In situ SEM analysis for deformation mechanism of micro/nanostructured 304 stainless steel with high strength and good plasticity

Bulk micro/nanostructured 304 austenitic stainless-steel plates with bimodal grain size distributions were prepared by Alumina Thermite Reaction at various temperatures and extents of rolling deformation. Rolling cogging of the sheet was performed with a rolling reduction of 40% at 1000[Formula: see text]C followed by rolling reduction of 80% at 700[Formula: see text]C. The strength and plasticity of the resulting micro/nanostructured 304 stainless steels with bimodal grain size distribution achieved the best matching, with tensile strength, yield strength, and elongation of 1410 MPa, 723 MPa and 15.3%, respectively. To better understand the deformation mechanism of this micro/nanostructured stainless steel sample, an in situ scanning electron microscopy technique was adopted. The crack initiation, propagation, and fracture were dynamically observed and recorded during the tensile deformation. Our results revealed that a stress concentration near the preset notch served as the initiation source and that microcracks were formed in the grain boundaries between micro- and nano-grains and then spread to the microcrystalline region until passing through the microcrystalline region or until passivation occurred in the microcrystalline region. The microcracks not only caused serious damage to the specimen but also generated back stress, which could lead to hardening of material, thereby enhancing the global ductility. Finally, the mechanism responsible for the enhanced plasticity and strength of the micro/nanostructured 304 stainless steel with a bimodal grain size distribution was analyzed and combined with the fracture morphology.