Nonequilibrium alloying of graphite-reinforced aluminum metal matrix composites

With corrosion behavior superior to pure aluminum (Al), nonequilibrium Al alloys produced by cosputter deposition have great potential for use as matrix metals in graphite (Gr)-reinforced Al composites. These composites offer enhanced properties as a result of their low density matrix and a high modulus reinforcing fiber. Cosputter-deposited Al alloys provide a means to improve corrosion resistance of Al and to remove a conventional processing step that accelerates degradation of the composite once the Gr fibers are exposed. Additions of tungsten (W) and molybdenum (Mo) were found to enhance the localized corrosion resistance of Al. At concentrations of 19 at.% Mo or 26 at.% W breakdown potentials (E_b) increased by 1,115 mV_SCE and 1,245 mV_SCE over pure Al, respectively. Heat treatment of these alloys at 400°C for 1 h did not result in precipitation of a second phase. The alloys maintained excellent resistance to localized attack. Galvanic current diagrams predicted use of these alloys in composite structure would decrease galvanic corrosion rates by at least 2 orders of magnitude. This enhancement in galvanic corrosion resistance was a result of significant improvements in passivity which occurred with the nonequilibrium addition of Mo or W to Al. Galvanic current measurements confirmed the predicted reduction in galvanic corrosion rates.