Investigation on the Intrinsic Plasticity of Gradient Nano-Grained AISI 304 Stainless Steel Treated by Ultrasonic Impact Treatment

The gradient nano-grained (GNG) structure was formed on the top layer of AISI 304 with coarse grains (CG) by ultrasonic impact treatment. The impact velocity was 5 m/s and the coverage was changed from 3000% to 15000% to obtain different microstructure GNG /CG materials. The tensile test and small punch test were also performed to investigate the uniaxial and biaxial deformation behaviors of the GNG / CG materials. The results showed that the yield strength σ_s was enhanced in both uniaxial and biaxial deformation conditions. But the tensile strength σ_b was not nearly changed. The tensile fracture appeared to be dimple. The size of the dimples changed with gradient from the treated surface to the matrix. The strain localization was effectively suppressed in both conditions. AISI 304 of GNG / CG structure was elastically homogeneous but plastically gradient which offered unusual mechanical responses. So the GNG / CG 304 showed intrinsic plasticity like coarse grain one.