Interplay between deformation and fracture mechanism in gradient nanostructured austenitic stainless steel

The plastic deformation parameters are crucial for the grain size of gradient nanostructured (GNS) materials, and the multiscale grain structure within the GNS layer has a significant impact on fracture behavior. In this study, the detailed depth-dependent microstructures of GNS310S stainless steel fabricated by surface mechanical rolling treatment (SMRT) were investigated, and the relationship between the geometrically necessary dislocation (GND) density and the strain gradient was established through the local misorientation obtained by electron backscatter diffraction. The critical deformation parameters (strain gradient, shear strain, and strain rate) of SMRT-induced grain refinement at different scales were quantitatively evaluated. The tensile results show the yield strength and tensile strength of GNS310S are improved by 103% and 23.1% respectively compared to CG310S. Additionally, we elucidate the underlying tensile fracture mechanism of GNS310S stainless steel through detailed microstructural analysis. Due to the poor ductility of the near-equiaxed nanograins, cracking occurred on the surface of the SMRT specimen at low strain during the tensile testing. These cracks evolved into competitive shear offsets and multi-cracks with increased strain on the specimen surface, eventually leading to a hybrid fracture mode. In contrast, the subsurface nanotwinned lamellae layer displayed good ductility during tension, which can accommodate the deformation through lamellae widening and forming secondary twinning. This nanotwinned lamellae layer can suppress the inward propagation of surface cracks when the strain was not large. This work provides new insights into the fabricating parameters of the GNS stainless steel, as well as the deformation and fracture mechanisms.