| The high cycle fatigue behavior of squeeze cast SiC whisker-reinforced aluminum composites based on either A356 Al or A390 Al matrices has been studied. Squeeze cast Al/SiCw specimens, which contain roughtly 17 vol. pct. whiskers from two different sources, have been examined for their high cycle fatigue strength under fully reversed test conditions using a staircase method to determine the mean fatigue strength at 10^7 cycles. The results show 30 to 40% increases in the fatigue strength of the A356 Al-based composites when compared to the unreinforced matrix alloy, but much less fatigue strengthening in the A390Al-based composites. A fractographic analysis indicates that about 80% of the composite specimens fail as a result of crack initiation within regions which are characterized by low volume fractions of the SiC whiskers. These reinforcement-free regions assume two forms: continuous $quot;veins,$quot; which are the more deleterious to fatigue, and discontinuous $quot;unreinforced areas,$quot; which are deleterious only in certain shapes and sizes. Both finite element analysis and Eshelby-based analysis indicate that the localized stresses within the unreinforced regions appear to be high enough to initiate fatigue cracks, especially if unreinforced ares are elongated and their major axis is aligned to the stress axis. The fractographic analysis also identifies the importance of primary Si particles in limiting the fatigue strength of the A390Al-based composites. In addition, low level of Mg in the ureinforced region, which is caused by a $quot;Mg ettering$quot; effect, results in the low hardness and significantly decreases the fatigue strength. |
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