The deformation behavior of the gradient nanostructured microstructure of low-carbon steel under the tensile stress

A microstructure within the gradient structure was successfully prepared using the ultrasonic impact treatment (UIT), achieving good strength to toughness synergy. Specifically, the diffraction peak on the surface of the gradient microstructure had a certain broadening effect. Furthermore, the grain size in the gradient deformation layer with a thickness of about ~150 mu m was coarsened from about 100 nm in the top layer (~10 mu m) to about 10 mu m in the matrix layer (~250 mu m). Meanwhile, the tensile strength and yield strength of the sample after the UIT were increased by 12.1% and 60.0% respectively. This result was mainly attributed to the fact that the gradient structure could inhibit strain localization in tensile deformation, improving the mechanical properties through the synergistic effect of surface strengthening and bulk toughness. Digital image correlation (DIC) technology was used to observe the displacement characteristics of the sample during the tensile deformation process and the strain change near the fracture. The results showed that the gradient layer could disperse the stress and inhibit the strain localization. The geometrically necessary dislocation (GND) density of the GNS sample after tensile fracture was lower than that of the original sample.