Si particles is the main fracture mechanism. Corresponding crack path profiles are shown in Fig.5 and Fig. 6. At low ΔK, fatigue surfaces in the SiC/ Al-7Si composite exhibit a zig-zag crack propagation path, with crack deflection around the particles microcacking inside in the 4.5μm SiC along the side of the crack(Fig.5(a)), whereas in the 20μm SiC composite they exhibit only crack deflection around the large particles. The results has shown that in comparison with the composite with 20μm SiC, the FCG rate at 4.5μm SiC was significantly decreased by the development of a higher level of crack closure. The increased crack closure was accompanied by extensive small particle cracking evident. The opening of a microcrack formed from particle cracking under the combined action of the crack-tip stress can result in a local volume expansion around the microscrack, referred to as microcrack-induced crack closure. The effect of the closure force is similar to the reduction of crack-tip stress intensity by transformation toughening. For the composite with 20μm SiC, the major effect of the large SiC reinforcement is to alter the fracture path, which enhanced crack closure through roughness-induced crack closure. However, roughness-induced crack closure diminishes at high stress intensities. The fatigue cracks do not avoid but run through the SiC particles. Therefore, the effectiveness of particles in crack deflection should be much smaller than the matrix alloy and 4.5μm SiC composite and would be further reduced because of the higher tendency of the large SiC fracture. 5．Conclusions (1) The 4.5μm SiC/Al-7Si composite has the best fatigue crack growth resistance, i.e. lower fatigue crack growth rate and higher threshold stress intensity factor range ΔKth. The ΔKth is 3.878 a m ΜΡ • for the 4.5μm SiC reinforcement composite, 3.630 a m ΜΡ • fore the 20μm, and 3.605 a m ΜΡ • for the Al-7Si alloy, respectively. The values of the fatigue crack growth rate for the 20μm are lower than the matrix alloy in the near threshold region and Paris regime, however, it become higher than those of Al-7Si alloy at the fast fracture state. (2) For the unreinforced alloy, crack propagating around Si particles is observed in the near-threshold regime. Crack deflection caused by Si phase, gives rise to a higher degree of roughness of crack surface, and then results in the occurrence of crack closure effect. (3) For the 4.5μm SiC reinforcement composite, crack deflections around SiC and microcracking inside SiC particles are the principle mechanisms of interaction between SiC particles and crack tip. Crack path analyses indicates that the extent of microcracking inside SiC particles induces a high level of fatigue crack closure ,which effectively reduces the crack growth driving force to slow down fatigue crack growth. (4) For the composite with 20μm SiC particle, crack deflection caused by the SiC particles attributes the lower crack growth due to the high levels of roughness-induced crack closure at the low ΔK, then the propensity for the fracture of large SiC particles
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