Mechanical behavior and microstructure evolution of deep gradient structural steel under dynamic impact

The gradient structure has superior strength-ductility synergy due to its unique design strategy. This synergistic effect is mainly attributed to the heterogeneous deformation induced (HDI) strengthening caused by plastic strain gradient. The quasi-static mechanical behavior of gradient structures has been extensively revealed, but research on their mechanical property under dynamic conditions is almost blank, and the mechanical behavior of gradient structures under dynamic impact is not clear, which limits the application scope of gradient structures. Therefore, this study used the cyclic torsion method to prepare deep gradient structures on low carbon steel and the compression tests were conducted under a wide strain rate range (10^-4-10³ /s). The results indicate that the gradient structure significantly improves the yield strength of low carbon steel. At a strain rate of 0.001 /s, the yield strengths of GS1 and GS2 samples were 407.5 and 483.6 MPa, respectively. Compared to the initial state (203.2 MPa), the yield strengths increased by 100.5% and 137.9%, respectively. Moreover, as the strain rate increases, the yield strength also gradually increases, exhibiting significant sensitivity to positive strain rate. Under dynamic conditions, the strain rate sensitivity of GS1 and GS2 samples increases sharply, reaching 0.129 and 0.1097, respectively, which is almost an order of magnitude higher than quasi-static conditions. In addition, the degree of gradient will have an impact on the deformation mechanism: samples with a higher degree of gradient mainly consume the work hardening ability of the central soft domain during the deformation process, while the hardening effect of the edge region is weakened. Under dynamic conditions, due to the adiabatic temperature rise effect, the dislocation density of the edge hard domain is actually reduced. This study deeply reveals the mechanical response and deformation mechanism related to strain rate in heterogeneous regions of gradient structures. On the one hand, it provides reference for the design of gradient structures, and on the other hand, it provides theoretical value for the future application of gradient structures in extreme service environments such as dynamic impact.