environment [5-7]. Of these variables, particle size is one of microstructural parameters influencing the fatigue crack growth behavior [8, 9]. Sugimura et al. [10] have reported that increasing the volume fraction of SiC particulates in cast Al-Si alloy promotes higher FCG rate. The effects of SiC volume fraction and particle size on the fatigue behavior of powder metallurgy 2080 Al alloy have been investigated by Chawla et al. [11]. They find that increasing volume fraction (from 10% to 30%) and decreasing particle size (from 23μm down to 5 μm) result in an increase in fatigue resistance. Xu et al. [12] have pointed out that the retardation of FCG was found when crack propagated from low volume fraction of SiC to high volume fraction of SiC. The crack deflection and branching decreased FCG rates. Although a number of studies on fatigue resistance and fatigue crack growth (FCG) in SiCp/Al composites and Al-Si-Mg alloys have been done , the effect of particle size on fatigue crack growth behavior of this kind of composites produced by spray deposition has been limited. Thus it is necessary to carry out a fundamental research work in this respect. The purpose of the present study is to understand the effect of particle size on fatigue crack growth behavior and closure mechanism of spray-deposited SiCp/Al composites. 2. Experimental 2.1 Material and microstructure Al-7Si-Mg alloy fabricated by spray deposition was used in the unreinforced condition and with 15 vol.% SiCp reinforcement. The chemical composition (mass. %) is Si 7, Mg 0.3, Mn 0.01, Cu 0.01, balance Al. Extrusion at 450℃ was used to produce plates with dimensions of 10×120mm2. Fig. 1 shows the microstructures of the sections parallel (L-plane) to the extrusion direction in the unreinforced alloy and the composites. It can be seen that SiC particles tend to be partially aligned along the extrusion direction. Some clustering tendency observed in the 4.5μm SiCp/Al-7Si composite decreases as the size of the particulates increases.
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