Towards strength-ductility synergy in a medium Mn steel through developing heterogeneous dual-morphology microstructure

Strong and ductile steels are highly desired for reliable yet lightweight applications. The present work designed a heterogeneous medium Mn steel with lamellar and ultra-fine granular dual-morphology microstructure. The dual-morphology steel developed by warm rolling exhibits superior strength-ductility synergy up to 65 GPa%. Transformation-induced plasticity (TRIP) effect is priorly triggered in granular austenite, which improves flow stress and further promotes TRIP effect in ultra-thin austenite lamellae. This contributes to a sufficient TRIP effect prevails at the early stage of deformation and continuously reaches saturation at a true strain of 0.2. The hetero-deformation-induced (HDI) strengthening dominates the subsequent strain hardening. The significant differences between domains with distinct morphology, crystal structure and dislocation density produce strong strain partitioning, giving rise to efficient HDI hardening. The shift of strain localization induced by the TRIPmartensite phases with high dislocation density further motivates additional HDI stress at high strain levels. Delamination toughening additionally postpones abrupt fracture and improves ductility. The activation of multiple strengthening and toughening mechanisms as well as their synergetic effect facilitate an ideal overall strain-hardening ability and prominent property combinations. The dual-morphology design strategy provides new insight into developing strong and ductile metallic materials at economic costs.