Initial-microstructure-dependent transition of strain accommodation mechanisms in a γ′-strengthened superalloy during superplastic deformation

Achieving superplastic deformation in highly alloyed γ′-strengthened superalloys is essential for improving hot workability. This work investigates how initial microstructures with different grain sizes and γ′ precipitate characteristics affect strain accommodation mechanisms in a GH4151 superalloy during superplastic deformation at 1100°C. Three initial microstructures were prepared: ultrafine-grained (UFG, 3.9 μm), fine-grained (FG, 13.3 μm), and coarse-grained (CG, 71.9 μm). The UFG sample exhibited the best superplastic response, with a strain-rate sensitivity of approximately 0.40 and an elongation of about 510% at 10⁻³ s⁻¹. Across the three initial microstructures, the flow stress increased from the UFG to CG conditions, whereas ductility decreased. Microstructural analyses reveal a clear transition in the dominant strain accommodation mechanism. In the UFG sample, grain boundary sliding (GBS) dominates, accompanied by local intragranular accommodation through γ′ shearing, stacking faults and twinning. In the FG sample, continuous dynamic recrystallization (CDRX)-assisted strain redistribution becomes prevalent. In the CG sample, intragranular dislocation slip prevails, leading to 111 texture formation and intergranular cracking. These results demonstrate that the coupled variations in grain size and γ′ precipitate characteristics influence the transition of strain accommodation mechanisms and the resulting superplastic deformability. This work provides a mechanistic basis for microstructural design to improve the hot workability of γ′-strengthened superalloys.