Comparative study of shock-wave hardening and substructure evolution of 304L and Hadfield steels irradiated with a nanosecond relativistic high-current electron beam

Abstract

We present the results of a comparative study of the shock-wave hardening regularities and mechanisms revealed for bulk (thickness h = 6 and 9.3 mm) targets made of austenitic 304L stainless steel and Hadfield steel. A high-current relativistic electron beam (45 ns, 1.35 MeV, 34 GW/cm2) produced by the SINUS-7 accelerator was used for generation of a shock wave. It is revealed by 2D-computer simulation for type 304 steel that the direct ablation of the target material leads to generation of shock wave with duration of ∼0.1 μs and amplitude of ∼20 GPa, and the strain rate during its direct propagation and reflection from the free rear surface decreases from ∼2 down to ∼0.4 μs−1. It is found experimentally that in the absence of a rear spall (h = 9.3 mm) the shock-wave loading of both steels leads to formation of three hardened layers: a front layer with a maximum microhardness at a depth of 0.5–1 mm from the bottom of ablation hole, which is in a reasonable agreement with the predictions of the heat-transfer calculations, as well as intermediate and rear-side layers. In case of 304L stainless steel, the depth distributions of microhardness and fraction of twinned grains are consistent with each other, while in the Hadfield steel, the correlation is within the front and intermediate hardened layers only. It is shown by microstructural characterization and analysis of hardening mechanisms that in the case of 304L stainless steel, both front and rear-side hardening are significantly associated with the formation of new intra-phase boundaries by deformation twinning. In the Hadfield steel, unlike the 304L stainless steel, the unusual rear-side hardening is mainly due to increasing the dislocation density under submicrosecond single cycle of compression followed by tension with peak stress of ∼3 GPa.