Martensitic transformation dominated tensile plastic deformation of nanograins in a gradient nanostructured 316L stainless steel

We systematically investigated the tensile deformation mechanisms and mechanical properties of a gradient nanograined AISI 316 L stainless steel prepared by means of surface mechanical grinding treatment. The gradient nanograined samples exhibit a good synergy of high strength and remarkable tensile ductility at room temperature. Microstructural observations revealed that martensitic transformation co-operating with grain boundary migration accommodates the tensile plastic deformation of nanograins in the topmost layer of gradient nanograined 316 L SS, which is distinct from the traditional partial dislocation associated deformation twinning in homogeneous coarse-grained counterpart under tension loading. Accompanied by the decrease in grain boundary migration rate, the martensite content increases significantly with increasing tensile strain, reaching ∼50% at a true strain of 50%. The newly formed martensites as strengthening phase not only provide dynamic work hardening, but also effectively suppress the strain localization, resulting in the substantial tensile ductility of the gradient nanograins.