J. M. Rivas

Hypervelocity impact cratering: Microstructural characterization

Microstructures below impact craters in metal targets, specifically aluminum 1100 and OFHC copper, exhibit evolutionary features characteristic of variations in deformation extending from the crater wall into the target. Regions proximate to the crater wall where adiabatic temperature effects are prominent, exhibit recovery and recrystallization in both aluminum and copper. Dislocation cells, with cell sizes increasing as the deformation decreases, are observed. In copper, a wide zone exhibiting microband clusters with unidirectional, elongated, cell-like microstructures coincident with traces of {l_brace}111{r_brace} was observed. Similar features have not been observed in other circumstances involving deformation, particularly high-strain-rate deformation in copper, and are probably related to shock-wave phenomena which is a prominent part of the impact crater development.

Novel deformation processes and microstructures involving ballistic penetrator formation and hypervelocity impact and penetration phenomena

Abstract Light metallography and transmission electron microscopy techniques affording unique observations of microstructural issues in connection with a related set of novel, high-strainrate deformation processes provide some fundamental insight into the following areas: shock-wave-induced twinning, explosive welding, shaped charge development, explosively-formed penetrator phenomena, hypervelocity impact cratering in metal targets, and long, dense rod penetration/perforation of thick metal targets. Although shock wave phenomena are precursors in all these processes, deformation twins are rarely observed in the residual, process microstructures. In the case of hypervelocity impact craters, no deformation twins are observed in the crater-related target microstructures. Microbands that appear to be related to twins are observed. Melt-related phenomena are observed only in the explosive weld-wave interfaces. Jetting phenomena related to shaped charges and crater rim formation are dominated by dynamic recrystallization, which provides a mechanism for extreme plastic flow in the solid state. Differences observed between rod penetration of rolled homogeneous armor and Ti-alloy thick targets manifest themselves in distinct microstructural differences that also do not include melt phenomena.

Microstructural aspects of hypervelocity impact cratering and jetting in copper

Light and transmission electron microscopy techniques have been applied in observations of hypervelocity impact craters in two different copper targets: a 38 μm grain size mill-processed target, and a 763 μm grain size annealed target, the smaller grained target being impacted with a 1100 aluminium sphere and the larger grained target being impacted with a soda-lime glass sphere, at velocities near 6 km s−1. Both target craters exhibited dynamic recrystallization near the crater wall. The jetting associated with these two craters was very different. Considerably more plastic flow and a larger rim characterized the larger grained target. No significant melt-related phenomena were observed either near the crater wall or in the jetted rim for either crater. Consequently, the principal features of crater formation involve extreme plastic flow in the solid state. Microbands were observed to occur profusely in a zone below the smaller grained mill-processed target crater while more profuse and extremely long, unidirectional bundles of microbands (which were coincident with traces of {1 1 1} planes) occurred below the annealed larger grained target crater. These observations attest to the dominant and unique role played by deformation microbands in cratering in copper, because essentially no deformation twins were observed in either target.

Measuring hypervelocity impact velocity from micrometeoroid crater geometry

Summary Guided by half-space computer simulations showing hypervelocity impact crater formation for an iron particle impacting an aluminum target and characteristic crater geometry changes with impact velocity over the range 8–40 km s −1 , we examined normal surface crater views and cross-sectional views through craters (>0.5 mm diameter) from samples retrieved from the NASA LDEF satellite and examined in the scanning electron microscope (SEM). While geometrical features suggested in the computer simulations were indeed observed for micrometeoroid craters in 6061-T6 aluminum targets and 303 stainless steel targets, there was no consistent estimate for impact velocities in any of the experimental samples, and velocity estimates based on measuring ratios of ejecta width/crater diameter and ejecta height/crater depth as well as ejecta height/crater diameter varied from 8 to 42 km s −1 ; over the same range simulated. These results point to the need to create reference data from actual hypervelocity impact experiments in the laboratory, and systematic observation of residual crater geometries in the SEM. These experiments also demonstrate the uncertainty in assuming a fixed impact velocity for all impact craters in space materials as well as an apparent futility in attempting to correlate impacting particle velocity with post-mortem characteristics of a given crater.

Direct observations and comparison of crater cross-section microstructures in copper targets for aluminium projectiles impacting at 1.4 and 6.7 km s−1

Light and transmission electron microscopy observations of impact crater-related microstructures in copper targets have revealed dramatic differences in the extent and type of microstructures. For a crater formed by a 6.4 mm diameter aluminium (1100) spherical projectile impacting at 1.4 km s−1, a narrow (∼20 μm) recrystallized zone extended axially outward from the crater wall, with dislocation cells which increased in size extending from this zone. By comparison, a crater formed by a 3.2 mm diameter aluminium (1100) spherical projectile impacting at 6.7 km s−1 exhibited a recrystallization zone extending more than 200 μm axially from the crater wall, a connecting zone of increasingly dense microbands, having an axial width of about 2000 μm. This zone converged upon a region of dislocation cells which increased in size away from the crater wall. These observations highlight important microstructural differences in cratered metal targets in the hypervelocity impact regime in contrast to the lower-velocity regimes where shock-wave and related ultra-high-strain-rate effects are unimportant.

Electron microscope observations of impact crater debris amongst contaminating participates on materials surfaces exposed in space in low-Earth orbit

Debris particles extracted from a small sampling region on the leading edge of the Long Duration Exposure Facility (LDEF) spacecraft have been examined by analytical transmission electron microscopy and the elemental frequency observed by energy-dispersive X-ray spectrometry and compared with upper atmosphere (Earth) particle elemental frequency and the average elemental compositions of interplanetary dust particles. A much broader elemental distribution was observed for the exposed spacecraft surface debris milieu. Numerous metal microfragment analyses, particularly aluminium and stainless steel, were compared with scanning electron microscope observations of impact crater features, and the corresponding elemental spectra on selected LDEF aluminium tray clamps and stainless steel bolts. The compositions and melt features for these impact craters and ejecta have been shown to be consistent with microcrystalline debris fragments in the case of aluminium, and these observations suggest an ever changing debris milieu on exposed surfaces for space craft and space system materials.