C. S. Niou

Shock-induced deformation twinning in tantalum

Abstract Shock-wave deformation of tantalum to a pressure of 45 GPa and duration of 1.8 μs generates profuse twinning. The post-shock mechanical response is significantly affected, with shock hardening exceeding the expected hardening due to the transient shock strain ϵ s = ( 4 3 ) ln ( V V 0 ) ; this enhanced hardening, and other alterations in response, are attributed to the barriers presented to plastic deformation by the deformation twins. A constitutive model is proposed that predicts the threshold shock stress for mechanical twinning; it is based on the application of the Swegle-Grady relationship between shock stress and strain rate to constitutive equations describing the critical stress for slip and twinning. This constitutive model incorporates grain-size effects and predicts a threshold twinning stress that is a function of temperature and grain size; predictions of the model are in qualitative agreement with experimental results.

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.

Comparison of jetting-related microstructures associated with hypervelocity impact crater formation in copper targets and copper shaped charges

Abstract Shaped charge jet and slug formation is characterized prominently by dynamic recrystallization which may occur in deformation-recrystallization cycles, providing a mechanism for extreme plastic flow in jetting. There was no evidence for melting or melt-related phenomena. Hypervelocity impact crater development is also dominated by dynamic recrystallization in a narrow flow zone where target material is jetted into the crater rim. The crater rim can particulate by velocity gradients along the jetting rim just like the shaped charge jet. Like the shaped charge, there was no significant melt phenomenon associated with the cratering process, and extreme plastic, high-strain-rate flow occurs in the solid state. Microbands are created in a zone removed from the crater wall in copper targets in response to hypervelocity impact which, like deformation twins, are coincident with the trace of primary 111 planes. Their density and extent increase with both impact velocity and grain size. Neither microbands nor deformation twins are observed in recovered shaped charge slug and jet fragments.

Dynamic recrystallization in detonating tantalum shaped charges: a mechanism for extreme plastic deformation

Abstract In a systematic examination and comparison of two different tantalum starting liner cones and corresponding recovered slugs and jet fragments for detonated shaped charges, using light and transmission electron microscopy observations, the slug and jet grain sizes were reduced from the liners by nearly a factor of 100. The slug and jet cross sections exhibited different concentric zones of varying grain and substructures, and these also varied with a difference in the starting liner cone grain structure. These observations provide unambiguous evidence for dynamic recrystallization as a prominent mechanism for deformation in the detonating shaped charge.

Characterization and comparison of microstructures in the shaped-charge regime: copper and tantalum

Abstract Light microscopy scanning electron microscopy, and transmission electron microscopy techniques were employed, along with a novel technique for building up small, recovered jet fragments using electrodeposition of copper, to examine specific segments of fabricated shaped charge liner cones and corresponding, residual jet fragments. Oxygen-free electronic copper and tantalum shaped charged regimes (linerr cones and recovered jet fragments) were compared, and a reduction in the average grain size of recovered jet fragments as compared to the starting liner cones was a consistent observation. The average grain sizes for all cones was 35μm, and the maximum grain reduction occurred for an annealed, equiaxed tantalum cone, which resulted in a residual jet fragment grain size between 1 and 5μm. This is indicative of dynamic recrystallization during jet elongation and microstructure evolution.

Exfoliation and related microstructures in 2024 aluminum body skins on aging aircraft

Exfoliation, a directional attack along elongated grain boundaries, has been examined in some detail in rolled 2024 aluminum sheet and plate for KC-135 aging aircraft body skin samples utilizing optical (light) metallography, scanning electron microscopy, and transmission electron microscopy. A detailed analysis and comparison of precipitates within the grains and in the grain boundaries were performed, as well as an examination of elemental depletion profiles across grain boundaries. These observations suggest that corrosion-related anodic sites play a far less significant role in the propagation of exfoliation than do the hard corrosion products creating wedging stresses within the elongated grain boundaries, which seem to demonstrate unique and unusual structural or energetic features or both.

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.

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

Abstract 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.

Comparison of beginning and ending microstructures in metal shaped charges as a means to explore mechanisms for plastic deformation at high rates

Optical metallography and transmission electron microscopy (TEM) observations were made of a variety of forged or sputtered copper, molybdenum, and tantalum shaped charge components. The beginning shaped charge liner grain sizes and sub-structures were compared with those observed in residual (ending), recovered and corresponding jet fragments and slugs. The wide range of microstructures and evolutionary features of observed microstructures can be characterized by low-energy dislocation structure (LEDS) principles which are altered because the shaped charge deformation corresponds to hot working, and dynamic recovery and recrystallization play a prominent role. There is a prominent relationship between the starting liner grain size, Do, and the ratio Do/Ds, where Ds is the ending (slug or jet), steady-state grain size. As a consequence of this relationship, it appears that the volumetric stored energy, which depends upon the grain size and dislocation density (or degree of deformation), is the critical issue in controlling shaped charge jet stability.

Energy partitioning and microstructural observations related to perforation of titanium and steel targets

This paper presents an analysis of two target materials and the associated energetics related to the initial penetration into the target and perforation as the penetrator exits the target. Impact tests were conducted for tungsten alloy rods striking rolled homogeneous armor (RHA) and titanium alloy plates. Rod impact velocities were nominal 1,500 and 2,000 m/s. Target thicknesses were chosen so that the rods would overmatch the targets and lose some 200 m/s during penetration. The tests utilized flash x-rays to determine rod residual lengths and velocities and target plug features to include thicknesses and velocities. From these observables, experimental determination of the corresponding kinetic energies (KEs) and estimates for the fracture energies were obtained. Also, in each case, target material adjacent to penetration channel walls was examined by optical and electron microscopy and x-ray diffraction to gain further insight into deformation processes (cavity expansion) during penetration. The analytic penetration model gave results that were in good agreement with the experimental observables. In addition, it was observed that the RHA follows traditional plastic flow of cavity expansion while titanium alloy shows deformation features that deviate significantly. The paper discusses possible causes for these differences.