to increase stiffness when present in sufficiently small size (about 1 µm), high concentration, and homogeneous distribution. Invented by an interdisciplinary team, the new materials, termed XD alloys, exhibit specific moduli up to 40 percent greater than the base metal.²⁹ The process by which these alloys are produced is proprietary, but the prospects for such materials appear very encouraging because, once formed, they can be processed (for example, cast, rolled, extruded, or welded) by conventional means.

To date, the XD process for producing ceramic-stiffened metallic materials has been demonstrated for Al, Cu, Fe, Mg, Ni, Pb, and Ti alloys, and also for Ti and Ni aluminides. Their outstanding properties apparently result not only from the uniform dispersion of fine ceramic particles but also from the “cleanliness” of the ceramic particle-metal interface, a result not readily achieved in conventional powder metallurgy processing. Figure 5 compares the modulus of XD-Al alloys with SiC-reinforced Al, and Figure 6 shows the improved high-temperature performance of XD-7075 Al over the conventional 7075 alloy. Other demonstrated advantages of XD-aluminum alloys

FIGURE 5 Relation between Young’s modulus and concentration of ceramic phase for XD-Al and SiC-Al alloys. Insert shows typical microstructure of XD-Al material. From Christodoulou, Brupbacher, and Nagle.²⁹