Condition-driven mechanism transitions: Superplastic deformation and microstructural response of Inconel 718

This study aims to elucidate the fundamental superplastic deformation mechanisms and microstructural evolution of a fine-grained Inconel 718 sheet, which is crucial for optimizing its superplastic forming process window. Within the range of 910-970 °C and strain rates of 6×10 -4 to 4×10 -3 s -1 , the alloy exhibits superior superplastic elongation of up to 750%, strain rate sensitivity ( m ) values of 0.33-0.44, and a true activation energy close to that of lattice self-diffusion. The flow stress depends on the dynamic competition between work hardening and flow softening. Work hardening originates from dislocation multiplication, while flow softening mainly arises from dynamic recrystallization and cavity nucleation. The evolution of the δ phase (Ni 3 Nb) has a dual influence on flow stress. It initially acts as a strengthener through pinning effects, but later produces a softening effect as its dissolution weakens pinning and increases grain boundary mobility. Moreover, grain boundary sliding (GBS) serves as the dominant strain-producing mechanism, which is accommodated by intragranular dislocation activity, with the specific mode being determined by the obstacle grain size. Dislocation glide predominates for accommodating GBS in the case of small obstacle grains, while sub-grain boundary formation becomes dominant for large ones. A decrease in temperature or an increase in strain rate promotes a transition in the accommodation mechanism from dislocation glide to sub-grain boundary formation. This work provides a profound understanding of the relationship between deformation conditions and microstructural evolution, ultimately revealing the condition-dependent transitions in deformation mechanisms governing superplasticity of Inconel 718.