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The morphology evolution of lamellar δ 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 δ phase. The results showed that lamellar δ phase hindered the movement of dislocations, which led to a dislocation aggregation zone around the fracture location of the δ phase. Strain in the region thus increased, which promoted the occurrence of dynamic recrystallization and fracture of the lamellar δ 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 δ phase. Under the combined action of deformation, fracture, and dissolution, the lamellar δ phase gradually transformed into granular short rods and spheroids. [ABSTRACT FROM AUTHOR]