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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 aluminum 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 materials. 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.

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 performance, ease of production, and ease of maintenance (including 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 nanostructured systems, and the development of aluminum-based composites. The nanostructured aluminum approach is exemplified 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:strengthening 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 composite powder is then degassed and blended with microscale Al 5083. This trimodal composite powder is then consolidated 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, including 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

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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 Science and Engineering: A 527(1-2): 305-316.



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