Microbands and shear-related microstructural phenomena associated with impact craters in 6061-T6 aluminum

Normal incidence impact craters in thick 6061-T6 aluminum targets formed by spherical soda-lime glass projectiles at velocities ranging from 1.7 km s−1 to 5.2 km s−1 were examined in cross-section using light microscopy and transmission electron microscopy. Microbands were observed below the crater wall, especially near the crater bottom, and both their frequency and linear extent (at the crater bottom) increased with increasing impact velocity. The absence of deformation twins and propensity of microbands was shown to be related in part to the T6 treatment which creates a high precipitate and dislocation density which, combined with a relatively small grain size, suppresses twin formation. The shearing of very large precipitates in a narrow zone of severe plastic flow adjacent to the crater wall attests to the extreme solid-state flow and jetting associated with the crater formation. There was no evidence for either dynamic recrystallization or crater melt-related phenomena in the range of impact velocities investigated.