Stella Quinones

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.

The low-velocity-to-hypervelocity penetration transition for impact craters in metal targets

Abstract Projectile/target behavior for 1100 Al/Cu, soda-lime glass/Cu, soda-lime glass/1100 Al, ferritic stainless steel/Cu, and ferritic stainless steel/1100 Al for spherical (3.18 mm diameter) projectiles at impact velocities ranging from 0.8 to ∼6 km s −1 has been examined by light metallography, SEM, and TEM. At a reference velocity of 1 km s −1 , the crater depth/crater diameter ratio ( p / D c ) is observed to be linearly related to bulk density ratios ( ρ p / ρ t ) 1/2 and elastic modulus ratios ( E p / E t )( ρ p / ρ t ) 1/2 , and to vary from about 0.2 to 2.95. The hypervelocity ( u o >5 km s −1 ) threshold value for p / D c is also shown to be linearly related to these functionalities and ranges from p / D c =0.4 for the 1100 Al/Cu system and 0.85 for the ferritic stainless steel/1100 Al system. The residual crater microstructures are all characterized by a zone of dynamic recrystallization at the crater wall (which thickens with impact velocity), and decreasing dislocation density beyond this zone; consistent with residual hardness profiles whose amplitudes decrease with distance from the crater wall. Computer simulations and validation of these simulations utilizing the ranges of experimentally measured crater geometries with impact velocity were developed which fairly accurately represented residual crater shapes and related features. These results also demonstrate the importance of appropriate projectile/target strength ratios in computer simulations; and illustrate the potential for extrapolations to new systems, and for impact velocities well beyond those achievable in the laboratory.

Correlations of Computed Simulations with Residual Hardness Mappings and Microstructural Observations of High Velocity and Hypervelocity Impact Craters in Copper

An AUTODYN 2D (version 3.0), PC-compatible hydrocode utilizing Lagrangian and Eulerian processors (the latter including a fracture criterion) with a Johnson-Cook constitutive relationship has been applied to simulating experimentally developed impact craters in 1.3 cm thick OFHC copper targets for 1100 aluminium spheres (nominally 3.2 mm diameter) impacting at velocities ranging from 1.08 to 6.01 km/s. Good validation was achieved not only for crater dimensions but especially for the simulation of crater shapes and other features - including fractured or particulated crater rims, target spallation at 4.4 and 6.01 km/s and residual stress contours extending from the crater wall; which were related to residual, experimental hardness profiles and crater-related microstructures observed by optical and transmission electron microscopy. This comprehensive validation of 2D hydrocode simulations allowed extrapolations for impact craters well into the hypervelocity regime : 12 and 24 km/s, where dynamic recrystallization was demonstrated to contribute significantly to hypervelocity impact crater formation.

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.

Experimental observations and computer simulations of spherical aluminum-alloy projectiles impacting plane limestone targets

The stress distribution within spherical aluminum-alloy (2024) projectiles impacting plane limestone (calcite) targets was observed by converting residual microhardness maps obtained for cross-sections of recovered projectiles impacting from 0.8 to 1.3 km/s. A maximum residual yield stress zone was observed to migrate toward the rear of the impacting projectiles with increasing impact velocity. The maximum occurred at a normalized depth z′/am ≅ 0.5 (where am is the contact radius); consistent with the theoretical result for elastic impacts. Computer simulations showed good agreement with experiment, and demonstrated that elastic assumptions were valid well into the plastic deformation regime.

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.

Cracking associated with micrometeoroid impact craters in anodized aluminum alloy clamps on LDEF

The Long Duration Exposure Facility (LDEF) is a reusable hollow-cylindrical satellite sustaining a total of 57 different experiments. The 130 sq m of spacecraft surface area included anodized 6061-T6 Al alloy bay frames and clamps for holding experiment trays in the bay areas. Attention is presently given to the micrometeoroid impact crater features observed on two tray clamps recovered from the LDEF leading-edge locations. It is found that even very subtle surface modifications in structural alloy anodizing can influence micrometeoroid impact crater cracking, notable radial cracking due to the ejecta-rim of the impact craters.

Nanotexturing of surfaces to reduce melting point.

This investigation examined the use of nano-patterned structures on Silicon-on-Insulator (SOI) material to reduce the bulk material melting point (1414 C). It has been found that sharp-tipped and other similar structures have a propensity to move to the lower energy states of spherical structures and as a result exhibit lower melting points than the bulk material. Such a reduction of the melting point would offer a number of interesting opportunities for bonding in microsystems packaging applications. Nano patterning process capabilities were developed to create the required structures for the investigation. One of the technical challenges of the project was understanding and creating the specialized conditions required to observe the melting and reshaping phenomena. Through systematic experimentation and review of the literature these conditions were determined and used to conduct phase change experiments. Melting temperatures as low as 1030 C were observed.