MECHANICAL-BEHAVIOR OF MULLITE FIBER-REINFORCED ALUMINUM-ALLOY COMPOSITES

Discontinuously reinforced aluminum alloys are viewed as candidate mat erials for elevated temperature applications because of their attracti ve high temperature strength properties and wear resistance. The eleva ted temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attenti on in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fi bers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Young's modulus of the composite exceeds that of the unreinforced all oy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The pri mary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the degradation in the matrix stiff ness. Reinforcing the alloy with mullite fibers results in only a mode rate improvement in strength at room temperature but the elongation to failure is reduced considerably. Increasing the amount of shot, altho ugh not appreciably degrading strength, further reduces the ductility. Shot is found to play an important role in the damage evolution by fr acturing early in the loading process, and thus, the composite integri ty when subjected to slow stable crack growth, as in fatigue, for exam ple, could be adversely affected.