Dual gradient structure enables simultaneous enhancement of strength and ductility

Improving the strength of austenitic stainless steels without sacrificing ductility is crucial for broadening their engineering applications. Here, a dual-gradient structured 304 stainless steel (304 SS) was produced via cyclic twisting and ultrasonic nanocrystalline surface modification. This tailored structure enabled an excellent combination of yield strength (∼680 MPa) and uniform ductility (∼24%). The enhanced strength originated from synergistic contributions of grain refinement, α' -martensite formation, increased dislocation density and hetero-deformation induced (HDI) strengthening. Meanwhile, the retained ductility was primarily attributed to the strain-induced martensitic transformation (i.e., TRIP effect) at the surface zones and HDI work hardening due to the pile-up of geometrically necessary dislocations (GNDs) at the core and middle zones.