APPENDIX H 143
Those aluminum alloys that are easily weldable are therefore preferred in these applications, even if some penalty is paid in terms of strength and ballistic performance. The trade-offs between weight, structural performance, ballistic perfor- mance, ease of production, and ease of maintenance (includ- ing resistance to corrosion) play a very significant role in the choice of alloy for vehicular applications. Because most of these alloys are used as rolled plate, work-hardening alloys such as the 5000 series (Al 5083 being the prime example) have some advantages. Aluminum alloys used as armor in Army vehicles also include Al 2024, Al 2519, Al 5059, Al 6061, Al 7039, and Al 7075. Promising new commercial alloys include Al 2139, which is a commercial alloy with significant strength (around 600 MPa at high strain rates) and reasonable ductility.
There is significant potential for the development of novel aluminum-based materials with very high strengths through alloying approaches, the development of nanostruc- tured systems, and the development of aluminum-based com- posites. The nanostructured aluminum approach is exempli- fied by the so-called trimodal aluminum material developed
by Li and Zhao and their coworkers.4 This aluminum-based material exhibits a very high strength (950-1,000 MPa) when loaded at high strain rates, although the ductility (as of 2009) is relatively low. The material achieves dramatic mechanical properties at impact rates of deformation through a combination of three microstructural approaches: strength- ening through a nanocrystalline core architecture; additional strengthening through length-scale-dependent reinforcement with micron-size ceramic particles; and enhanced ductility through the incorporation of a certain volume fraction of micron-scale grains. The resulting trimodal aluminum- based material achieves high specific strengths under very high rates of deformation and shows promise as a protective material, although the ductility remains a major concern.
The material is produced by cryomilling Al 5083 aluminum powders with boron carbide ceramic particulates. This com- posite powder is then degassed and blended with microscale Al 5083. This trimodal composite powder is then consoli- dated with conventional powder metallurgy techniques such as cold isostatic pressing plus extrusion to generate a bulk trimodal aluminum-based composite.
Figure H-2 presents stress versus strain curves obtained on a trimodal aluminum alloy at strain rates of 3,200 s–1 and 11,000 s–1 using a compression Kolsky bar. Strength levels of this magnitude are remarkable for an aluminum-based material. The mechanical response of the most common current armor steel (RHA) measured at similar strain rates is also shown in Figure H-2—note that this steel is nearly three times as dense as the aluminum alloy. The specific strength of the trimodal material is also shown in Figure H-2.
Mechanical milling, temperature and consolidation lead to a peculiar microstructure for this material; as a result its strength is derived from, in addition to the normal load transfer characteristics of the composite, four strengthening mechanisms. They are (1) grain boundary strengthening, via the refinement of grain size, (2) particle-size strengthening through ceramic reinforcement, (3) dispersoid strengthening, and (4) work-hardening owing to prior plastic work from extrusion and cryomilling. This material can be considered to be a sophisticated alloy, a nanostructured material, or a specific metal-matrix composite—the value is in the use of all of the associated strengthening mechanisms.
Advanced aluminum-based materials of this type, in- cluding wrought alloys such as Al 2139 and aluminum-based metal-matrix composites, discussed below, show promise of dramatic improvements as protection materials in terms of mass efficiency. The key research questions in terms of the utility of such advanced materials are those concerning the failure processes within the material: ductility, resistance
4Li, Y., Y.H. Zhao, V. Ortalan, W. Liu, Z.H. Zhang, R.G. Vogt, N.D.
Browning, E.J. Lavernia, and J.M. Schoenung. 2009. Investigation of aluminum-based nanocomposites with ultra-high strength. Materials Sci- ence and Engineering: A 527(1-2): 305-316.
Figure H-1.eps bitmap
FIGURE H-1 Specific stiffness versus specific strength of various materials, including metals and ceramics. The position occupied by rolled homogeneous armor is identified, as is the conventional alu- minum alloy 5083. Note the substantially greater specific strength that can be obtained by using aluminum-based nanocrystalline matrix composites such as the so-called trimodal aluminum materi- als. SOURCE: Zhang, H., J. Ye, S. Joshi, J. Schoenung, E. Chin, G. Gazonas, and K. Ramesh: Superlightweight nanoengineered aluminum for strength under impact. Advanced Engineering Ma- terials. 2007. 9. 335-423. Copyright Wiley-VCH Verlag GmbH &
Co. KGaA. Reproduced with permission.
144 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
to crack growth, resistance to spall, and resistance to shear band development.
MAGNESIUM AND MAGNESIUM ALLOYS
Magnesium has a remarkably low density of 1,700 kg/
m3 (in comparison, the density of Al is 2,800 kg/m3, that of Ti is 4,950 kg/m3 and those of steels are 7,800 kg/m3). The density of magnesium approaches that of polymers. Magne- sium and magnesium alloys, which are among the lightest structural metals, are becoming increasingly important in the automotive and hand-tool industries. The rapid growth in the commercial use of magnesium is intimately tied to the increasing cost of energy. The low density makes these materials very attractive for defense applications, but magne- sium alloys historically have had relatively low strengths (in the range 250-300 MPa) in comparison to aluminum alloys.
There has also been lingering (and somewhat exaggerated) concern about the flammability of magnesium and about the relative ease with which these alloys can be corroded in severe environments. However, these potential problems are relatively easily mitigated by proper design and the appropri- ate protocols for maintenance.
A substantial effort was begun over the past decade to generate high-strength magnesium alloys using a variety of approaches, including solid solution strengthening and precipitation strengthening. Commercial magnesium alloys
that can substitute for some aluminum alloys include AZ315 and ZK60, and several alloys containing rare earths show promise. Most of the innovation in this area is currently oc- curring outside this country, particularly in China and Japan, which may present a long-term risk for the United States.
A recent workshop at the Johns Hopkins University on the potential of magnesium and magnesium alloys as protection materials highlighted a variety of opportunities. One of the more promising strengthening approaches appears to be the development of ultra-fine-grained or nanostructured mag- nesium alloys through severe plastic deformation. A major research effort in developing a fundamental understanding of strengthening mechanisms in magnesium alloys promises to be fruitful, and the opportunities presented by low-density alloys should not be missed.
Since magnesium is a hexagonally close-packed mate- rial, the plastic deformation of this metal is much more com- plex than that of cubic metals like aluminums and steels. Two features of the plastic deformation are particularly important:
the development of deformation twins and the development of strong textures. Both topics require careful investigation in order to increase the utility of magnesium-based materials as components of protection material systems.
5Mukai, T., M. Yamanoi, H. Watanabe, and K. Higashi. 2001. Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure.
Scripta Materialia 45(1): 89-94.
Figure H-2.eps bitmap
FIGURE H-2 High-strain-rate compressive response of a trimodal aluminum alloy, in comparison with that of rolled homogeneous armor at similar strain rates (103 s–1). Curve 4 represents not experimental data but the prediction of a model based on composite micromechanics.
SOURCE: Zhang, H., J. Ye, S. Joshi, J. Schoenung, E. Chin, G. Gazonas, and K. Ramesh: Superlightweight nanoengineered aluminum for strength under impact. Advanced Engineering Materials. 2007. 9. 335-423. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
APPENDIX H 145
CERMETS
The term “cermet” describes a structure that is a composite mixture of a metal phase and a ceramic phase.
The combination of ceramic and metal in cermets works synergistically to improve the toughness of the composite material: The ceramic phase is a strengthening (a “hard”
material) phase with the function of breaking or eroding the penetrator, and the ductile metal phase inhibits failure. The metals usually used are aluminum, magnesium, and titanium.
Because of the synergism between the two materials, which in concert can defeat an incoming kinetic energy penetra- tor, cermets have a significant potential for expanded use in lightweight armor development. Cermets can be divided into two subgroups: ceramic-matrix composites and metal-matrix composites (MMC), depending on whether the ceramic is in continuous or matrix phase. Figure H-3 shows a micrograph of an MMC with a dispersed SiC phase in an aluminum matrix.6
A number of metal-matrix composites show potential for protective material applications. Typically these materials consist of ceramic particulate or ceramic fiber reinforcements within a ductile metal matrix, with the volume fractions of the reinforcements ranging from 5 to 50 percent. The typical result of incorporating a ceramic reinforcement into a metal- lic matrix is enhanced strength and some loss of ductility.
Most of the MMCs used commercially are aluminum-based and ceramic-reinforced,7 and these have been investigated thoroughly. However, there is also potential for magnesium- based systems and steel-based systems. Such MMCs could also lead to the development of functionally graded materials that have microstructures graded to provide optimum resis- tance to a specific threat. The high-strain-rate mechanical properties and dynamic failure processes in MMCs (see, for example, Li and Ramesh, 1998,8 and Li et al., 20009) have not been investigated in detail, and further work in this area is likely to be very useful in the development of armor packages in which the MMC may be used as a backing for a ceramic material.
The conventional method for fabrication of MMCs is to compress a porous compact of ceramic powder to ap- proximately 65 percent of its theoretical density, leaving an open and continuous pore phase, which can be readily infiltrated with molten metal, usually aluminum. Finally, the compact undergoes a heat-treatment process at a somewhat more elevated temperature, causing a reaction between the
6Uribe, Y., and H. Sohn, unpublished research.
7Lloyd, D. 1994. Particle reinforced aluminum and magnesium matrix composites. International Materials Reviews 39(1):1-23.
8Li, Y., and K. Ramesh. 1998. Influence of particle volume fraction, shape, and aspect ratio on the behavior of particle-reinforced metal–matrix composites at high rates of strain. Acta Materialia 46(16): 5633-5646.
9Li, Y., K. Ramesh, and E. Chin. 2000. The compressive viscoplastic response of an A359/SiCp metal-matrix composite and of the A359 alumi- num alloy matrix. International Journal of Solids and Structures 37(51):
7547-7562.
aluminum metal and the ceramic, forming a strong interphase bond. In situ processes for making cermets—such as Lanx- ide’s PRIMEX process, Martin Marietta’s XD process, self- propagating high temperature, and reactive gas injections—
have also been developed.10,11,12,13 The PRIMEX process involves cermet fabrication under a pressureless condition, in which a spontaneous infiltration of molten aluminum into a porous ceramic preform in the presence of magnesium and nitrogen occurs without using vacuum or externally applied pressure.14 A cermet material in the form of silicon carbide-aluminum was produced by Lanxide Armor Products and was employed to protect against artillery fragments and small arms. It has largely been replaced, however, by an improved material developed by M Cubed Technologies, in which the SiC+C and B4C+SiC are infiltrated with molten silicon to form a tough SiC bonding phase that provides su- perior performance as a cermet armor protection material. A lightweight cermet material was also developed at Lawrence Livermore National Laboratory, using boron carbide for the ceramic compact, backfilled with aluminum metal, and sub-
10Mortensen, A., and I. Jin. 1992. Solidification processing of metal matrix composites. International Materials Review 37: 101-128.
11Ibrahim, A., F. Mohamed, and E. Lavernia. 1991. Particulate reinforced metal matrix composites—A review. Journal of Materials Science 26(5):
1137-1156.
12Koczak, M., and M. Premkumar. 1993. Emerging technologies for the in situ production of MMC’s. The Journal of the Minerals, Metals, and Materials Society 45(1): 44-48.
13Asthana, R. 1998. Reinforced cast metals: part I solidification micro- structure. Journal of Materials Science 33(7): 1679-1698.
14Aghajanian, M., A. Rocazella, J. Burke, and S. Keck. 1991. The fabri- cation of metal matrix composites by a pressureless infiltration technique.
Journal of Materials Science 26(2): 447-454.
Figure H-3.eps bitmap
FIGURE H-3 Optical micrograph of Al-SiC cermet. Aluminum is the light-gray matrix, with discrete silicon carbide particles.
SOURCE: Unpublished research. Permission granted by K.T.
Ramesh.
146 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS
sequently heat-treated to form a delta phase chemical bond between the ceramic and the metal. The processing of B4C and Al composites, especially when the B4C content is high (above 55 vol percent), faces the problem of poor wettability of the aluminum on B4C at b temperatures, especially near the melting point of aluminum (660°C). Aluminum begins to wet the B4C surface at temperatures just above 1000°C, which results in an increase in the driving force of chemical reactions. The high temperatures (1000°C to 1200°C) used for improved infiltration increase the wettability of the ma- terials, but at the same time, chemical reactions between Al and B4C can result in the formation of intermediate phases, such as binary AlB2, b-AlB12, AlB10, borides, and ternary Al-borocarbides AlB24C4, Al3B48C2, and Al3BC.15 Al3C4 is also formed. It has been reported that about 30 vol percent of new phases are formed from initially 38 vol percent alumi- num and 62 vol percent B4C.16 Al4C3 is the most undesirable phase because of its hygroscopic nature and pure mechanical properties. Some products of the interfacial reactions are not desirable and can cause premature failure and poor ballistic performance, while other interphases are desired and even required to form a good interfacial bond and bring significant strengthening and high tensile strength of the cermet.
It is understood, however, that for an armor cermet mate- rial to be of high quality, a clean metallurgical interface be- tween the ceramic reinforcement and metal matrix is highly desirable, since it allows a more effective strengthening from the reinforcement.17 To avoid formation of intermediate interphases, low-temperature cryomilling was developed to synthesize a composite powder with clean metallurgical in- terfaces and without voids.18 In addition, to increase the duc- tility, which is always sacrificed when strength is increased, a trimodal Al-B4C cermet was developed, in which coarse- grained aluminum was introduced into the nanocrystalline Al reinforced with B4C particles.19 A trimodal composition with 10 wt percent B4C, 50 wt percent coarse-grained Al 5083, and the remainder nanocrystalline Al 5083 exhibited 1,065 MPa yield strength under compressive loading while still showing 0.04 true strain deformation.
15Lee, K., B, Sim, S. Cho, and H. Kwon. 1991. Reaction products of Al- Mg/B4C composite fabricated by pressureless infiltration technique. Journal of Materials Science and Engineering A 302(2): 227-234.
16Beidler, C., W. Hauth, and A. Goel, 1992. Development of a B4C/Al cermet for use as an improved structural neutron absorber. Journal of Testing and Evaluation 20(1): 57-60.
17Lloyd, D. 1992. Particle reinforced aluminium and magnesium matrix composites. International Materials Reviews 39(1): 1-23.
18Schoenung, J., J. Ye, J. He, F. Tang, and D. Witkin. 2005. B4C rein- forced nanocrystalline aluminum composites: Synthesis, characterization, and cost analysis. Pp. 123-128 in Materials Forum Volume 29. J.F. Nie and M. Barnett, eds. Institute of Materials Engineering Australia Ltd.
19Ye, J., B. Han, Z. Lee, B. Ahn, S. Nutt, and J. Schoenung. 2005. A trimodal aluminum based composite with super-high strength. Scripta Materialia 53(5): 481-486.
As noted by Chin,20 in addition to particulate-reinforced cermets with excellent work-hardening characteristics under dynamic loading, the functionally graded armor composites (FGACs) were developed. In FGACs, ballistic space and mass efficiency of cermets were enhanced by tailoring the through-thickness incorporation and distribution of various reinforcement morphologies, sizes, and chemistries to miti- gate shock damage. The idea of improving FGAC perfor- mance is to disrupt the shock wave in order to minimize col- lateral damage during a ballistic event. The FGAC structure is composed of a series—a hard (ceramic) layer interspersed with a high strain-to-failure material such as aluminum. The hard outer surface is usually designed to be the ballistic im- pact layer, and behind this layer is a thin-bonded layer of the ductile material. The design feature is such that in successive layers going toward the back surface, the volume fraction of the ductile material is increased and the volume fraction of the hard layer is decreased. Thus, the strain-to-failure ratio is increased as the depth of the penetration increases. The perturbations will be tailored throughout the microstructural design, which prolongs projectile-through-target-material dwell time. The extended dwell time promotes the break- ing up of the projectile prior to the occurrence of complete penetration or unacceptable collateral damage of the armor material.21 There is a clear realization of the importance of and need for a better understanding of the character of the interfaces in FGAC because of the softening of the material due to interfacial and particle damage from high-rate loading.
The self-propagating high-temperature synthesis meth- odology is another important technique used to produce metal-matrix composites where dissimilar phases (metal and ceramics) are integrated through a self-propagating exother- mic reaction.22 The development of nanoscale, multilayer, self-propagating exothermic reaction foils, which can be ignited by a simple electrical spark, is important for joining FGACs to a wide range of structural surfaces as well as for modular armor repair.
In summary, cermet materials exhibit light weight and excellent ballistic properties suitable for personnel armor use. However, cermets have not been extensively utilized in armor protection applications, in part due to high fabrication costs but also because the optimal composite properties have not always been fully realized, owing to poorly understood interfacial bonding and properties. The field of armor cer- mets is, therefore, ripe for exploitation using combinations of the common refractory ceramic materials (alumina, silicon carbide, boron carbide) and light metals such as magnesium, titanium, and aluminum. Cermets have been successfully
20Chin, E. 1999. Army focused research team on functionally graded armor composites. Materials Science and Engineering A 259(2): 155-161.
21Ibid.
22Michaelsen, C., K. Barmak, and T.Weihs. 1997. Investigating the thermodynamics and kinetics of thin film reactions by differential scanning calorimetry. Journal of Physics D: Applied Physics 30(23): 3167-3186.
APPENDIX H 147
used in armor protection applications because of their rug- gedness and ability to withstand impact, but the best proper- ties of each component phase are often not fully realized in the composite structure. Cermets can be fabricated in a rela- tively straightforward manner and in a wide variety of forms, but most MMCs, like aluminum, have been developed with
relatively low-melting metal phases. For higher-temperature components, special fabrication techniques are needed.
Mechanistic research on high-temperature ceramic-metal bonding in cermets, the fabrication of these structures, and their relationship to projectile defeat and armor performance can productively be researched.