Towards the ultimate strength of iron: spalling through laser shock

The ultimate strength of materials is reached at strain rates approaching the Debye frequency, when the deformation time at the atomic scale approaches the time for atoms to move away from the equilibrium to their extreme separation position (~5.5 × 1013 s−1 for iron). We conducted high-power pulsed laser experiments on single, poly-, and nanocrystalline iron, generating tensile pulses with strain rates approaching the Debye frequency, 106 s−1 – 107 s−1, and nanosecond time durations. We find iron strengths varying between 5 and 10 GPa, a factor of ten higher than the static tensile strength. Ultrafine-grained iron samples exhibit a lower tensile strength, ~4-6 GPa, than single crystal iron, ~10 GPa. MD simulations show that this is due to differences in the initiation sites for voids, primarily at grain boundaries for the nano- and polycrystalline conditions. Sparse runaway voids (~5 μm diameter) and evidence of surface melting are observed for the single crystal iron and are likely due to strain-induced melting when sufficient deformation occurs. The process of separation leading to spalling is modeled by molecular dynamics, and the mechanisms observed in the experimentally recovered specimens are determined: in single crystals voids nucleate at the intersection of twins, while in nanocrystalline specimens grain boundaries are the principal sources of void nucleation. Analytical calculations are applied to the dislocations generated by the emission of shear loops from the void surfaces and the geometrically necessary dislocation densities are found to be consistent with predictions from molecular dynamics calculations.