Superlightweight Nanoengineered Aluminum for Strength under Impact

The search for better armor materials is intimately tied to the history of mankind, and the rise and fall of civilizations is sometimes related to the development of new armor systems . An ideal armor material is expected to have three attributes: sufficient strength at high impact velocities, sufficient toughness to carry normal structural loads, and low weight. The degree to which each attribute is desirable depends both on what is being protected (a vehicle or an individual) and on the specific threat (e.g. an anti-vehicle weapon or a bullet). Materials that have all three attributes have been extraordinarily difficult to find, and much current vehicular armor uses combinations of multiple materials, one of which carries the bulk of the structural load and another, which has the appropriate impact response. Constructing such “armor packages” with sufficiently low weight often defines the limits of vehicular weight, leading directly to limits on the ability to rapidly project military power and constraints on geopolitical operations during global conflicts. Materials used for armor have evolved from natural sources such as wood and leather to synthetics that include fibers, monolithics and hybrid composites of metals, ceramics and polymers. Recent advances in armor materials include aramid fibers for ballistic vests and boron carbide for small arms protective inserts (SAPI) plates. In a monolithic armor design, the material is modified to attain a balanced range of properties to prevent penetration and fragmentation. This is a typical compromise between hardness and ductility in most materials. In modern armor designs, discrete layers of materials are functionally optimized to sequentially shatter/erode the projectile followed by containing all residual debris. This strategy maximizes the gains from each contributing component. However, the performance of such an armor system is bounded by the weighted average of the macro-components in the armor recipe. In this paper, a selectively dispersed nano/micro component material (TriMod) is demonstrated to exhibit extraordinary dynamic flow strength that far exceeds any of the respective individual constituents. This watershed innovation has created a new path to design native armor materials with properties exceeding that of layered armor designs, without compromises. Aluminum (Al) alloys are the most widely used metals in technologies where weight reduction is a major design consideration, and specific approaches such as solid solution strengthening and age-hardening have been developed to strengthen Al-based materials. However, the total strength that is attainable with these approaches is relatively small, and the best traditional aluminum armor materials (such as Al-5083, with 4.4 wt.% Mg, 0.7 wt.% Mn, and 0.15 wt.% Cr and the balance Al) do not have significant advantages over the better armor steels (Fig. 1). We describe here a nanoengineered aluminum-based material that has remarkable properties relevant to armor systems. The material achieves dramatic mechanical properties at impact rates of deformation through a combination of three micro-structural length scale approaches: strengthening through a nanocrystalline core architecture, additional strengthening through length-scale deC O M M U N IC A IO N S