Figure 1. Optical micrographs of longitudinal planes of the (a) 4.5μm SiCp/Al-7Si composite (b) 20μm Al-7Si/ SiCp composite 2.2 Specimen preparation All specimens were solution treated at 535℃for 2.5h, quenched in eater and then aged at 160℃ to achieve peak-aged condition. The aging time was 7h for the Al-7Si alloy and 4.5μm SiCp/Al-7Si composite, 11h for 20μm SiCp/Al-7Si composite. Compact tension (CT) specimens, of width W=48mm, height H=57.6mm, thickness B=10mm, were machined from the extruded plates of the materials with the loading axis parallel to the extrusion direction and with the plane of the notch perpendicular to the loading axis. 2.3 Mechanical properties The room temperature tensile properties of the two kinds of the composites and the Al-7Si alloy are listed in Table 1. Incorporation of the reinforcing particles promotes the elastic modulus 15.84~18.94%. The composite containing large reinforcing particles is higher in yield strength, defined as the stress corresponding to a plastic strain of 0.2%, but lower in elastic modulus than that containing small particles. The composites display a slight decrease of the ultimate tensile strength and elongation with respect to the matrix alloy. Table 1. Mechanical properties of the Al-7Si alloy and composite at room temperature SiC content (vol. %) SiC Size （μm） Elastic modulus, E (GPa) 0.2% proof strength, σ0.2 (MPa) Ultimate tensile strength, σb (MPa) Elongation, A (%) 0 0 72.3 242.5 336.2 11.2 15 4.5 86.0 240.5 328.8 9.0 15 20.0 83.7 270.9 321.3 8.6 2.4 Procedure The tests according to ASTM E647 were conducted in laboratory air at room temperature (298K) and relative humidity 40-50% at constant stress ratio of 0.1, using a sinusoidal waveform signal at 25Hz.Region Ⅰdata were obtained following the guidelines for near-threshold testing. A computer controlled load shedding technique
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