Investigating microstructure dynamics and strain rate sensitivity in gradient nanostructured AISI 304 L stainless steel: TEM and nanoindentation insights

In recent years, gradient nanostructured (GNS) materials have gained significant attention due to their superior strength-ductility balance and enhanced functional properties compared to their coarse-grained counterparts. This research examines the microstructure evolution and nanomechanical responses of GNS AISI 304 L austenitic stainless steel using transmission electron microscopy (TEM) and nanoindentation techniques. Through surface mechanical attrition treatment (SMAT), a gradient nanostructured layer with ultrafine grains (∼15 nm) and nanoscale martensite (up to ∼40 %) within the austenite matrix has been successfully created on the steel’s surface. This treated surface exhibits a hardness of ∼6.7 GPa, nearly double the original value. The GNS layer demonstrates single-step (γ → α’) and two-step (γ → ε → α’) martensitic transformations, deformation twinning (γ -twin), a decrease in the density of deformation bands, compressive residual stress, lattice strain, and martensite content, along with an increase in grain size. Strain rate sensitivity (SRS) increases with austenitic grain size and inversely correlates with martensite proportion as depth increases in the GNS layer. A significant amount of ultrafine martensite is primarily responsible for the limited SRS in the topmost layer.