TITANIUM AND TITANIUM ALLOYS
Titanium is a hexagonally close-packed metal with a density of 4,950 kg/m3; it can have a specific strength (the ratio of the yield strength to the density) that is greater than that of some (but not all) steels. Commercially pure titanium has a yield strength of about 400 MPa, with strong strain hardening and substantial rate sensitivity at high strain rates.1 Strengths of this magnitude are not sufficient to provide significant benefit in comparison to that of rolled homogeneous armor (RHA) for protection material applications, given the density of titanium. However, titanium alloys can have much greater strengths, and in particular the Ti-6Al-4V alloy has a strength approaching 1 GPa in the solution treated and aged condition. As a consequence, there is at least one specification of Ti-6Al-4V for armor applications,2and there are several specific components of military vehicles in which this titanium alloy has been substituted for steel, with significant weight savings.3 Titanium alloys have good corrosion resistance, offer good ballistic protection with some weight savings, and can be welded.
The primary obstacles to the expanded use of titanium as protection materials are twofold. First, and most important, is cost: the extraction, processing, and forming of titanium all result in a final component that is significantly more expensive than a component made of steel. Second, titanium alloys, like many hexagonally close-packed metals, have a relatively high susceptibility to adiabatic shear localization. These factors have resulted in the greater use of aluminum and aluminum alloys as substitutes for steels.
ALUMINUM AND ALUMINUM ALLOYS
Aluminum and aluminum alloys were developed early in the twentieth century, and beginning around the time of World War II, they were pressed into service to reduce the weight of protective materials (beginning with armor for aircraft). The introduction in the late 1950s of the T113 (later M113) personnel carrier using an aluminum alloy structure resulted in the deployment of a significant amount of aluminum alloys to the armored fleet. Whereas pure aluminum is very soft, conventional aluminum alloys can have yield strengths that easily compete with the simpler steels. Specific approaches such as solid solution strengthening and age hardening have been developed to strengthen aluminum alloys. Note that the range of strengths attainable with steels is very large, and there are no conventional aluminum alloys that can compete with the highest-strength steels in terms of yield strength. However, when one considers the specific strength (that is, the strength per unit weight, or sy/r), some of the commercial aluminum alloys can be very competitive.
FIGURE H-1 shows the typical specific strengths and specific stiffnesses of many metals and ceramics—the specific stiffnesses are of interest when deflection-limited design is important, as with some ceramic tiles, whereas specific strength is important for some strength-limited applications. Ceramics generally have higher specific strengths than metals and metal alloys, and ceramics indeed have a major role to play in protection material systems. The figure shows, among the metals, the relative locations of RHA and one aluminum alloy (Al 5083, which is 4.4 wt percent Mg, 0.7 wt percent Mn, and 0.15 wt percent Cr; the balance is Al). This alloy is commonly used in military vehicles such as personnel carriers.
A critical question for metals that meet both structural and armor roles in vehicles involves weldability, since this has a large impact on both production cost and maintenance. The welding of steels is a finely developed technology, but the weldability of aluminum alloys is much more variable.
1Meyers, M., G. Subhash, B. Kad, and L. Prasad. 1994. Evolution of microstructure and shear-band formation in α-hcp titanium. Mechanics of Materials 17(2-3): 175-193.
3Montgomery, J., M. Wells, B. Roopchand, and J. Ogilvy. 1997. Low-cost titanium armors for combat vehicles. Journal of the Minerals, Metals and Materials Society 49(5):45-47.
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Appendix H Metals as Lightweight Protection Materials TITANIUM AND TITANIUM ALLOYS ALUMINUM AND ALUMINUM ALLOYS Titanium is a hexagonally close-packed metal with a Aluminum and aluminum alloys were developed early density of 4,950 kg/m3; it can have a specific strength (the in the twentieth century, and beginning around the time of ratio of the yield strength to the density) that is greater than World War II, they were pressed into service to reduce the that of some (but not all) steels. Commercially pure titanium weight of protective materials (beginning with armor for has a yield strength of about 400 MPa, with strong strain aircraft). The introduction in the late 1950s of the T113 hardening and substantial rate sensitivity at high strain rates.1 (later M113) personnel carrier using an aluminum alloy Strengths of this magnitude are not sufficient to provide sig- structure resulted in the deployment of a significant amount nificant benefit in comparison to that of rolled homogeneous of aluminum alloys to the armored fleet. Whereas pure alu- armor (RHA) for protection material applications, given minum is very soft, conventional aluminum alloys can have the density of titanium. However, titanium alloys can have yield strengths that easily compete with the simpler steels. much greater strengths, and in particular the Ti-6Al-4V al- Specific approaches such as solid solution strengthening and loy has a strength approaching 1 GPa in the solution treated age hardening have been developed to strengthen aluminum and aged condition. As a consequence, there is at least one alloys. Note that the range of strengths attainable with steels specification of Ti-6Al-4V for armor applications,2 and there is very large, and there are no conventional aluminum alloys are several specific components of military vehicles in which that can compete with the highest-strength steels in terms this titanium alloy has been substituted for steel, with signifi- of yield strength. However, when one considers the specific strength (that is, the strength per unit weight, or sy/r), some cant weight savings.3 Titanium alloys have good corrosion resistance, offer good ballistic protection with some weight of the commercial aluminum alloys can be very competitive. savings, and can be welded. Figure H-1 shows the typical specific strengths and spe- The primary obstacles to the expanded use of titanium as cific stiffnesses of many metals and ceramics—the specific protection materials are twofold. First, and most important, stiffnesses are of interest when deflection-limited design is cost: the extraction, processing, and forming of titanium is important, as with some ceramic tiles, whereas specific all result in a final component that is significantly more ex- strength is important for some strength-limited applications. pensive than a component made of steel. Second, titanium Ceramics generally have higher specific strengths than met- alloys, like many hexagonally close-packed metals, have a als and metal alloys, and ceramics indeed have a major role relatively high susceptibility to adiabatic shear localization. to play in protection material systems. The figure shows, These factors have resulted in the greater use of aluminum among the metals, the relative locations of RHA and one and aluminum alloys as substitutes for steels. aluminum alloy (Al 5083, which is 4.4 wt percent Mg, 0.7 wt percent Mn, and 0.15 wt percent Cr; the balance is Al). This alloy is commonly used in military vehicles such as personnel carriers. 1Meyers, M., G. Subhash, B. Kad, and L. Prasad. 1994. Evolution of A critical question for metals that meet both structural microstructure and shear-band formation in α-hcp titanium. Mechanics of and armor roles in vehicles involves weldability, since this Materials 17(2-3): 175-193. 2MIL-T-9046J. has a large impact on both production cost and maintenance. 3Montgomery, J., M. Wells, B. Roopchand, and J. Ogilvy. 1997. Low-cost The welding of steels is a finely developed technology, but titanium armors for combat vehicles. Journal of the Minerals, Metals and the weldability of aluminum alloys is much more variable. Materials Society 49(5): 45-47. 142
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143 APPENDIX H 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-1 SpecificFigure versus specific strength of various stiffness H-1.eps Figure H-2 presents stress versus strain curves obtained materials, including metals and ceramics. The position occupied by bitmap on a trimodal aluminum alloy at strain rates of 3,200 s–1 and rolled homogeneous armor is identified, as is the conventional alu- 11,000 s–1 using a compression Kolsky bar. Strength levels minum alloy 5083. Note the substantially greater specific strength of this magnitude are remarkable for an aluminum-based that can be obtained by using aluminum-based nanocrystalline matrix composites such as the so-called trimodal aluminum materi - material. The mechanical response of the most common als. SOURCE: Zhang, H., J. Ye, S. Joshi, J. Schoenung, E. Chin, current armor steel (RHA) measured at similar strain rates is G. Gazonas, and K. Ramesh: Superlightweight nanoengineered also shown in Figure H-2—note that this steel is nearly three aluminum for strength under impact. Advanced Engineering Ma- times as dense as the aluminum alloy. The specific strength terials. 2007. 9. 335-423. Copyright Wiley-VCH Verlag GmbH & of the trimodal material is also shown in Figure H-2. Co. KGaA. Reproduced with permission. 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 Those aluminum alloys that are easily weldable are therefore the refinement of grain size, (2) particle-size strengthening preferred in these applications, even if some penalty is paid through ceramic reinforcement, (3) dispersoid strengthening, in terms of strength and ballistic performance. The trade-offs and (4) work-hardening owing to prior plastic work from between weight, structural performance, ballistic perfor- extrusion and cryomilling. This material can be considered mance, ease of production, and ease of maintenance (includ- to be a sophisticated alloy, a nanostructured material, or a ing resistance to corrosion) play a very significant role in the specific metal-matrix composite—the value is in the use of choice of alloy for vehicular applications. Because most of all of the associated strengthening mechanisms. these alloys are used as rolled plate, work-hardening alloys Advanced aluminum-based materials of this type, in- such as the 5000 series (Al 5083 being the prime example) cluding wrought alloys such as Al 2139 and aluminum-based have some advantages. Aluminum alloys used as armor in metal-matrix composites, discussed below, show promise of Army vehicles also include Al 2024, Al 2519, Al 5059, Al dramatic improvements as protection materials in terms of 6061, Al 7039, and Al 7075. Promising new commercial mass efficiency. The key research questions in terms of the alloys include Al 2139, which is a commercial alloy with utility of such advanced materials are those concerning the significant strength (around 600 MPa at high strain rates) failure processes within the material: ductility, resistance 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- 4Li, Y., Y.H. Zhao, V. Ortalan, W. Liu, Z.H. Zhang, R.G. Vogt, N.D. tured systems, and the development of aluminum-based com- Browning, E.J. Lavernia, and J.M. Schoenung. 2009. Investigation of posites. The nanostructured aluminum approach is exempli- aluminum-based nanocomposites with ultra-high strength. Materials Sci - fied by the so-called trimodal aluminum material developed ence and Engineering: A 527(1-2): 305-316.
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144 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Figure H-2.eps 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. bitmap 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. that can substitute for some aluminum alloys include AZ315 to crack growth, resistance to spall, and resistance to shear band development. 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, MAGNESIUM AND MAGNESIUM ALLOYS which may present a long-term risk for the United States. Magnesium has a remarkably low density of 1,700 kg/ A recent workshop at the Johns Hopkins University on the m3 (in comparison, the density of Al is 2,800 kg/m3, that of potential of magnesium and magnesium alloys as protection Ti is 4,950 kg/m3 and those of steels are 7,800 kg/m3). The materials highlighted a variety of opportunities. One of the density of magnesium approaches that of polymers. Magne- more promising strengthening approaches appears to be the sium and magnesium alloys, which are among the lightest development of ultra-fine-grained or nanostructured mag- structural metals, are becoming increasingly important in nesium alloys through severe plastic deformation. A major the automotive and hand-tool industries. The rapid growth research effort in developing a fundamental understanding in the commercial use of magnesium is intimately tied to of strengthening mechanisms in magnesium alloys promises the increasing cost of energy. The low density makes these to be fruitful, and the opportunities presented by low-density materials very attractive for defense applications, but magne- alloys should not be missed. sium alloys historically have had relatively low strengths (in Since magnesium is a hexagonally close-packed mate- the range 250-300 MPa) in comparison to aluminum alloys. rial, the plastic deformation of this metal is much more com- There has also been lingering (and somewhat exaggerated) plex than that of cubic metals like aluminums and steels. Two concern about the flammability of magnesium and about features of the plastic deformation are particularly important: the relative ease with which these alloys can be corroded in the development of deformation twins and the development severe environments. However, these potential problems are of strong textures. Both topics require careful investigation relatively easily mitigated by proper design and the appropri- in order to increase the utility of magnesium-based materials ate protocols for maintenance. as components of protection material systems. A substantial effort was begun over the past decade to generate high-strength magnesium alloys using a variety 5Mukai, T., M. Yamanoi, H. Watanabe, and K. Higashi. 2001. Ductility of approaches, including solid solution strengthening and enhancement in AZ31 magnesium alloy by controlling its grain structure. precipitation strengthening. Commercial magnesium alloys Scripta Materialia 45(1): 89-94.
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145 APPENDIX H 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 FIGURE H-3 Optical micrograph of Al-SiC cermet. Aluminum matrix.6 Figure H-3.eps is the light-gray matrix, with discrete silicon carbide particles. A number of metal-matrix composites show potential SOURCE: Unpublished research. Permission granted by K.T. bitmap for protective material applications. Typically these materials Ramesh. 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 aluminum metal and the ceramic, forming a strong interphase result of incorporating a ceramic reinforcement into a metal- bond. In situ processes for making cermets—such as Lanx- lic matrix is enhanced strength and some loss of ductility. ide’s PRIMEX process, Martin Marietta’s XD process, self- Most of the MMCs used commercially are aluminum-based propagating high temperature, and reactive gas injections— and ceramic-reinforced,7 and these have been investigated have also been developed.10,11,12,13 The PRIMEX process thoroughly. However, there is also potential for magnesium- involves cermet fabrication under a pressureless condition, based systems and steel-based systems. Such MMCs could in which a spontaneous infiltration of molten aluminum into also lead to the development of functionally graded materials a porous ceramic preform in the presence of magnesium that have microstructures graded to provide optimum resis- and nitrogen occurs without using vacuum or externally tance to a specific threat. The high-strain-rate mechanical applied pressure.14 A cermet material in the form of silicon properties and dynamic failure processes in MMCs (see, carbide-aluminum was produced by Lanxide Armor Products for example, Li and Ramesh, 1998,8 and Li et al., 20009) and was employed to protect against artillery fragments and have not been investigated in detail, and further work in this small arms. It has largely been replaced, however, by an area is likely to be very useful in the development of armor improved material developed by M Cubed Technologies, in packages in which the MMC may be used as a backing for which the SiC+C and B4C+SiC are infiltrated with molten a ceramic material. silicon to form a tough SiC bonding phase that provides su- The conventional method for fabrication of MMCs is perior performance as a cermet armor protection material. A to compress a porous compact of ceramic powder to ap- lightweight cermet material was also developed at Lawrence proximately 65 percent of its theoretical density, leaving Livermore National Laboratory, using boron carbide for the an open and continuous pore phase, which can be readily ceramic compact, backfilled with aluminum metal, and sub- infiltrated with molten metal, usually aluminum. Finally, the compact undergoes a heat-treatment process at a somewhat 10Mortensen, A., and I. Jin. 1992. Solidification processing of metal more elevated temperature, causing a reaction between the matrix composites. International Materials Review 37: 101-128. 11Ibrahim, A., F. Mohamed, and E. Lavernia. 1991. Particulate reinforced 6Uribe, Y., and H. Sohn, unpublished research. metal matrix composites—A review. Journal of Materials Science 26(5): 7Lloyd, D. 1994. Particle reinforced aluminum and magnesium matrix 1137-1156. 12Koczak, M., and M. Premkumar. 1993. Emerging technologies for composites. International Materials Reviews 39(1):1-23. 8Li, Y., and K. Ramesh. 1998. Influence of particle volume fraction, the in situ production of MMC’s. The Journal of the Minerals, Metals, and shape, and aspect ratio on the behavior of particle-reinforced metal–matrix Materials Society 45(1): 44-48. 13Asthana, R. 1998. Reinforced cast metals: part I solidification micro - composites at high rates of strain. Acta Materialia 46(16): 5633-5646. 9Li, Y., K. Ramesh, and E. Chin. 2000. The compressive viscoplastic structure. Journal of Materials Science 33(7): 1679-1698. 14Aghajanian, M., A. Rocazella, J. Burke, and S. Keck. 1991. The fabri - response of an A359/SiCp metal-matrix composite and of the A359 alumi- num alloy matrix. International Journal of Solids and Structures 37(51): cation of metal matrix composites by a pressureless infiltration technique. 7547-7562. Journal of Materials Science 26(2): 447-454.
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146 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS As noted by Chin,20 in addition to particulate-reinforced sequently heat-treated to form a delta phase chemical bond between the ceramic and the metal. The processing of B4C cermets with excellent work-hardening characteristics under and Al composites, especially when the B4C content is high dynamic loading, the functionally graded armor composites (above 55 vol percent), faces the problem of poor wettability (FGACs) were developed. In FGACs, ballistic space and of the aluminum on B4C at b temperatures, especially near mass efficiency of cermets were enhanced by tailoring the the melting point of aluminum (660°C). Aluminum begins through-thickness incorporation and distribution of various to wet the B4C surface at temperatures just above 1000°C, reinforcement morphologies, sizes, and chemistries to miti- which results in an increase in the driving force of chemical gate shock damage. The idea of improving FGAC perfor- reactions. The high temperatures (1000°C to 1200°C) used mance is to disrupt the shock wave in order to minimize col- for improved infiltration increase the wettability of the ma- lateral damage during a ballistic event. The FGAC structure terials, but at the same time, chemical reactions between Al is composed of a series—a hard (ceramic) layer interspersed and B4C can result in the formation of intermediate phases, with a high strain-to-failure material such as aluminum. The such as binary AlB2, b-AlB12, AlB10, borides, and ternary hard outer surface is usually designed to be the ballistic im- Al-borocarbides AlB24C4, Al3B48C2, and Al3BC.15 Al3C4 is pact layer, and behind this layer is a thin-bonded layer of the also formed. It has been reported that about 30 vol percent of ductile material. The design feature is such that in successive new phases are formed from initially 38 vol percent alumi- layers going toward the back surface, the volume fraction of num and 62 vol percent B4C.16 Al4C3 is the most undesirable the ductile material is increased and the volume fraction of phase because of its hygroscopic nature and pure mechanical the hard layer is decreased. Thus, the strain-to-failure ratio properties. Some products of the interfacial reactions are not is increased as the depth of the penetration increases. The desirable and can cause premature failure and poor ballistic perturbations will be tailored throughout the microstructural performance, while other interphases are desired and even design, which prolongs projectile-through-target-material required to form a good interfacial bond and bring significant dwell time. The extended dwell time promotes the break- strengthening and high tensile strength of the cermet. ing up of the projectile prior to the occurrence of complete It is understood, however, that for an armor cermet mate- penetration or unacceptable collateral damage of the armor material.21 There is a clear realization of the importance of rial to be of high quality, a clean metallurgical interface be- tween the ceramic reinforcement and metal matrix is highly and need for a better understanding of the character of the desirable, since it allows a more effective strengthening from interfaces in FGAC because of the softening of the material the reinforcement.17 To avoid formation of intermediate due to interfacial and particle damage from high-rate loading. interphases, low-temperature cryomilling was developed to The self-propagating high-temperature synthesis meth- synthesize a composite powder with clean metallurgical in- odology is another important technique used to produce terfaces and without voids.18 In addition, to increase the duc- metal-matrix composites where dissimilar phases (metal and tility, which is always sacrificed when strength is increased, ceramics) are integrated through a self-propagating exother- mic reaction.22 The development of nanoscale, multilayer, a trimodal Al-B4C cermet was developed, in which coarse- grained aluminum was introduced into the nanocrystalline self-propagating exothermic reaction foils, which can be Al reinforced with B4C particles.19 A trimodal composition ignited by a simple electrical spark, is important for joining with 10 wt percent B4C, 50 wt percent coarse-grained Al FGACs to a wide range of structural surfaces as well as for 5083, and the remainder nanocrystalline Al 5083 exhibited modular armor repair. 1,065 MPa yield strength under compressive loading while In summary, cermet materials exhibit light weight and still showing 0.04 true strain deformation. 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 15Lee, K., B, Sim, S. Cho, and H. Kwon. 1991. Reaction products of Al- not always been fully realized, owing to poorly understood Mg/B4C composite fabricated by pressureless infiltration technique. Journal interfacial bonding and properties. The field of armor cer- of Materials Science and Engineering A 302(2): 227-234. mets is, therefore, ripe for exploitation using combinations 16Beidler, C., W. Hauth, and A. Goel, 1992. Development of a B C/Al 4 of the common refractory ceramic materials (alumina, silicon cermet for use as an improved structural neutron absorber. Journal of Testing carbide, boron carbide) and light metals such as magnesium, and Evaluation 20(1): 57-60. 17Lloyd, D. 1992. Particle reinforced aluminium and magnesium matrix titanium, and aluminum. Cermets have been successfully composites. International Materials Reviews 39(1): 1-23. 18Schoenung, J., J. Ye, J. He, F. Tang, and D. Witkin. 2005. B C rein- 4 20Chin, forced nanocrystalline aluminum composites: Synthesis, characterization, E. 1999. Army focused research team on functionally graded and cost analysis. Pp. 123-128 in Materials Forum Volume 29. J.F. Nie and armor composites. Materials Science and Engineering A 259(2): 155-161. 21Ibid. M. Barnett, eds. Institute of Materials Engineering Australia Ltd. 19Ye, J., B. Han, Z. Lee, B. Ahn, S. Nutt, and J. Schoenung. 2005. A 22Michaelsen, C., K. Barmak, and T.Weihs. 1997. Investigating the trimodal aluminum based composite with super-high strength. Scripta thermodynamics and kinetics of thin film reactions by differential scanning Materialia 53(5): 481-486. calorimetry. Journal of Physics D: Applied Physics 30(23): 3167-3186.
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147 APPENDIX H used in armor protection applications because of their rug- relatively low-melting metal phases. For higher-temperature gedness and ability to withstand impact, but the best proper- components, special fabrication techniques are needed. ties of each component phase are often not fully realized in Mechanistic research on high-temperature ceramic-metal the composite structure. Cermets can be fabricated in a rela - bonding in cermets, the fabrication of these structures, and tively straightforward manner and in a wide variety of forms, their relationship to projectile defeat and armor performance but most MMCs, like aluminum, have been developed with can productively be researched.