Achieving exceptional strength-ductility synergy in the GH4698 nickel-based superalloy via heterogeneous grains and L12-γ′ nanoprecipitates with bimodal size distribution

Structural materials with the heterogeneous microstructures offer unique combinations of strength and ductility. In this study, a nickel-based superalloy with a heterogeneous microstructure consisting of retained deformed and recrystallized grains, and the L1₂-γ′ nanoprecipitates with a bimodal size distribution, is synthesized using cryogenic rolling at 77 K, followed by annealing and aging. The larger precipitates are primarily located in the retained deformed grains, while the smaller ones are uniformly distributed throughout both the retained deformed and recrystallized grains. Benefiting from the heterogeneous structure, the alloy exhibits an exceptional strength-ductility synergy, with an ultimate tensile strength of 1612 MPa and uniform elongation of 14.2 %. During deformation, the dislocations interact with L1₂-γ′ nanoprecipitates, stacking faults, and the Lomer-Cottrell locks, which contributes to the high strength-ductility synergy. The contributions of intrinsic strengthening, grain boundary strengthening, heterogeneous deformation induced (HDI) hardening, and precipitation strengthening were quantitatively assessed. Amongst these, HDI hardening and precipitation strengthening were identified as the primary contributors to the high strength. The heterogeneous deformation between the retained deformed grains and recrystallized grains plays a significant role in enhancing the ductility. Furthermore, the evolution of the Taylor factor and textures before and after deformation was investigated. Our findings offer a strategy to design heterogeneous grain structures composing L1₂-γ′ nanoprecipitates, which achieve high strength-ductility synergy in the nickel-based superalloys.