Multi-scale damage mechanism of hierarchically structured high-strength martensitic steels under shock loading

The multi-scale damage behaviors and underlying mechanisms of hierarchically structured high-strength martensitic steels are investigated via plate-impact recovery experiments, postmortem characterizations. Plate-impact experiments were conducted at compressive peak stresses ranging from approximately 5.5 to 11.0 GPa, and the tensile strain rates are ∼105 s−1. Significant improvement in the spall strength of the heat-treated martensitic steel is achieved (5.4 GPa), about 80% higher compared to the as-received bainite steel. The spall failure of the investigated martensitic steel exhibited a mixed-mode combination of brittle (micro-cracks) and ductile (voids) characteristics, known as quasi-cleavage spallation. As the impact velocity increased, voids nucleated successively at different types of boundaries as follows: boundaries between enriched zone and depleted zones, prior austenite grain boundaries, packet boundaries, block boundaries and lath boundaries. Shear cracking was found to be an important mechanism to accommodate the severe plastic deformation during spallation, giving the strict geometric constraints due to void nucleation occurring tend at grain boundaries. Additionally, molecular dynamics simulations considering realistic hierarchical structure of Martensitic steel have been developed to validate the experimental results. These findings and simulation methods contribute a better understanding of the spall behavior of martensitic steels, as well as guidance for future design of high-strength martensitic steels with exceptional reliability and safety under extreme conditions.