Elucidating microstructural evolution and strengthening mechanisms in nanocrystalline surface induced by surface mechanical attrition treatment of stainless steel

Surface mechanical attrition treatment (SMAT) is a high strain and strain rate severe plastic deformation (SPD) technique for surface nanocrystallization of metals. The aim of this study was to investigate the mechanism of nanocrystallization and strengthening in a medium stacking fault energy 316 L austenitic stainless steel during SMAT. The paramount role of microband and shear band formation in nanocrystallization is outlined, as opposed to deformation twinning previously reported in low SFE austenitic stainless steels. Shear bands undergo dynamic recrystallization and recrystallization twinning to produce ultra-fine grains in contrast to twin-twin intersections in low SFE stainless steel. The ultra-fine grains further sub-divide into smaller cells with initially low misorientation. Nanocrystallization occurs when misorientation between these cells increases with further strain. The additivity of strengthening by dislocation density and grain size is studied. Dislocation density was neglected in previous studies while studying strengthening mechanisms in SMAT processed materials. This study illustrates that dislocation density cannot be ignored as the strengthening mechanism in SMAT process. The grain size and dislocation density both significantly contribute to overall strengthening in SMAT processed microstructure.