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

Kwon, H.*; Lee, J. H.*; Zargaran, A.*; Harjo, S.   ; Gong, W.   ; Wang, J.*; Gu, G. H.*; Lee, B.-J.*; Bae, J. W.*; Kim, H. S.*

In this work, we harness a hierarchical microstructure to simultaneously tailor strengthening and deformation mechanisms in a CoCrFeNiMo (at%) ferrous medium-entropy alloy (MEA). A simple thermomechanical process (cold rolling and 90 s annealing) produces ultrafine recrystallized grains, non-recrystallized grains with substructures, and intragranular nanoprecipitates. This structure, with high dislocation density and fine grains, yields a high strength of 1.60 GPa but can risk premature fracture. To overcome this, Lders deformation, enabled by ultrafine grain boundaries and stress-induced martensitic transformation at pre-existing nucleation sites, is employed. Stable Lders band propagation delays strain hardening and enables large uniform ductility. As a result, a tensile strength of 1.84 GPa and uniform elongation of 20% are achieved, matching the best tensile properties among reported multi-principal element alloys.