Role of silicon carbide particles in fatigue crack growth in SiC-particulate-reinforced aluminum alloy composites

A study has been made of the mechanistic role of silicon carbide (SiC) particles during fatigue crack propagation in powder metallurgy AlZnMgCu metal matrix composites reinforced with 20 vol.% SiC particulates (SiCp), with varying sizes of reinforcement phase. Crack growth and accompanying crack tip shielding (principally by crack deflection, closure and bridging) are examined in peak-aged alloys over a wide spectrum of growth rates from 10−12 to 10−4 m cycle−1 and are compared with corresponding behavior in the unreinforced matrix alloy. Crack growth resistance in the composites is found to be both superior and inferior to that of the unreinforced alloy, depending on how the SiC and SiC-matrix interface fracture. At low stress intensity ranges ΔK, the predominant fracture of carbides close to the crack tip results in low levels of crack closure and rapid growth kinetics for fine SiCp distributions, whereas for coarse SiCp distributions the rougher fracture surface promotes crack closure from asperity wedging and improved crack growth resistance. With increasing ΔK, the fracture of large carbides farther ahead of the crack tip leads to the development of non-uniform crack fronts, thereby promoting crack bridging via uncracked ligaments and a small improvement in crack growth resistance. At high ΔK levels approaching K Ic, growth rates are faster in the composites owing to their low toughness. On the basis of these results, the overall fatigue crack growth performance of the SiCpAl composites compared with that of the traditional monolithic aluminum alloys is briefly discussed.