Gradient Microstructure Design in Stainless Steel: A Strategy for Uniting Strength-Ductility Synergy and Corrosion Resistance.

Martensite transformation and grain refinement can make austenitic stainless steel stronger, but this comes at a dramatic loss of both ductility and corrosion resistance. Here we report a novel gradient structure in 301 stainless steel sheets, which enables an unprecedented combination of high strength, improved ductility and good corrosion resistance. After producing inter-layer microstructure gradient by surface mechanical attrition treatment, the sheet was annealed at high temperature for a short duration, during which partial reverse transformation occurred to form recrystallized austenitic nano-grains in the surface layer, i.e., introducing extra intra-layer heterogeneity. Such 3D microstructure heterogeneity activates inter-layer and inter-phase interactions during deformation, thereby producing back stress for high yield strength and hetero-deformation induced (HDI) hardening for high ductility. Importantly, the recrystallized austenitic nano-grains significantly ameliorates the corrosion resistance. These findings suggest an effective route for evading the strength-ductility and strength-corrosion tradeoffs in stainless steels simultaneously.