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IN the last decade, discontinuously reinforced aluminum (Al) matrix composites (DRAs) have been developed to the extent of being viable structural materials for a number of “niche” applications in various industrial sectors.[1,2] These composites, being a subcategory of metal matrix composites (MMCs), are reinforced mainly by ceramic particles or whiskers of either silicon carbide (SiC) or alumina (Al2O3). Ceramic reinforcement has the advantage of a relatively low density (⬃3 g/cm3) and high elastic modulus (200 to 400 GPa).[3] For example, the elastic modulus of a SiC particulate is reported to be 380 GPa at the density of 3.21 g/cm3.[4] Since the late 1980s, DRAs have been processed by two principal methods: either casting or powder metallurgy.[5–8] Since the final product is a mixture of essentially two completely dissimilar raw materials, one metallic and the other ceramic, the processing of DRAs is very complex, costly, and requires dedicated equipment. Some of the processing problems could be alleviated if the reinforcement would coexist in a more-or-less equilibrium state with the Al matrix, most ideally according to a pertinent phase diagram, and would have mechanical properties similar to those of ceramics. Transition-metal trialuminide intermetallics could provide this kind of reinforcement for light-metal matrices. Their densities are very close to those of ceramics, and some of them have comparable elastic moduli (Table I). Most importantly, they are in thermodynamic equilibrium with the Al matrix, which means that there is a real chemical bonding between the Al and trialuminide reinforcement rather than an intermediate reaction zone, as with SiC.[12] Similar ideas could be applicable to the magnesium (Mg) matrices, although finding the appropriate intermetallic reinforcement could be more complicated but still feasible. An intermetallic-reinforced Al and Mg matrix composite would R.A. VARIN, Professor of Materials Science and Engineering, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted March 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

constitute a category of intermetallic-reinforced light-metal matrix composites. Based on Table I, the two trialuminides with the highest elastic moduli and melting points are Al3Ti and Al3Zr. Several attempts to fabricate Al/Al3Ti composites by semiconventional ingot metallurgy[22] or mechanical alloying[23–27] have been reported in the literature. These efforts have been directed to develop more dispersion strengthened–like alloys rather than classical discontinuously (particulate) reinforced composites. On the other hand, there is a dearth of experimental data in the literature related to the Al/Al3Zr composites.[12,28,29] The objective of the present research was to investigate the feasibility of the processing of the Al/Al3Zr and Al-Mg/ Al3Zr composites by conventional ingot metallurgy and, if successful, to assess their rudimentary mechanical properties such as hardness an