**Ford Research Laboratory, Ford Motor Co., Dearborn, MI 48121(Received February 3, 1998)(Accepted February 13, 1998)IntroductionParticle reinforced metal matrix composites (MMCs) have generated a significant amount of interestdue to their high strength, stiffness, weight savings and higher operating temperatures compared toconventional engineering materials (1–3). By the addition of a relatively inexpensive ceramic partic-ulate reinforcement, such as SiC, to conventional aluminum alloys, a substantial decrease in CTE andan increase in modulus and fatigue resistance can be achieved while maintaining cost at an acceptablelevel (4,5).A number of studies have examined the monotonic stress-strain behavior of particle reinforced Al(6–10), as well as the cyclic stress-strain behavior of these materials (11–15). In this study, we havecompared the cyclic stress-strain hysteresis behavior of the composite to that of the unreinforced alloyat different stress levels. The effect of reinforcement volume fraction on total and plastic strainaccumulated during fatigue was also examined. The implications of these findings regarding fatiguemechanisms are discussed.Materials and Experimental ProcedureA 2080 Al alloy (A1-3.6Cu-1.9Mg, age-hardenable) reinforced with 10, 20, and 30% SiC particles wasconsolidated via powder metallurgy processing and extruded (Aluminum Company of America, AlcoaCenter, PA). The average particle size of the reinforcement was approximately 5 mm. A thermome-chanical treatment (T8 treatment) developed by Krajewski et al. (16), consisting of solution treating,rolling, and peak aging, was conducted on the composite and the unreinforced alloy. The rolling stepwas crucial since it provided a homogeneous distribution of dislocations that served as heterogeneousnucleation sites for precipitation, and resulted in nearly identical S’ precipitates in the microstructuresof all the composites as well as the unreinforced alloy. Tensile and fatigue samples were machined byusing low-stress grinding. The gage diameter of tensile specimens was 5.08 mm and that of fatiguesamples was 4 mm. An extensometer with a gage length of 12.7 mm was used in all tests. Tensile tests