THE INFLUENCE OF SIC PARTICULATES ON FATIGUE-CRACK PROPAGATION IN A RAPIDLY SOLIDIFIED AL-FE-V-SI ALLOY

The fatigue crack propagation properties of a rapidly solidified alumi num alloy are compared with those of a metal matrix composite (MMC) ma de of the same base alloy with the addition of 11.5 vol pet SiC partic ulate. The high-temperature base material, alloy 8009 produced by Alli ed-Signal, Inc. (Morristown, NJ), is solidified and processed using po wder metallurgy techniques; these techniques yield a fine-grained, non equilibrium microstructure. A direct comparison between the fatigue cr ack propagation properties;of the reinforced and unreinforced material s is possible, because alloy 8009 requires no postprocessing heat trea tment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could aris e during processing and aging; The-experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack;p ropagation properties: slower crack growth rates for a given Delta K, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, Delta K -eff, the composite exhibits lower crack propagation rates than the un reinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the b ase;alloy. It is argued that the lower crack growth rates and higher i ntrinsic threshold stress intensity factor observed in the composite a re associated with crack deflection around SiC particles.