Lüders band-assisted high uniform ductility in ultrastrong ferrous medium-entropy alloy via hierarchical microstructure

In this work, we harness a hierarchical microstructure to tailor both the strengthening and deformation mechanisms of Co21Cr12.5Fe55Ni4Mo7.5 (at %) ferrous medium-entropy alloy (MEA) simultaneously. A simple thermomechanical processing (cold rolling and 90 s of annealing) creates a hierarchical microstructure composed of ultrafine recrystallized grains, non-recrystallized grains with rolling-driven substructures, and intragranular nanoprecipitates. The hierarchical microstructure with the high density of dislocations and ultrafine recrystallized grains leads to a high yield strength of ∼1.60 GPa, but it is well-known that the same features can make materials vulnerable to premature fracture. To solve this issue, Lüders deformation, which was induced by the ultrafine grain boundaries and stress-induced martensitic transformation facilitated by pre-existing martensite nucleation sites, was harnessed: stable propagation of the Lüders band delays massive strain hardening by regulating strain-induced martensitic deformation that ensues and enables a large uniform ductility. Resultantly, tensile strength of ∼1.84 GPa and uniform elongation of ∼20 % are achieved, on par with the finest tensile properties among multi-principal element alloys ever reported. Our results point to a paradigm to achieve a large uniform ductility via harnessing the Lüders deformation without compromising strength, based on the hierarchical microstructure.