Spheroidization Mechanism of Lamellar δ Phase in Inconel 718 during Delta Processing

The morphology evolution of lamellar delta phase in Inconel 718 during Delta Processing was characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. A spheroidization mechanism is proposed for transformation of the lamellar delta phase. The results showed that lamellar delta phase hindered the movement of dislocations, which led to a dislocation aggregation zone around the fracture location of the delta phase. Strain in the region thus increased, which promoted the occurrence of dynamic recrystallization and fracture of the lamellar delta phase. A diffusion channel for Nb atoms was simultaneously provided by the high-density dislocations, which led to dissolution and reduction in the amount of delta phase. Under the combined action of deformation, fracture, and dissolution, the lamellar delta phase gradually transformed into granular short rods and spheroids.