Simultaneous enhancement of stress- and strain-controlled fatigue properties in 316L stainless steel with gradient nanostructure

A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at -280 degrees C. The mean grain size is similar to 45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.