FATIGUE CRACK INITIATION AND MICROCRACK GROWTH IN SILICON-CARBIDE WHISKER REINFORCED 2124 ALUMINUM ALLOY (COMPOSITE, METAL MATRIX, FRACTURE)

Fatigue crack initiation sites and microcrack growth paths were observed in 20 volume percent SiC whisker reinforced Al 2124 and unreinforced Al 2124 alloy. The procedure used was to fatigue test a carefully polished specimen, taking surface replicas at several points during the course of the test with the specimen held at the maximum tensile stress to hold the cracks open. Shadowed surface replicas were then examined in a scanning electron microscope to determine the evolution of fatigue cracks from initiation to just prior to failure. By starting with the highest-cycle replica and noting the position of each crack, it was possible to work backward from the late observations to the origin of each crack observed. Fatigue crack initiation in SiC whisker reinforced 2124 Al alloy occurs primarily in the immediate vicinity of the whisker-matrix interface. Secondary sources of fatigue crack initiation include non-SiC intermetallics cracked during deformation processing and clusters of SiC with no intervening matrix material. Fatigue cracks propagate along favorably oriented interfaces where possible. Coalescence of cracks contributes to final failure, which is very rapid in the composite beyond an observed crack length of 30 microns at nominal stresses of 300-350 MPa.Additional experiments included the determination of monotonic and cyclic yield strengths for both the SiC/2124 composite and Al 2124 alloy. Both showed cyclic hardening. An investigation of fatigue crack closure was made by taking surface replicas at several points during a single fatigue cycle and observing the shadow tail length of each crack at each point in the load cycle. Some cracks were observed to close completely while others remained partially open even under compressive stress. Transmission electron microscopy was done on thin foils of the SiC/2124 and showed that fatigue increased and homogenized the dislocation distribution in the matrix. Auger electron spectroscopy of impact fractured SiC/6061 composite showed less Si and C than would be expected from the SiC volume fraction, suggesting that the impact fracture path may not always follow SiC-matrix interfaces.