Abstract

Herein, gradient nanostructures (GNs) are created in 300M ultrahigh-strength (UHS) steel by laser shock processing (LSP). Microstructure evolution and properties of GNs subjected to LSP with different pulse energies are thoroughly characterized on 3D profiler, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray stress analyzer, nanoindenter, and tensile tester. Results show successful creations of GNs in 300M steel after LSP treatments. With the increase in pulse energy, the size of the surface layer is refined from 15 nm (3 J) to 10 nm (7 J), and the corresponding grains are amorphized to some extent. Meanwhile, many substructure defects such as dislocation tangles and deformation twins (DTs) are noted in the subsurface. The dislocation density and the number of DTs increase with the pulse energy. Further, the high compressive residual stress is introduced to the 300M steel surface after LSP, and the corresponding hardness is improved substantially. The compressive residual stress, depth of the affected layer, and the hardness rise significantly with the pulse energy. Apart from improvements in strength and plasticity, the fracture morphology is changed from a typical ductile fracture to quasicleavage and ductile mixed fracture.