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8/3/2019 L. E. Murr et al- Novel Deformation Processes and Microstructures Involving Ballistic Penetrator Formation and Hypervelocity Impact and Penetration Phenomena

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Novel Deformation Processes and MicrostructuresInvolving Ballistic Penetrator Formation andHypervelocity Impact and Penetration PhenomenaL. E. Murr, E. Ferreyra T., S. PapJ. M. Rivas, C. Kennedy, A. Aya P u, E. I?. Garcia, J. C. Sanchez, W. Huang,a, and C.-S. NiouDepartment ofMetallurgical and Materials Engineering and Materials Research Institute,The University of Texas at El Paso, El Paso, TexasLight metallography and transmission electron microscopy techniques affording uniqueobservations of microstructural issues in connection with a related set of novel, high-strain-rate deformation processes provide some fundamental insight into the following areas:shock-wave-induced twinning, explosive welding, shaped charge development, explo-sively-formed penetrator phenomena, hypervelocity impact cratering in metal targets, andlong, dense rod penetration/perforation of thick metal targets. Although shock wave phe-nomena are precursors in all these processes, deformation twins are rarely observed in theresidual, process microstructures. In the case of hypervelocity impact craters, no deformationtwins are observed in the crater-related target microstructures. Microbands that appear tobe related to twins are observed. Melt-related phenomena are observed only in the explosiveweld-wave interfaces. Jetting phenomena related to shaped charges and crater rim formationare dominated by dynamic recrystallization, which provides a mechanism for extreme plasticflow in the solid state. Differences observed between rod penetration of rolled homogeneousarmor and Ti-alloy thick targets manifest themselves in distinct microstructural differencesthat also do not include melt phenomena. 0 Elsevier Science Inc., 1996

INTRODUCTION

There is a wide range of novel deformationprocesses, especially involving high-energy-rate processes at high strains (true strainsfrom 1 to 6) and high-strain rates (102-lo7ss*), that are often both practical andunique in the context of more conventionalprocesses, particularly those of metal work-ing, forming, and performance modifica-tion. Many processes are driven by high ex-plosives or other, related techniques forcreating intense shock waves. Explosive fab-rication processes include so-called standoff,contact, and impact operations that affectforming, drawing, hardening, compaction,welding, and cladding of metals [l-5]. Inrelated processes, penetrating devices are

shaped, self-forged, or simply acceleratedto breach a metal or material target whosedeformation is intrinsically coupled to thebehavior of the penetrator or to its owncharacteristic microstructure [l, 3, 5-71. Inthe past decade or so, the role of microstruc-ture in process efficiency or in understandingprocess mechanisms or fundamentals beganto influence design strategies and applica-tions of such high-rate phenomena. These ap-plications include industrial, military, andspace-related structures and systems.

In many high-rate deformation processes,shock heating at very high pressures, or thecombined high strains and high strainrates, produces localized, adiabatic heatingthat can promote alternative mechanismsfor deformation behavior as well as micro-

MATERI ALS CHARACTE RIZATION 37:245-276 (1996)0 Elsevier Science Inc., 1996655 Avenue of th e Americas, New York, NY 10010

2451044-5803/96/$15.00

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246 L. E. Murr t al.structural modification. Shock-wave effectsare precursors to most impact-related or ex-plosive contact phenomena and can createmicrostructural modifications that are im-portant in forming, working, welding, andpenetration. And, though there have beendecades of studies of plane shock wavemodification of metals and other materialssystems [l-6, 8-121, very little is knownabout oblique or spherical shock wave ef-fects that characterize more practical defor-mation processes [3,13].

In this study, we present an overview ofseveral related or interrelated high-strain-rate and often extremely high strain defor-mation processes, all involving shock waveprecursors as a consequence either of highexplosive drive systems or of high velocityor hypervelocity impact. Of particular in-terest in the development of this overviewis the effect of initial system microstruc-tures on process development, microstruc-tural alterations as a consequence of theprocess itself, and microstructure evolutionor its unique features that characterize thefinal process or its products. Microstruc-tural issues include plane-wave and ob-lique shock loading of both face-centeredcubic (fee) and body-centered cubic (bee)metals and alloys, explosive welding, shapedcharge and explosively formed penetratordevelopment, high velocity and hyperveloc-ity impact and impact crater formation, andthick plate penetration and perforation byexplosively shaped devices and long, denserods. Light and transmission electron mi-croscopy techniques constitute the princi-pal experimental approaches for illustratingand elucidating the associated microstruc-tural features and their development, whichare unique to the deformation processes ormodes to be described.

SHOCK-WAVE PHENOMENA

Essentially all explosively-driven processesand impact phenomena involve the cre-ation of shock waves that travel as a geo-metrical demarcation (or shock front) in theassociated materials or materials system. In

crystalline or polycrystalline materials, theadvancing shock front creates lattice de-fects that alter the microstructure. In thesimple, plane-wave shock regime illus-trated in Fig. l(a), a plane flyer plate im-pacting a plane (parallel) crystal surfacecreates a shock wave having a peak shockpressure, Ps = pt (Ct + St UP,)Upm, where ptis the target density [the crystal latticeshown in Fig. l(a) 1, Ct is the bulk sound ve-locity in the target, St is a material constant,and UP is the modified projectile (or flyerplate) velocity in the compressed region af-ter impact [l, 51. This shock wave actuallypropagates the same in both the target andthe flyer plates. Consequently, the peakpressure is the same in both. As shown inFig. l(a), the advancing shock front leaveslinear dislocation arrays (as well as otherdefects). For many polycrystalline metalsand alloys, the dislocation density increaseswith increasing peak pressure [1,3]. Conse-quently, hardness and yield strength areusually increased when a shock wavepasses through a crystalline or polycrystal-line material.Because a shock wave will move at bulksound velocity, it is a precursor to plasticdeformation. Consequently, the associatedplastic deformation will involve the shock-wave-altered microstructure, which may ormay not be influenced in a significant wayby the original or starting microstructure(including, of course, the grain size and dis-location density).

In the arrangement shown in Fig. l(a),detonation occurs at the apex of a line-wave generator that, because of its anglerelative to the main explosive charge, willproduce a plane front of detonation prod-ucts. These in turn will detonate the maincharge simultaneously over its surface, ac-celerating the flyer plate parallel to theplane target. If, as illustrated in Fig. l(b),the detonation wave (VD) travels parallel tothe target plate surface, the collision is ob-lique, and the flyer and target will weld to-gether. At this welding interface, a jet occursthat is often very small, but the associatedplastic deformation at the contacting sur-faces is often so severe that the flyer plate


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Novel Deformation Processes and Microstructures 247and target flow together to create a bond orweld zone. This zone is often characterizedby waves whose period or wave spacing, X,and height are related to the detonation ve-locity, VD, or the associated pressure. Melt-ing often occurs at the weld interface andwithin the weld-wave structure, and thesefeatures are illustrated in Fig. 2 for the ex-plosive welding of copper. It is possible tovirtually eliminate the wavy structures inexplosive welding through variations of Voand l3 (Fig. 2). Correspondingly, the meltfraction is changed. Explosive welding isunique because virtually any metal, as wellas other materials, can be joined over hugesurface areas, and multiple materials, com-posites, or laminates can be created [3,4].The simple schematics represented inFig. 1 illustrate some of the important fea-tures that can occur when a geometricallyplane material impacts a plane target at dif-ferent angles and velocities (or pressures).In Fig. l(a), a plane shock wave propagatesinto the target and there is no joining orwelding of the two. In Fig. l(b), the platesare joined or welded, differently for differentimpact conditions noted earlier. It shouldbe apparent, then, that these phenomenaoccur in any impact at high velocity. More-over, if the impacting plate geometrychanges, the process also changes. Finally,a thermal wave trails the shock front,whose magnitude (or dynamic temperaturerise) increases with increasing pressure. Theprincipal effect of this phenomenon is that,for very high pressures, the microstruc-tures created in the shock front can be re-covered or annihilated by the trailing ther-mal pulse. These thermal phenomena arefurther complicated by actual deformation(or residual straining) of the target (or theflyer), which produces adiabatic heating:ATx(e)(E), where AT is the temperature riseand E and i are the corresponding strain andstrain-rate, respectively. Of course, for shock-loading-related phenomena, the correspond-ing strain rates are very large (106-10s s-l).These features-high strain, high-strainrate, and associated high temperatures-are, of course, conducive to dynamic recov-ery and recrystallization of microstructures

created in the shock front. This occurs dur-ing plastic deformation and in static recov-ery and recrystallization from residualheat, either shock heating or adiabatically-developed heating.

DEFORMATION TWINNINGAlthough, as illustrated schematically inFig. l(a), shock wave propagation in crys-talline materials may create dislocations toaccommodate the plastic distortion in theshock front, this simple glide phenomenonmay not be able to fully accommodate largelattice shear, and localized, massive shearmay occur, which is crystallographically

L

FIG. 1. (a) Plane shock wave generation and propa-gation in a crystalline solid. (b) Explosive welding ofan explosively-driven flyer plate impacting a target orbase plate. Explosive welding parameters and para-metric relations are shown.


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FIG. 2. Light microscope views of explosive weld waves in a copper/copper weld zone [Fig. l(b)]. (a) Solidifica-tion microstructures in weld-wave vortex and behind the wave crest (arrows) indicative of melt zones. (b) Lowermagnification view of weld waves in a copper/copper explosive weld, showing flyer plate and base plate grainstructure and extreme plastic flow and grain distortion along the weld zone (interface).


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250 L. E. Mwr etal.lapping fault bundles forming twin faultsare formed, and their density and spacingchange with peak pressure [1, 31. In high-stacking-fault free-energy metals such ascopper and nickel, dislocations form cellstructures whose walls densify with pres-sure as the cell size decreases [ 1,3].

Deformation twins are often unique forshock loading of metals and other materialsbecause they do not form in most deforma-tion processes, and, if they do form, theirpropensity is not as great as in shock load-ing. In contrast with fee, where deforma-tion twins or twin faults form coincidentwith the (1111 slip planes, twins in bee formon {112) planes or plane segments. And, be-cause of the multiplicity difference between(112) and (111) planes and the glide-relatedtwinning mechanisms, the appearance oftwins is different. These features are appar-ent on comparing plane shock-induced de-formation twins in fee nickel shown in Fig.3 with plane shock-induced deformationtwins in bee molybdenum shown in Fig. 4,for the same (100) grain surface orientation.In Fig. 3, the twin faults are narrow vol-umes that intersect at 90. The twin widthsare relatively uniform and are apparent atthe intersection that produces a recogniz-able contrast (arrow). The [ill} twin planesare inclined -55 with the (100) grain sur-face and produce a rather regular refine-ment of the nickel grain structure. This pro-duces a residual hardening (AH) and increasein yield strength (a) because

and u0 refers to the unrefined structure (a sin-gle crystal), D s the grain size, A is the parti-tioning or grain refinement parameter notedin Fig. 3, and I< and K are material constants.These same features generally apply inFig. 4, but the contribution of twinning tograin partitioning is different because thetwin density is smaller and crystallographi-tally (and geometrically) different fromthat shown in Fig. 3. In Fig. 4, the twinboundary is generally uniform but not onthe same (coherent) or parallel (112} plane.The twin boundary projection is, nonethe-less, reasonably uniform and corresponds

to an angle of inclination of the (112) planeof ~66 with the (100) grain surface.

Figures 5 and 6 show, for comparison,deformation twins in bee tantalum and atantalum -2.5 wt.% tungsten alloy. In thisparticular case, the Ta-2.5%W alloy was theflyer plate in a plane-wave arrangementsimilar to that of Fig. l(a), whereas the Taplate was the target. The peak shock pres-sure was -45 GPa, and the pulse durationwas essentially the same as for Figs. 3 and 4(~2 ks). Figure 5(a) shows a light (optical)metallographic view of the equiaxed Ta tar-get after shock loading. The arrow in Fig.5(a) indicates some nonconcurrent, longdeformation twins. A crystallographicallysimilar situation is shown in Fig. 5(b) in thetransmission electron microscope (TEM).Figure 6 shows similar views for deforma-tion twins in the Ta-2.5%W alloy. Figure6(c, d) also shows more specific twin fea-tures, which include extra twin reflections[arrow in Fig. 6(c)] and a corresponding dark-field image utilizing these reflections andother twin reflections that coincide with thematrix reflections for the (100) surface orien-tation. The details of twin spots in diffractionpatterns have been described by Bulloughand Wayman [14,15] and Murr [16].It might be of interest to note that theVickers microhardness for the unshockedTa target material corresponding to Fig. 5averaged 109, whereas the shocked-resid-ual hardness was 176, or a AH of 62%. Itmight also be important to note that shock-induced deformation twinning is grain sizedependent; the propensity for twinning in-creases with increasing grain size (and pres-sure). Simultaneously imposed strain alsocan complicate this phenomenon. Further-more, below some critical grain size, there isoften little or no twinning [5, 17, 181. Conse-quently, the microstructure (grain size) canoften be manipulated to avoid twinning, ifdesired, in certain circumstances. Other re-finements to the grain size can often affect asize reduction below the critical grain sizefor twinning. They include dense precipi-tates and dispersoids as well as heavydislocation arrangements associated withthese microstructural features [19].


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Novel DeformationProcessesand Microstructures 251Metals such as tantalum and its alloys

have been of interest in recent years be-cause their high-strain-rate formability andhigh density (16.7g/cm3 for Ta) are attrac-tive in military penetration devices such as

shaped charges and explosively-formedpenetrators (EFPs). These general conceptsas well as simple impact cratering anddense rod perforation of a material targetare illustrated schematically in Fig. 7. In all

FIG. 4. Plane-shock-wave-induced deformation twins in molybdenum corresponding to a peak pressure of 27GPaand a 2~s pulse duration. The arrow illustrates a principal (112) trace direction along [042].


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252 L. E. Murr et al.

FIG. 5. Deformation twins in plane-wave shock-loaded tantalum at 45GPa, 2t.1~pulse duration. (a) Light micro-im ah of polished and etched shocked sample showing equiaxed grains with crystallographic markings. (b) Trans-miS! ion electron micrographic bright-field image showing twins on two different 1112) planes in dir ections(ma rked A and B) for a (100) grain surface orientation. Two other possible directions are shown by the arrows.


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Novel Deformation Processes and Microstructures 253

FIG. 6. Deformation twins in plane-wave shock-loaded Ta-2.5%W alloy at 45GPa, 2~s pulse duration, as in Fig. 5.(a) Light micrograph of polished and etched shocked sample showing crystallographic markings. (b) Transmis-sion electron micrographic bright-field image showing deformation twins in a (100) grain surface orientation sim-ilar to that of Fig. 5(b). (c) Selected-area electron diffraction (SAD) pattern for (a), showing [loo] zone. (d) Trans-mission electron micrographic dark-field image of (a), using twin reflections noted in (c) by arrows. The small spotis an extra twin reflection, whereas the primary twin reflection is coincident with a [loo] zone matrix reflection.


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254 L. E. Murr et al.of the deformation processes illustrated inFig. 7(a-e), a shock wave is a precursor. InFig. 7(b-c), the arrows denoting the detona-tion (or shock) front are variously obliqueto the liner material, with the EFP in Fig.7(c) being closest to a plane shock wave.The impact conditions in Fig. 7(d, e) pro-duce an even more complex, sphericalshock wave, which forms the penetratingcrater whose depth is related generally to2, mt, where pPand pt are the penetra-tor and target densities, respectively, and I,is the penetrator dimension (length or di-ameter). It is thus apparent why heavy pen-etrator materials are of interest.Figure 7(a) shows a special shock loadingarrangement for creating an oblique shockwave (-55) relative to the axis of metalrods, which can be simultaneously shockloaded and uniaxially strained. This ar-rangement can provide some indications ofmicrostructures created by shock precursorgeometries more realistically related to thedeformation processes illustrated in Fig.7(b-e), and recent results for copper rodshaving different grain sizes have been re-ported by Sanchez et al. [20].

Figure 8 shows some examples of defor-mation twins in copper rods shock loadedin the oblique arrangement shown sche-matically in Fig. 7(a). Figure B(a, b) atteststo the increasing deformation twin densitywith increasing grain size noted previously,whereas Fig. 8(c) and its selected-area dif-fraction (SAD) pattern inset illustrate thetwin nature in the TEM. The twins in Fig.B(c) are actually tilted between 5 and 10[the (110) surface is tilted by this amount].They would normally appear as thin stripsbecause the {111) twin planes are ideallyperpendicular to the (110) surface orienta-tion. These crystallographic features aredemonstrated in a more exact (110) grainsurface orientation with deformation twinscoincident with two nonconcurrent {111)slip systems along directions (Fig.9). The SAD pattern inset in Fig. 9(a) and itssimulated pattern inset in Fig. 9(b) showprominent twin reflections at /3 po-sitions. The corresponding dark-field im-age in Fig. 9(b) originates for two unique

/3 twin reflections near the (113)matrix reflection in the simulated diffrac-tion pattern inset.One of the interesting features of Figs. 8and 9 is that the deformation twinning isvery similar to that observed for plane-wave shock loading, although there are ex-ceptions: the peak pressure is lower thanthe critical twinning pressure in plane-wave shock loading (N12GPa in contrastwith N20GPa). Sanchez et al. [20] have alsonoted that there is some intermixing of mi-crobands [21, 221 with the deformationtwins, which also seem to increase with in-creasing grain size and residual strain (atconstant grain size). Nonetheless, plane-wave shock loading appears to provide atleast phenomenological details of deforma-tion twinning in a variety of processes in-volving high-pressure shock waves. Thereare, however, some intriguing exceptions,which we will describe later.

SHAPED CHARGEDEFORMATION PROCESSES

As shown schematically in Fig. 7(b), ashaped charge involves a conical liner withapex angles ranging from 42 to 68. Theshock or detonation front (with pressuresof 2O-80GPa) initially obliquely shock loadsthe liner prior to its collapse. The outer 80%of the collapsed liner flows to form the slugwhile the inner 20% of the liner flows toform an elongating, high-speed jet thatelongates under the velocity gradient andnecks into periodically particulated frag-ments. In the jet elongation, the strains canexceed lOOO%, and the associated strainrates are usually in the range lo4 to lo5 s-i.These extreme deformation features con-tribute to adiabatic heating, with tempera-tures in the deforming and elongating jetexceeding 0.7T~ in metals such as copper.Under these processing conditions, the de-forming jet can extend by dynamic recrys-tallization that, as Chokshi and Meyers [23]originally proposed, could be phenomeno-logically described as superplastic stretchingas the recrystallized grains are refined to sizes


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Novel Deformation Processes and Microstructures 255~lprn. In reality, this process could be con-sidered a continuous dynamic recrystalliza-tion process with recovery and recrystalliza-tion actually providing a flow mechanism.

Figure 10 shows, for comparison, thegrain structure for a starting, undeformed,equiaxed tantalum shaped charge liner

/ DETONATOR

cone [Fig. 10(a)], along with a typical resid-ual microstructure of a soft-recovered jetfragment [Fig. 10(d)] [24,25]. It is apparenton comparing Fig. 10(b) with Fig. 10(f) thatthere is a grain size reduction from roughly40Frn to 1u.m in the jet center. However,the jet cross section actually exhibits con-

C

d

FIG. 7. Deformation process schematics. (a) Cylindrical/oblique shock loading of copper rods placed in a cylin-drical metal holder. The explosive is C-4. (b) Detonating shaped charge. Dashed lines depict the metal liner cone.Arrows denote the shock front or detonation propagation. Shading represents the high explosive (H.E.). (c) Simplebackward folding EFP. Arrows denote detonation (shock) front propagation. The liner (black) is a circular saucer-shaped plate. (d) High velocity/hypervelocity impact and crater formation; 1.4~s the impact velocity. (e) Sequen-tial views of long rod (length/diameter = lo), high-velocity penetration and perforation of thick (T) plate.


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256centric zones of very small grains, with av-erage grain sizes less than 0.4pm (a reduc-tion from the original cone of lo*). Thisfeature is also observed to some extent inthe recovered slug, but the slug also exhib-its an evolutionary microstructure acrossits cross section that does not experience re-peated deformation in the same context asthe elongating jet. Figure 11 shows thesemicrostructural features where only at the

L. E. Murr et al.slug center is the microstructure character-istic of that in the jet cross section and dy-namically recrystallized [compare Figs. 10(f)and 11(d)]. However, the slug representsan evolutionary and connected microstruc-ture as a consequence of temperature andstrain variations from the slug center to itsperimeter. At the slug perimeter [Fig. 11(b)],the microstructure is characterized by largedislocation cells with misorientations Cl.

FIG. 8. Deformation twins in oblique-shock-loaded copper rods: (a) small grain size (D = 29km) light micro-graph; (b) large grain size (D = 375km) light micrograph; (c) transmission electron micrographic bright-field im-age with superimposed SAD pattern showing /3 twin reflections perpendicular to the [li2] twin trace di-rection. The (110) grain surface is tilted to provide a projection of the twins.


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FIG. 9. Deformation twins in oblique-shock-loaded copper rod. (a) Transmission electron micrographic bright-field image showing two sets of twins along the [i12] and [li2] directions indicated by dashed lines in the diffrac-tion pattern simulation inset in (b). The SAD pattern inset in (a) shows /3 twin reflections, which are alsonoted in the pattern simulation. (b) Dark-field transmission electron micrographic image obtained using the (173)reflection and its associated /3 twin reflections.


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258These cells become elongated and the mis-orientations increase to -2 at e in Fig.11(b). From this zone [e in Fig. 11(b)], thedislocation cells begin to recover and re-crystallize, and the misorientations averagefrom 9 to 12 at the slug center [Fig. 11(d)].Dynamic recrystallization therefore playsa minor role in the slug development,whereas it is the dominant feature of jet plas-ticity. It should also be noted that, unlike his-

L. E. Murr et al.torical models for shaped charge jetting,there are no melt- or solidification-relatedmicrostructures; refer to Fig. 2 for example.Consequently, shaped charge jetting ischaracterized by extreme solid-state plasticflow, with melt fragments occurring (inter-mittently) for certain process circumstancesalong the jet axis [26]. It is also notable thatno deformation twins are observed in ei-ther the Ta jet fragments or slug (Figs. 10

FIG 10. Shaped charge components and their microstructures: (a) liner cone section view showing isolation ofspecimen from inner cone wall for light microscope and transmission electron micrographic imaging: (b) light mi-crograph of tantalum starting liner cone; (c) transmission electron micrographic image of liner cone in (b); (d) soft-recovered Ta jet fragment; (e) light micrograph of jet fragment center; (f) transmission electron micrographic bright-field image of jet fragment center in (e). Compare (b) and (c) and (e) and (f). Magnifications are corresponding.


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Novel Deformation Processes and Microstructures

b

FIG. 11. Shaped charge components and their microstructures (continued): (a) tantalum slug (scanning electronmicrographic composite view); (b) light micrographic view for strip through slug cross section (arrow) [lowercasec and e show positions of microstructures shown correspondingly in (c) and (e).]; (c) light micrographic viewof slug center zone; (d) transmission electron micrographic image of (c); (e) light micrograph of slug zone markede in (b); (f) transmission electron micrographic image of (e).


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260 L. E. Murr et al.and 11). They are annihilated in the dy-namic recovery and recrystallization pro-cesses. However, Meyers et al. [17] have ar-gued recently that, because of the grain sizeeffect on twinning, this phenomenon con-tributes to the improved jet stability as thegrain size is reduced. Alternatively, as sug-gested by Murr et al. [25, 271, the use ofvery small grain size liner cones or highlydeformed starting liner cones also may ac-celerate the dynamic recrystallization pro-cess, thereby promoting enhanced jet stabilityand longer breakup (particulation) behaviorby providing significant stored energy to theplastic deformation of the collapsed liner.Regardless of the specificity of the argu-ments for shaped charge jet development,the experimental realities can only provideus with a view of the starting liner micro-structure and the residual, recovered jetfragment microstructure, between whichthe dynamic event (or events) must be re-constructed. Figure 12 shows a schematiccartoon of how this process might developin the shaped charge. It is worth reiteratingthat the remarkable feature of shapedcharge jet development is that the enormousdeformation associated with the elongatingjet is accomplished primarily by solid-stateplastic flow. This is also true of extremelyfine wire drawing of, say, lcm diametercopper rod to 30p.m wire with only infre-quent annealing, which is a lower strain-rate analogue.

EXPLOSIVELY FORMEDPENETRATOR PROCESSING

Explosively-formed penetrators are one ofnumerous self-forged projectiles that differfrom shaped charges primarily in the verylarge angle of the liner cone (>120). Theexplosively formed penetrator can be for-ward or backward folding, and there arespecial cylindrical devices for assuring sym-metrical folds, particularly in the tail of theEFP. Figure 7(c) shows a simple modifica-tion of the shaped charge detonation re-gime in Fig. 7(b). However, in contrast withthe shaped charge, the EFP is dominated by

what is equivalently a self-formed slug. Thisslug does not form by the uniform, extremeplastic strain characteristic of the shapedcharge, and the strains within the EFP usu-ally do not exceed a true strain of 4. Thestrain rate is somewhat lower than theshaped charge jet (~10~ s-l). There is also avelocity gradient along the forming EFP,which produces tensile stresses along itsaxis in flight. Although it might be expectedthat there may be some microstructuralsimilarities to the shaped charge, there isonly a meager volume fraction of dynamicrecrystallization. Dynamic recovery andevolutionary dislocation cell structuresdominate in tantalum EFPs, for example.There are isolated examples of recrystal-lized grains intermixed with recovery mi-crostructures. The extremes in microstruc-ture, illustrated in the light metallographicviews in Fig. 13, are not observed in theshaped charge, although there are somesimilarities between the Ta shaped chargeslug microstructures-Fig. 11(e) for exam-ple-in contrast with Fig. 13(b) [28-321. Fig-ure 13(d) illustrates a Ta-2.5%W EFP thatrevealed a few isolated examples of resid-ual deformation twins in the tail section [tothe left in Fig. 13(d)]. Figure 14 reproducesa sequence of transmission electron micro-graphic bright- and dark-field images illus-trating these features. These twins shouldbe compared with those formed by plane-wave shock loading illustrated in Fig. 6.The general absence of deformation twinsin tantalum and their apparent alteration inTa-2.5%W EFPs (Fig. 14) may be an indica-tion of their thermal instability, because theEFP temperature overall does not exceed0.4TM (where TM, the melting point, is-3000C for Ta) and could reach this tem-perature only in isolated, high-strain re-gions.

HIGH-VELOCITY AND HYPERVELOCITYIMPACT CRATER PHENOMENA

Just as the shock wave collapses the linercone in the shaped charge or drives theself-forging EFP, a spherical shock wave as-


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61

FIG. 2. Cartoon and associated transmission electron micrographic bright-field images depicting the dynamicrecrystallization features implicit for the shaped charge jetting process. The schematic part is from Ref. [25]. Thecartoon shows continual (repeated) deformation (shading) and recrystallization with the straining jet. At 2, theoriginal cone is shocked. At 3, the jet begins to recrystallize, At 7, the jet necks and, at 8, it particulates. The trans-mission electron micrographic images are of tantalum.


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FIG. 13. EFP components and microstructures. (a) Soft-recovered tantalum EFP cut in half and polished to revealmic restructures observed in the light microscope in (b) and (c). (d) Soft-recovered Ta-2.5%W symmetrical (three-fold ) EFP.


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FIG. 14. T ransmission electron micrographic bright-field (a) and dark-field (b) images showingtwir IS in the3 ail section (arrow) in Fig. 13(d). The dark-field image was made by using apertured reflecin tl- e SAD pattern inset; a and b show (ilO) and (002) reflections, respectively, for [IlO] zone. 7diffr action, and extra (kinematical) twin reflections occur between the two matrix reflections.

defo rmation:tion (arrow)win , double


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264 L. E. Murr et al.sociated with a high-velocity or hyperve-locity (impact velocity, u0 > 5km/s) parti-cle impacting a target drives the crater thatforms. Because, as noted earlier, the shockpressure is the same in both the impactingparticle and the target at the instant of con-tact, the reflection of shock waves in a finite(and ideally spherical) impactor can createextremely high temperatures as well ascomplete spallation. Melting, fragmenta-tion, and even vaporization can be compet-ing mechanisms affecting the impactingparticle. A semi-infinite target, on the otherhand, will crater with the symmetry of theimpact-generated shock wave, which isideally hemispherical in the target, with theshock wave attenuation contributing to theactual crater size. As the actual crater forms,the target material flows plastically up alongthe wall to form a jet, which creates a rimabove the target surface, a process that isphenomenologically similar to explosiveweld jetting (Fig. 2) and shaped charge jet-ting [Fig. 7(b)]. These features are repre-sented very generally in Fig. 15. Figure15(b) shows a computer simulation for asteel impactor striking an aluminum targetat 25km/s. The impactor is vaporized in thismodel, and the rim jetting is apparent. Fig-ure 15(c, d) shows a normal view into a cra-ter in copper and a corresponding sectionalview through the same crater, respectively,made by a soda-lime glass sphere (3.2mmdiameter) impacting at 5.8km/s. Figure15(d) shows the impact axis through thecrater half-section, with sequential nota-tions a, b, c, and d below the crater wall.Figure 16 shows a corresponding sequenceof transmission electron micrographic bright-field images taken along the impact axis, asnoted by extracting and polishing 3mmdiscs sequentially from the crater wall bot-tom. Figure 16(e) corresponds to a zoneroughly 14mm below the crater bottom inFig. 15(d), whereas Fig. 16(f) is typical ofthe unimpacted copper plate, which had alarge grain size of 763pm. Unlike the com-puter-simulated crater in Fig. 15(b), thetransmission electron micrographic se-quence in Fig. 16 illustrates that a residualmicrostructure below a hypervelocity cra-

ter in a copper target extends far beyondthe crater wall region and is a consequenceof the spherical shock wave. Near the craterwall (0.2-0.4mm), as shown in Fig. 16(a),there is a zone of dynamically recrystallizedand considerably refined grains (0.7pm incontrast with ~700~m for the original tar-get plate). This is an even larger grain re-finement than that observed in the tanta-lum shaped charge-compare Fig. 10 forexample. Beyond the recrystallized zone,there is an extensive zone of severe plasticdeformation that contains very fine disloca-tion cells and a high dislocation density, asshown typically in Fig. 16(b). This zone be-comes intermixed with microbands and awide region where profuse microbanding isobserved. Beyond this microband zone [fromabout 3 or 4mm in Fig. 15(d)], the micro-structure is characterized by evolutionarydislocation cell structures that increase incell size and decrease in dislocation density.There is a corresponding variation inhardness (Vickers) with considerable soft-ening in the dynamically recrystallizedzone near the crater wall, rising to a peak inthe microband zone and then decreasingsomewhat regularly as the dislocation cellsize increases [Fig. 16(d, e)]. These featureswere originally observed by Quinones et al.[33] and Rivas et al. [34,35] for copper andaluminum craters. Interestingly enough, sim-ilar phenomena were observed even earlierin the examination of laser-induced crater-ing in single-crystal and polycrystal iron tar-gets [36].There are several interesting features in-cluded in the microstructural evolution be-low the copper crater in Fig. 16. The first isthe zone of dynamically recrystallized andultrarefined grains at the crater wall. Thesecond is the observation of a zone of mi-crobands somewhat removed from the cra-ter wall, and the absence of deformationtwins. The third is the absence of any melt-related phenomena at or near the crater wall.

The observation of a zone of microbands[Fig. 16(c)] rather than shock-induced de-formation twins or twin faults associatedwith hypervelocity impact craters in cop-per is especially intriguing because it has


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Novel Deformation Processes and Microstructuresalso been demonstrated that this zone is ve-locity (or pressure) dependent and increaseswith impact velocity, u,,. Furthermore, asshown in Fig. 17 and 18, crater-related mi-crobands tend to increase in density withincreasing grain size, are oriented coinci-dent with a primary slip system ((ill}), andhave misorientations of -2-3 and widthsranging from about 0.2 to 0.4Fm. Huangand Gray [22] believe that the microbandboundary is a dislocation double wall. It isapparent from Fig. 18 that microbands aresomehow related to deformation twins interms of primary (111) slip, but their mech-anism of formation is obviously different. Itis interesting to note that the dislocation

a

265cells in Fig. 11(f) look very similar to themicrobands in Figs 16(c) and 17(f). Even thewidth and misorientation of the cells arethe same. However, the cells are not coinci-dent with a slip system (which, of course,would be {llO) for the Ta in contrast with(111) for copper).

Microbands have also been identified inother cratered fee targets which, as a conse-quence of their propensity to twin, mightnormally be expected to contain deforma-tion twins, as in the case of copper. Figure19 shows a hypervelocity impact crater in a6061-T6 aluminum target. Note the absenceof any significant rim (owing to tensile frac-ture in the rim jet) and the crater surface

FIG. 15. Hypervelocity impact crater phenomena. (a) Hypervelocity impact crater schematic [Fig. 7(d)]. (b) Com-puter-simulated impact crater half-section for a steel projectile impacting an aluminum target at 25km/s. The pro-jectile is assumed to be vaporized. (c) Normal view of impact crater in a large-grain (735pm) copper target for3.2mm diameter soda-lime glass sphere (u, = 5.8km/s). (d) Sectioned crater in (c). The notations along the impactaxis [dashed line in (d)] correspond to the specific zone of microstructure illustrated in Fig. 16.


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FIG. 16. Sequential and evolutionary residual transmission electron micrographic images of microstructuresalong the impact axis [in Fig. 15(d)] below the crater wall bottom in copper. (a) Dynamic recrystallization zonenear the crater wall. (b) Heavy dislocation zone beyond (a). (c) Microband zone beyond heavy grain distortion.Microbands are oriented along the trace of (111) for a (110) oriented grain. (d) Zone of dislocation cells beyond themicroband zone, which continues beyond about 12mm from the crater bottom in Fig. 15(d). Note the dislocationcell changes from (d) to (e). (f) Undeformed copper starting-target microstructure for reference. The centerline is acoherent annealing twin boundary with dislocations at the boundary plane.


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FIG. 17. Comparison of microbanding associated with hypervelocity impact cratering in copper targets of differ-ent grain size: (a) original target microstructure, D = 38um; (b) microband zone about 2mm from the crater bot-tom formed by a 3.2mm aluminum sphere impacting the plate in (a) at 6.lkm/s; (c) transmission electron micro-graphic view of microbands in (b); (d) original target microstructure, D = 763um; (e) microband zone about 3mmfrom the crater bottom in Fig. 15(d); (f) transmission electron micrographic image of microbands in (e). Mi-crobands in (c) and (f) are along traces of 1111). Magnification of(f) is the same as that of (c).


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268 L. E. Murr et al.differences in Fig. 19(a, b) in contrast with pearance of the distortions of grain flowFig. 15(c, d). Note also the microband simi- lines below the sectional view in Fig. 19(b)larities on comparing Fig. 19(c, d) with Figs. in contrast with the computer simulation in,16(c) and 17(c, f), as well as the general ap- Fig. 15(b).

FIG. 18. Comparison of (a) microband features with (b) shock deformation-induced microtwin features. The mi-crobands in (a) are coincident with the [1?2] trace for (ill) planes (arrow), which are perpendicular to the [ill] direc-tion shown within the SAD pattern inset. The grain surface orientation is (110). The microbands exhibit a 2 overallmisorientation shown by spot splitting (arrow) in the SAD pattern inset in (a). (b) Twin-faults coincident with (ill)planes along [li2] (arrow) are exactly coincident with microbands in (a). The SAD pattern inset shows prominent/3 twin reflections (arrow). The [ill] direction is perpendicular to the [1?2] arrow. The grain surface is (110).


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Novel DeformationProcessesand MicrostructuresFinally, Fig. 20 summarizes hyperveloc-

ity impact cratering in metal targets, espe-cially in the context of the important micro-structural issues associated with the crateringprocess. The light micrograph in Fig. 20shows a cross section of a rim area in thecrater of Fig. 15(c, d). The upper-rim sur-face forms by a kind of shear band flow,and these overlapping flow zones consist ofthe dynamically recrystallized (and re-fined) grain structure. It appears that, justlike shaped charge jetting, the rim jettingrepresented in Fig. 20 occurs by a velocitygradient, which imposes a tensile stress inthe flow direction. The rim edges can there-fore neck down and eject or particulate acircumferential rim zone. There is no evi-dence for melting, and crater flow (or, morespecifically, target material flow) into thejetting rim is a solid-state plastic flow phe-nomenon identical with shaped charge jetflow and elongation. Or, in the case of cra-tering in 6061-T6 aluminum shown in Fig.19(a, b), the rim can fracture extensivelyunder this resolved tensile stress and ejectthe bulk of the rim material. This process ofrim fracture accounts for essentially all ofthe crater-related (target) mass loss. Notethe microbands and original grain structurein the underside of the rim section shownin Fig. 20. This relatively undeformed andlifted target surface area is also illustratedin the computer-simulated crater in Fig.15(b) and accounts for the balance of dis-placed target mass in forming the crater.

The schematic representation shown inFig. 20 is an effort to depict very generallythe hypervelocity cratering process in thickmetal targets (where there is no spa11 orperforation). The microstructural zones de-pict those prominent features shown in Fig.16 and the rim section view shown in Fig.20. An attempt is made to depict the fate ofthe impacting projectile, which, in the caseof aluminum impacting a copper target,produces a thin, melt/solidification film onthe crater wall. However, there is no rem-nant glass for a soda-lime glass projectile.There have been no examples of projectilematerial interacting with (alloying with)the target. There are examples of the projec-

tile spalling into many fragments, some ofwhich are embedded in the crater wall. Wehave observed this phenomenon for 3.2mmdiameter stainless steel projectiles impact-ing the copper targets shown in Fig. 17(a,d) for example (u, > 5km/s).

LONG ROD PENETRATIONOF THICK TARGETS

The novel features of long rod penetration,as illustrated in Fig. 7(e), in contrast withsimple cratering shown in Fig. 7(d), includethe extension of the penetrator and thecomplete perforation of a finite-thicknesstarget. Grace [37] has recently examinedthe interaction of a dense, penetrating rodwith the target in the context of penetratorerosion and has applied this approach inexplaining differences in penetration/per-foration between conventional rolled, ho-mogeneous armor (RHA) plate and a Ti-6AI-4V target plate [38]. In addition, Huanget al. [39] have shown that there is a signifi-cant microstructural difference associatedwith the residual penetration channel region.These differences have also been augmentedby TEM observations of channel-related mi-crostructures, and Figs. 21 and 22 illustratesome of these features.

In Figs. 21(a) and 22(a) are shown compar-ative cross sections resulting for a W5%Ni,2%Fe alloy rod (7.8mm in diameter and78mm in length; with a density of 17.6g/cm3),impacting at 1.5km/s. These two targets,RHA [Fig. 21(a)] and Ti-alloy [Fig. 22(a)],were w45mm and 70mm in thickness, re-spectively. There is a readily apparent dif-ference in the cratering process in Fig. 21(a)in contrast with Fig. 22(a). The target sur-face is lifted three times as much in formingthe RHA crater and perforation in compari-son with the Ti-alloy. In addition, the lightmicrograph of the RHA microstructure inFig. 21(b) is considerably different and de-formed in comparison with the corre-sponding images in Fig. 22(b, c) for the Ti-alloy, where there is no apparent grain orphase distortion even 5 to 10p.m from thepenetration channel wall [compare Fig.


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2 7 0

21(b, d) and Fig. 22(b, c)]. The transmissionelectron micrograph in Fig. 21(c) shows thecomplex and heavily deformed RHA (car-bon steel) starting plate microstructure; be-cause of the heavy deformation in the chan-nel wall region creating highly magneticthin foils, no corresponding transmissionelectron micrographs have been obtainedfor this region. However, the transmissionelectron micrographs of the Ti-alloy chan-nel wall region shown in Fig. 22(e) exhibit asignificantly increased dislocation densityin the Ti-alloy duplex (CI + p) microstruc-ture, possibly shock-wave-induced [com-pare with Fig. 22(d, f)].Because the microstructures associatedwith different penetration channels in Figs.21 and 22 are so different, it seems likelythat the actual mechanism of penetrationmight be different. Such differences are

L. E. Murr et al.somewhat apparent in hypervelocity cra-tering on comparing, in retrospect, Fig.15(c, d) for copper with Fig. 19(a, b) for a6061-T6 aluminum target.

SUMMARY AND CONCLUSIONS

We began this study of novel, high-strain-rate deformation processes by focusing ontheir common feature of shock wave or det-onation propagation, either as a means todrive the process or as a consequence of theprocess. Because, ultimately, this shock wavephenomenon affects a metal or metal sys-tem, a brief survey of shock wave-induceddeformation twinning unique to manyshock processing regimes was presented,which included twinning in both fee andbee metals and which also included plane-

FIG. 19. Microbands associated with a hypervelocity impact crater in a 6061-T6 aluminum target for a 6.4mm di-ameter soda-lime glass sphere impacting at 5.2km/s. (a, b) An example of a crater at 4.6km/s. (c, d) Microbandsbelow the 5.2km/s crater wall. (c) A light micrographic image. (d) A transmission electron micrographic bright-field image. The orientation in (d) is (110), and microbands are coincident with the trace of [li2].


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Novel DeformationProcesses and Microstrtlctureswave and oblique shock wave arrangements.Deformation twinning increases with grainsize in both fee and bee metals and can re-fine or partition polycrystalline metals, re-sulting in significant increases in bothhardness and yield stress. It can signifi-cantly increase the initial stored energy andcan thereby affect microstructure develop-ment, recovery, and recrystallization phe-nomena.

In oblique shock loading of copper rods,Sanchez et al. [20] have also noticed thatthere is an intermixing with microbands,particularly at large grain sizes and whenthere is a simultaneous (and residual) strain.Gray [40] has also shown that twinning inplane-wave shock-loaded copper can change

271

predominantly to microbands when there isa large, associated strain.

The explosively-welded system interfacein copper, for example, was shown to con-sist of prominent melt/solidification zonesassociated with the vortex and crest of weldwaves. There is also a narrow region of ex-treme deformation characterized by recrys-tallization phenomena, probably a mixingof dynamic and static recrystallization.Although shaped charges can be effec-tively examined only as recovered frag-ments (jet fragments and the slug), thecomparison of starting liner cone micro-structures with the residual component mi-crostructures has suggested that dynamicrecrystallization plays a prominent role in

_r_,. ,- -j crywr &+le metbsjectc

FIG. 20. Hypervelocity impact crater rim flow and a generalized cartoon showing the dynamically recrystallizedzone that this represents, as well as other crater-related microstructures. The peak shock pressure at the point ofimpact is denoted P, and the shading represents its amplitude attenuation into the target. The projectile can melt,spa11 into fragments, vaporize, or undergo combinations of these phenomena. Note, in the light micrograph of therim section, that the outer rim (surface) flow zone is characterized by extremely small grain structure (by dynamicrecrystallization). This is in contrast with the large grain structure below the rim, which contains some groups orbundles of microbands.


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L. E. Murr et al.

FIG. 21. W-alloy rod penetration of an RHA target at 1.5km/s: (a) target sectional views showing penetrationchannel; (b) light micrographic view of channel-related microstructure in (a); (c) transmission electron micro-graphic bright-field view of the complex RHA microstructure containing lamellar and other distributed carbidesand dislocations; (d) light micrograph corresponding to a zone in the target far removed from the penetration chan-nel: (c) represents a region in this zone. Compare (b) and (d) where the magnification is the same as that of (b).


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FIG. 22. W-alloy rod penetration of a Ti-6AI-4V target at 1.5km/s: (a) target sectional view showing penetraichannel; (b) light micrographic image of channel-related microstructure in (a); (c) typical target microstructureremoved from the penetration channel; (d) transmission electron micrographic bright-field image from region(c); (e) transmission electron micrographic bright-field image essentially coincident with the channel wall inthrough a tangential section; (f) transmission electron micrographic bright-field image near the bottom of thecrograph in (b), roughly lmm from the channel wall.

tionI farn in

(b)mi-


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274 L. E. Murr et al.jet development while the slugs are domi-nated by dynamic recovery and exhibit dy-namic recrystallization only along the slugcenter (axis). There are no significant meltphenomena, and the shaped charge processoccurs by extreme plastic flow in the solidstate. Dynamic recrystallization can provide aunique mechanism for this plastic flow.

Explosively-formed penetrators or self-forming or forging projectiles differ fromshaped charge phenomena because of awider distribution of smaller strains andslightly lower strain rates. The microstruc-tures associated with EFPs are widely dis-tributed and contain significantly less evi-dence for dynamic recrystallization thaneven the shaped charge slug. Residual mi-crostructures in the EFP are dominated bydynamic recovery microstructures. Theseinclude prominently elongated dislocationcells, but there are a significant evolution ofdislocation cells to recovery microstruc-tures and some intermixing of dynamicallyrecrystallized grains. There have been somelimited observations of deformation twinsin EFPs, but they appear to be thermally al-tered and may be unstable at relatively lowprocessing temperatures.

Hypervelocity impact craters in metaltargets such as copper provide a unique op-portunity to link high-strain-rate and high-strain deformation microstructures with theundeformed target. There is a narrow zonenear copper crater walls that looks exactlylike shaped charge jet fragments and ischaracterized by dynamic recrystallization.In addition, there are also no significant meltphenomena in copper crater microstruc-tures, and the flow of material out of thetarget and into the rim is identical with theextreme solid-state plastic flow that charac-terizes shaped charge jet development. Dy-namic recrystallization also provides a mech-anism for this solid-state flow. Below thisrecrystallized zone, there is a zone of heavydeformation and grain distortion, withhigh dislocation densities and small dislo-cation cell sizes. This zone becomes an ex-tensive zone of microbands. No deformationtwins have been observed, and dislocationcells with increasing size and decreasing dis-

location density characterize the microstruc-ture beyond the microbands. Microbandsare coincident with traces of (111) planes infee metal targets and are apparently relatedto deformation twins. Microbands are alsoapparently related to grain size and are im-pact-velocity (or pressure) dependent. Re-sidual microstructural effects in hypervel-ocity impact craters can extend well beyondthe crater wall, to distances of roughly onecrater diameter below the crater bottom, forexample.

Finally, we observed significant micro-structural differences between long-rod pen-etration of an RHA target and that of a Ti-6AI-4V target. There was a zone of extremeplastic deformation surrounding the RHA,whereas only dense dislocation arrays veryclose to the penetration channel wall wereobserved in the Ti-alloy target. There was noevidence for melt phenomena in either tar-get; nor was there evidence for recrystalliza-tion near the wall in the Ti-alloy target.This research was supported in part by ArmyContract DAAA21-94-C-0059 through the ArmyResearch, Development, and Engineering Center,Picatinny Arsenal, New Jersey, and NASA-Johnson Space Center Grant NAG-9-481. APatricia Roberts Harris Fellowship supported J. C.Sanchez, and other students taking part in thisresearch have been supported by a Mr. and Mrs.Macintosh Murchison Endowed Chair. We thankMike Hespos of ARDEC, Picatinny, for providingEFPs, Dr. Lou Zemow, for providing shapedcharge components, and Dr. Fred H&z of NASA-JSC, for providing archived crater samples. Dr.Fred Grace of the Army Research Laboratory,Aberdeen, Maryland, provided the thick, rod-penetrated targets examined in this research.

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Blazynki, ed., Applied Science Publishers, NewYork (1983).5. M. A. Meyers: Dynamic Behavior of Materials.Wiley, New York (1994).

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Tantalum, A. Crawson and E. Chen, eds. TMS,Warrendal(l996) in press.S. A. Quinones, J. M. Rivas, E. P. Garcia, L. E.Murr, F. Horz, and R. P. Bernhard: Microstructureevolution associated with hypervelocity impactcraters in OFHC copper. In Metallurgical and Mate-rials Applications of Shock-Wave and High-Strain-Rate Phenomena, L. E. Murr, K. P. Staudhammer,and M. A. Meyers, eds., Elsevier Science, B.V., Am-sterdam, p. 293-312 (1995).J. M. Rivas, S. A. Quinones, and L. E. Murr: Hyper-velocity impact cratering: microstructural charac-terization. Ser. Metall. Mater. 33(1):101-107 (1995).J. M. Rivas, S. A. Quinones, E. I. Garcia, and L. E.Murr: Microstructural evolution associated withhypervelocity impact crater formation in metallictargets. In Metallurgical and Materials Applicationsof Shock-Wave and High-Strain-Rate Phenomena, L.E. Murr, K. P. Staudhammer, and M. A. Meyers,eds., Elsevier Science, B.V., Amsterdam, p. 313-323 (1995).M. Hallouin, F. Cottet, J. I. Romain, L. Monty, andM. Gerland: Microstructural transformations iniron induced by intense laser shock loading. In Im-

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Received March, 2996; accepted June 1996.

l. e. murr et al- novel deformation processes and microstructures involving ballistic penetrator...

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