Surface Recrystallization and Twin Formation in a Single Crystal Superalloy

The samples of single crystal superalloy DD6 were grit blasted and then heat treated in the temperature range of 1100-1250°C for 4h and the DD6 alloy ‘standard heat treatment’ in vacuum furnace, respectively. The results showed that cellular recrystallization occurred in the surface layer after heating at 1100°C for 4 hours. While equiaxed recrystallization grains occurred near the surface of the samples annealed at 1200°C for 4 hours, meanwhile, cellular recrystallization located between equiaxed recrystallization grains and the original region. With the improvement of the heating temperature, the size of cellular recrystallization decreased, while the size of equiaxed recrystallization grains increased, and the shape of the coarse γ′ phase in the cellular recrystallization changed from lamellar to equiaxial. Fully equiaxed recrystallization grains nucleated after standard heat treatment. Furthermore, the twins occurred in fully equiaxed recrystallization grains, and that the γ′ phase of the twin plane appeared different from that of the equiaxed recrystallization boundary. On the contrary, the twin formation was not observed in the cellular recrystallization grains. Therefore, the differences in twin behavior between the fully equiaxed recrystallization and cellular recrystallization grains were discussed.