CRACK PROPAGATION IN CORROSION-FATIGUE OF METAL-MATRIX COMPOSITES

The effect of aqueous environment on the fatigue crack propagation of several metal-matrix composites (MMC) has been investigated.; The fatigue cracking behavior and mechanisms in Gr-Al, Al(,2)O(,3)-Al, and SiC-Al materials were determined. In only the SiC-Al MMC materials were meaningful crack propagation data (da/dN vs. (DELTA)K) obtained. The other materials tended to exhibit cracking in the fiber/matrix direction, primarily at the interfaces.; Environmental effects tended to raise da/dN at a given (DELTA)K in SiC-Al about a decade, but there was no discernible difference between the effect of distilled water and 3.5 percent salt solution.; Transitions in the da/dN curves over a range of (DELTA)K were linked to microstructural features in a model containing the subgrain size and interwhisker spacing.; An orientation dependence was discovered (which stemmed from preferred orientations developed during processing) in the SiC-Al MMC.; Fractographic examination revealed the existence of whisker clumping and growth faults along the SiC whiskers, which could lead to stress concentrations and facilitate cracking in fatigue.