By using the A356 aluminum alloy (Al-7Si-0.3Mg) with dispersed eutectic silicon particles and the single-phase Al-1Si-0.3Mg alloy, this study was carried out to investigate how eutectic silicon influences the crack propagation behavior under resonant vibrations in natural and overaging conditions. In the natural aging condition, the cracking modes of these two alloys resemble each other and the cracks through slip bands occur. Besides slip band cracking, as the cracks in the A356 alloy pass through the interdendritic zones with eutectic silicon clustering, they propagate through broken silicon particles or silicon particle/matrix interfaces. So, the resistance to crack growth of the Al-1Si-0.3Mg alloy is slightly higher than A356 alloy. For the overaging condition, the crack paths of these two alloys obviously differ from each other. The cracking mode of the Al-1Si-0.3Mg alloy largely displays the striation feature. However, owing to the existence of brittle silicon particles, the cracking exhibits a preferred direction towards the silicon particles and runs through broken silicon particles or silicon particle/matrix interfaces. Thus, the cracking resistance of A356 alloy is significantly lower than Al-1Si-0.3Mg alloy. Moreover, the crack growth resistance of the Al-1Si-0.3Mg alloy in the natural aging condition is slightly higher than that under the overaging condition. In contrast, for the A356 alloy, the former shows much higher resistance than the latter.
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