Microstructural evolution associated with hypervelocity impact crater formation in metallic targets

This investigation is a fundamental study of the microstructural evolution associated with hypervelocity impact crater formation in metallic targets and ballistic rod penetration of thick metallic plates. Light and transmission electron microscopy techniques have revealed significant differences in the type and extent of microstructures observed in OFHC copper and aluminum alloy targets impacted at different velocities. Observations of the area surrounding the crater indicate several regions of different microstructural features. These features have a direct correlation with the applied pressure, strain and strain rate in the target material, and are also complicated by the complex geometry of the deformation process. In most of the targets analyzed, there is a narrow (dynamic) recrystallized region, adjacent to the crater wall (or channel wall, for the complete penetration experiments). This region is characterized by small grains which are an indication of plastic deformation enhanced by adiabatic heating in that zone. There is a transition region, mainly characterized by distorted grain structures and dislocation cells (within those grains) beyond the recrystallized zone. The dislocation cell diameter increases as the distance from the crater (or channel) wall increases. Below this transition region, extensive areas of microband clusters have been observed in OFHC copper targets. Upon impact, a pseudo-spherical shock wave propagates into the target material which induces localized deformation in specific crystallographic orientations. The observed microbands have been attributed to this shock wave. Moreover, the microbands have been observed to be coincident with the traces of $\{111\}$. Also, the density of microband clusters changes along the central axis of the target below the crater. The distribution of microbands coincides also with microhardness values obtained from these targets. Microhardness increases in the region where the density of microbands is maximum. Similar microbands have been observed by previous researchers in explosively deformed metals but this is the first time in which such microstructures are observed in connection with hypervelocity impact cratering and ballistic rod penetration in copper. There are also extensive reports in the literature concerning deformation twinning under shock (explosively driven) deformation. However, no significant deformation twins were observed in any of the targets analyzed. This investigation has provided an insight into the microband formation mechanism by conducting a detailed transmission electron microscope analysis of microbands, associated with impact loading, for the first time. Microbands appear to form by a shearing mechanism which activates the primary slip systems in conjunction with a stress-screening process which creates volumes essentially free of dislocations about 0.1 $\mu$m wide. In a more general scenario, this analysis has contributed to the general knowledge of the behavior of metals subjected to impact loading. In this regard, it may be important to use microstructural zone changes as a function of impact velocity as a means to extrapolate the potential effects on the microstructures of materials exposed to a wider range of impact velocities than those achieved by laboratory guns