Deformation behavior and formability of gradient nano-grained AISI 304 stainless steel processed by ultrasonic impact treatment

The deformation behavior and formability of gradient nano-grained (GNG) AISI 304 stainless steel in uniaxial and biaxial states were investigated by means of tensile test and small punch test (SPT). The GNG top layer was fabricated on coarse grains (CG) AISI 304 by ultrasonic impact treatment. The results showed that the CG substrate could effectively suppress the strain localization of NC in GNG layer, and an approximate linear relationship existed between the thickness of substrate (h) and uniform true strain before necking (ε_unif). Grain growth of NC was observed at the stress state with high Stress triaxiality T, which led to better ductility of GNG/CG 304 in SPT, as well as similar true strain after the onset of necking (ε_neck) compared with coarse 304 in tensile test. Ei-values of GNG/CG 304 with different structures were nearly the same at different punch speeds, and good formability of GNG/CG 304 was demonstrated. However, punch speed and microstructure needed to be optimized to avoid much lost of membrane strain region in biaxial stress state.