Micro-damage behaviors of Al–6Mg alloy impacted by projectiles with velocities of 1–3.2 km/s

Microstructure changes and micro-damage behaviors of a Al–6Mg alloy under the impact of GCr15 steel projectiles have been studied by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that dynamic recrystallization in the material just adjacent to the crater can be induced by the hypervelocity impact. Dislocation cells formed during the impact process act as the nuclei of dynamic recrystallization, and the temperature rising caused by the heat transformed from the adiabatic plastic deformation work offers the temperature condition for the recrystallization. The degree of recrystallization depends on the impact velocity. Three kinds of micro-damages: micro-cracks, microvoids and adiabatic shear bands, in the region around the crater are usually produced by hypervelocity impacts. When the impact velocity is lower, grains within adiabatic shear bands are obviously fragmentated, and when the velocity is higher, whirlpool-like shear bands and wavy ones are formed just near the crater wall and the bottom, respectively. Micro-cracks are formed in front of the shear band, and are observed to propagate along the bands. Microvoids are observed in the region ahead of micro-cracks.