The role of microstructural variability on the very high-cycle fatigue behavior of discontinuously-reinforced aluminum metal matrix composites using ultrasonic fatigue

The fatigue behavior of two 2009/SiC/15p-T4 DRA composites has been investigated in the very high-cycle fatigue regime (VHCF, 107 ⩽ Nf ⩽ 109 cycles) using ultrasonic fatigue to achieve the very high cycle counts. One composite was produced to have a very homogeneous spatial distribution with minimal particle clustering and the other was produced to have a relatively heterogeneous distribution with significant particle clustering. Fatigue cracks initiated predominantly at AlCuFe inclusions in the homogeneous material and no crack initiation was observed at SiC particle clusters. Conversely, fatigue cracks initiated predominantly at clusters of SiC particles in the heterogeneous material and no crack initiation at inclusions was observed. Fatigue lives in both composites approaching 109 cycles exhibited minimal variation in lifetime and subsurface crack initiation was observed in all cases. Differences in the crack initiation behavior between the two composites were attributed primarily to variations in the spatial distribution of the reinforcement phase.