Ultrahigh tensile strength achieved in a lightweight medium Mn steel via prominent work hardening

We develop a new ultrastrong medium Mn steel with a density reduced to 7.39 g cm^(-3).It has a novel tri-phase microstructure comprising a hierarchical martensitic matrix(α’),dispersed ultra-fine-retained austenite grains(γ),and both compressed and{200}orientedδ-ferrite lamellas,the latter’s formation is due to the alloying of high Al and Si contents for reducing density.As a result,both ultrahigh ultimate tensile strength of 2.1 GPa and good ductility of 16%are achieved after an extraordinary plastic strain hardening increment of about 1.4 GPa.The in-situ synchrotron-based high-energy(HE)X-ray diffraction(XRD)examinations during the tensile deformation revealed that the initial presence of residual com-pressive stress inδ-ferrite could increase the stress required to initiate the plastic tensile deformation of the specimen,leading to the isolatedδ-ferrite lamellas mostly deformed elastically to coordinate the plastic deformation of the martensitic matrix during yielding.During the plastic deformation,the gradual release of residual compressive stress inδandα’,the dislocation multiplication in all the three phases and the successiveγ-to-α’transformation all contribute to such a prominent work hardening increment.This study facilitates the development of novel strategies for fabricating ultrastrong but light steels.