Investigation of Surface Nanostructuring, Mechanical Performance and Deformation Mechanisms of AISI 316L Stainless Steel Treated by Surface Mechanical Impact Treatment

Nanostructured materials exhibit superior properties with respect to their bulk counterpart. Recently, a new processing method for surface nanostructuring of metallic materials called surface mechanical impact treatment (SMIT) was developed. In this study, the surface microstructural features due to the refinement process of AISI 316L stainless steel by means of SMIT and subsequent mechanical performance were investigated. The effects of SMIT processing parameters, i.e. ball size and treatment duration, were studied in terms of microstructural evolutions using X-ray diffraction, transmission electron microscopy, optical microscopy, and field emission scanning electron microscopy analyses, and mechanical properties through hardness and tensile tests. A gradient nanostructured surface layer was successfully formed on the surface of the treated samples. The mean grain size was measured to be ~ 20 nm in the topmost surface layer and increased with increasing depth. Microstructural examinations showed that the twins and their intersections (rhombic blocks) formed in the surface layers. It was found that the mechanical performance of the treated samples is effectively enhanced. The surface hardness of the treated samples increased about 3 times while the yield strength of the samples increased with increasing SMIT time and size of the ball up to 2.5 times. The grain refinement mechanisms, mechanical properties, and fracture behavior were subsequently analyzed and discussed.