Characterization of gradient properties generated by SMAT for a biomedical grade 316L stainless steel

Highlights: • Gradient microstructure including nanostructured layer is generated by SMAT. • Nanoindentation is performed in the cross-section of the SMATed stainless steel. • Grain size refinement is the main factor that results in high measured hardness. • Pile-up is observed in the SMAT-affected region as a result of low strain hardening. • The contact area is corrected due to pile-up in order to recalculate the hardness. -- Abstract: Gradient microstructure generated by surface mechanical attrition treatment (SMAT) has beneficial effects for treated mechanical components, such as improved fatigue behaviour and tensile yield stress. Little effort has been devoted to characterize the local properties of each region in the gradient microstructure generated by SMAT. In this paper, the gradient microstructure was first highlighted by microscopic observation using Electron Back-Scatter Diffraction (EBSD). Nanoindentation was used to characterize the loading-unloading behaviour at different depths beneath the treated surface. All the indentation patterns were observed using Atomic Force Microscopy (AFM) and a pile-up phenomenon was detected. The nanoindentation results were analysed taking into account this pile-up effect on the dimension of the imprints. The results reveal that grain refinement plays a dominant role on the hardness values, while the effect of residual stress is less significant. In addition, correcting the indentation contact area appears to be necessary in the near surface region which is strongly affected by SMAT.