An outstanding synergy of high strength and ductility/toughness by combining gradient dislocation structure and DIM transformation

Constructing heterostructures is a feasible approach to achieving high strength and ductility/toughness in metals. Here, various gradient dislocation structures (GDS) with different dislocation density distributions were engi-neered in commercial 304 stainless steel by cyclic-torsion (CT) treatments. With increasing the twist angle of CT treatments, the evident enhancement of strength is along with a slow reduction of ductility following a positive deviation from the inverse linear rule, showing an exceptional strength-ductility synergy. This condition is validated by the simultaneous enhancement of yield strength and static toughness. The enhanced strength was attributed to the introduction of gradient distributed dislocations and the hetero-deformation induced (HDI) stress (300-500 MPa). The retention of high ductility and toughness was provided by the continuous work -hardening from the core to the surface, resulting from the HDI work-hardening and the promoted deforma-tion induced martensite (DIM) transformation (gamma -> alpha ') and dynamical strain partitioning effect by the gradient dislocation structure. Moreover, the detwinning mechanism rather than twinning was active for the GDS samples in the tension and for the coarse-grained sample in both CT and tension, and the effect of detwinning on the mechanical properties was also discussed.