Rejuvenation of a Crept Second-Generation Nickel-Based Single Crystal Superalloy with Emphasis on Dendritic Scale Heterogeneity

Rejuvenation heat treatments (RHTs) are essential for repairing service-induced creep damage in high-value turbine blades made of nickel-based single crystal superalloys. This study systematically evaluated several RHT protocols for a second-generation superalloy subjected to 2.0 pct creep strain at 980 °C/250 MPa, with emphasis on the dendritic scale heterogeneity. The rejuvenation efficacy was found to be substantially influenced by this heterogeneity. A Sub-solvus RHT restored the microstructure in the dendrite cores but left the microstructures only partially recovered in the interdendritic regions, causing preferential strain accumulation therein during the subsequent creep and thus limiting the restoration of creep properties. Conversely, a Super-solvus RHT, while achieving complete recovery across the dendritic scale, induced the detrimental recrystallization originating from the strain concentrated within the interdendritic regions, which destroyed the creep resistance of the alloy. Critically, introducing a cyclic Stress relief annealing prior to the Super-solvus RHT effectively suppressed the recrystallization by reducing the stored energy, particularly in the interdendritic regions. This combined approach enabled complete microstructural restoration in both the dendrite cores and interdendritic regions, thereby fully recovering the creep performance. This work underscores the critical role of the dendritic scale heterogeneity in determining rejuvenation efficacy and provides guidelines for developing optimized RHT strategies to extend the service life of single-crystal components.