Uncovering the Mechanism of Recrystallization in a Single-Crystal Superalloy: The Role of Porosity and Localized Strain in Nucleation

Preventing recrystallization during rejuvenation heat treatment is crucial for extending the service life of single-crystal nickel-based superalloy components. This study investigates the mechanism of recrystallization (RX) in a tensile pre-strained, single-crystal nickel-based superalloy, STAL15, with a particular focus on the roles of porosity and resultant localized strain. Using an innovative interrupted heat treatment (IHT) technique, combined with high-resolution scanning electron microscopy and electron backscatter diffraction, the microstructural evolution was tracked in the same regions of interest through four sequential IHT cycles. The findings reveal that nucleation of RX occurs at two solidification pores (S-pores) located in the gauge center of the pre-strained sample, which exhibit the highest level of localized strain. Although the macroscopic tensile plastic strain was 4.5 pct, RX is not observed in pore-free regions except at the sample surface. The dominant mechanism for nucleation of RX is identified as subgrain coalescence/rotation (SCR), enhanced by a sharp orientation gradient from the high localized strain around the S-pore. This process is probably further assisted by the preferential dissolution of secondary γ′ particles near the pore rim, which is believed to accelerate static recovery and subgrain coarsening. The subsequent growth of the successfully formed RX nucleus is dominated by the formation of annealing twins, which rapidly expand to consume the localized strain area around the S-pore.