Effect of hybrid reinforcement on crack initiation and early propagation mechanisms in cast metal matrix composites during low cycle fatigue

Metal matrix composites (MMCs) suffer cyclic plastic deformation in the structural application. Microcrack initiation and early propagation during low cycle fatigue were studied in three types of materials: cast hybrid MMC with SiC particles and Al₂O₃ whiskers, cast MMC with Al₂O₃ whiskers, and cast Al alloy. Furthermore, the role of hybrid reinforcement in the mechanisms of microcrack initiation and early propagation were examined. In Al alloy, microcracks were observed to initiate in the Al grain, but when the matrix was reinforced, the initiation location changed to the whisker/matrix and particle/matrix interfaces and the hybridization effect reduced the resistance to crack initiation. Moreover, the two MMC materials exhibited similar interface debonding in fracture, which created additional secondary microcracks due to continued fatigue cycling. Numerous voids were formed ahead of the crack tip, and the microcracks intersected with other nearby microcracks. However, in the Al alloy, the microcracks propagated through the boundaries between Si particle clusters and the Al grain through void nucleation and coalescence or through striation formation in the Al grain.