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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.
OCR for page 146
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.
