ni-based superalloy CMSX-4 Plus. Phase transformation temperatures were determined using differential scanning calorimetry and thermodynamic calculations to guide the selection of super-solidus heat treatment parameters. Microstructural evolution, porosity, and chemical segregation were characterized by advanced electron microscopy and energy-dispersive X-ray spectrometry, while DICTRA kinetic simulations provided complementary insights. Super-solidus heat treatments were found to promote rapid dissolution of eutectic regions and substantial reduction of chemical segregation, with homogenization strongly dependent on temperature and holding time. The optimized super-solidus hot isostatic pressing process, comprising two holding stages at 1345 °C and 1350 °C under 100 MPa isostatic pressure, achieved near-complete elimination of porosity and residual eutectics, while preserving the single crystal structure, in a fraction of the time required by conventional treatments. These findings establish a scientific and practical foundation for accelerated homogenization of high-refractory-content single crystal superalloys, supporting improved processing efficiency and microstructural quality in advanced turbine applications.