The fracture toughness of 6061Al alloy metal matrix composite reinforced with 15 vol% fine SiC particles of 600 nm in size under a high temperature over-aged condition was studied. And the toughness of the 6061Al matrix alloy and the same matrix alloy reinforced with coarse SiC particles of 9.5 µm were also measured for comparison. Fracture toughness tests were conducted on a servo-hydraulic testing machine by using three point bending specimens. The results showed that the toughness of the fine particle reinforced composite was obviously higher than that of the coarse particle reinforced composite in terms of crack initiation toughness, crack propagation energy and total absorbed energy. The mechanisms controlling the fracture toughness of the two composites reinforced with fine and coarse SiC particles were discussed based upon the measured toughness data and microstructure examinations by means of an optical microscope and a scanning electron microscope.

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In-situ SEM observations of fracture processes in two 6061Al alloy composites reinforced with coarse and fine SiC particles, respectively, were carried out to clarify their fracture mechanisms. It was found that in the coarse particle reinforced composite, voids were formed in the matrix around SiC particles ahead of the main crack tip, then coalesced with each other, and finally connected with the main crack tip, causing propagation of the main crack. However, in the fine particle reinforced composite, multiple micro-cracks were formed at the boundaries between SiC particle clusters and surrounding matrix or within the clusters, then connected with each other, and finally joined with the main crack tip, leading to crack branching and growth of the main crack. Crack branching, multiple cracking and crack deflection were proposed to contribute to the enhanced fracture toughness in the fine particle reinforced composite compared with the coarse particle reinforced composite.

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