Failure mechanisms in SiC-fiber reinforced 6061 aluminum alloy composites under monotonic and cyclic loading

Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061 alloy metal-matrix composite (SiC_f/Al-6061) during monotonie and cyclic loading are examined at room temperature, both for the longitudinal (0 deg or L-T) and transverse (90 deg or T-L) orientations. It is found that the composite is insensitive to the presence of notches in the L-T orientation under pure tension loading due to the weak fiber/matrix interface; notched failure strengths are ∼1500 MPa compared to 124 MPa for unreinforced 6061. However, behavior is strongly dependent on loading configuration, specimen geometry, and orientation. Specifically, properties in SiC_f/Al in the T-L orientation are inferior to unreinforced 6061, although the composite does exhibit increasing crack-growth resistance with crack extension (resistance-curve behavior) under monotonie loading; peak toughnesses of ∼16 MPa√m are achieved due to crack bridging by the continuous metal phase between fibers and residual plastic deformation in the crack wake. In contrast, such bridging is minimal under cyclic loading, as the ductile phase fails subcritically by fatigue such that the transverse fatigue crack-growth resistance is superior in the unreinforced alloy, particularly at high stress-intensity levels. Conversely, fatigue cracks are bridged by unbroken SiC fibers in the L-T orientation and exhibit marked crack deflection and branching; the fatigue crack-growth resistance in this orientation is clearly superior in the composite.