Characterization of mechanical behavior of nanocrystalline layer induced by SMAT using micro-pillar compression technique coupled with finite element analysis

Abstract : Micro-pillar compression tests were used to study the mechanical behavior of a stainless steel treated by SMAT (Surface Mechanical Attrition Treatment), particularly the induced nanocrystalline surface layer. Micro-pillars were first machined using a Focused Ion Beam (FIB) on the cross-section of the SMATed specimen. They were then deformed with a flat head mounted on a nanoindenter to obtain loading-displacement curves for various micro-pillars machined at different distances from the SMATed surface. Finite element analysis was used to study the effect of experimental parameters including taper angle (the angle between the tangent of wall and the axis of pillar), aspect ratio (the ratio of height and diameter of the pillar), and misalignment between the pillar axis and the compression direction. Based on the results of finite element analysis, the constitutive behavior in the form of stress-strain curve was identified for different layers from the SMATed surface including nanocrystalline layer. According to the obtained stress-strain curves, the mechanical strength of the stainless steel is significantly improved after SMAT. Deformed micro-pillars after compression tests were examined using FEG SEM to analyze deformation mechanisms. Observation results show that the deformation mechanisms of the material appear to be different according to the distance from the treated surface.