Characterization of the gradient properties of a 316L steel generated by SMAT using micro-pillar compression tests

Abstract : Surface Mechanical Attrition Treatment (SMAT) is one of the most promising mechanical surface treatment techniques. It is based on the repetitive multi-directional impacts between the surface of a material and spherical shot boosted by an ultrasonic generator. For a mechanical part treated by SMAT, a gradient microstructure can be generated from the treated surface to the interior region, with the presence of a top surface nano-sized grains layer. This nanostructured layer, even if it is generally thin, can have significant effect on the performance of materials, since engineering components are mostly loaded on their surface. The nanostructured layer obtained by SMAT is so thin that it is difficult to prepare samples to perform uniaxial mechanical tests. In this work, micro-pillar compression technique is used to investigate the mechanical properties of a gradient microstructure generated by SMAT for a biomedical grade 316L stainless steel under both monotonic and cyclic loadings. A particular attention was paid to the mechanical property of the 5 microns thick nanostructured layer. The results show that the strength for the micro-pillars located in the nanostructured layer is much higher than that in the non-affected SMAT region. In addition, micro-pillars located at different distances from the treated surface show different cyclic behaviors and different deformation mechanisms. The effectiveness of this micro-pillar compression technique coupled with EBSD observation is demonstrated.