High cycle fatigue of squeeze cast Al/SiCw composites

The high cycle fatigue behavior of squeeze cast SiC whisker-reinforced alum inum composites based on either A356 Al or A390 Al matrices has been studie d. Squeeze cast Al/SiCw specimens, which contain roughly 17 vol.% whiskers from two different sources, have been examined for their high cycle fatigue strength under fully reversed test conditions using a staircase method to determine the mean fatigue strength at 10(7) cycles. The results show 30-40 % increases in the fatigue strength of the A356 Al-based composites when co mpared to the unreinforced matrix alloy, but much less fatigue strengthenin g in the A390 A-based composites. A fractographic analysis indicates that a bout 80% of the composite specimens fail as a result of crack initiation wi thin regions which are characterized by low volume fractions of the SiC whi skers. These reinforcement-free regions assume two forms: continuous 'veins ,' which are the more deleterious to fatigue, and discontinuous 'unreinforc ed areas,' which are deleterious only in certain shapes and sizes. Both fin ite element analysis and an Eshelby-based analysis indicate that the locali zed stresses within the unreinforced regions appear to be high enough to in itiate fatigue cracks, especially if unreinforced areas are elongated and t heir major axis is aligned to the stress axis. The fractographic analysis a lso identifies the importance of primary Si particles in limiting the fatig ue strength of the A390 Al-based composites. (C) 2000 Elsevier Science S.A. All rights reserved.