High cycle fatigue of squeeze cast aluminum/silicon carbide whiskers composites

The high cycle fatigue behavior of squeeze cast SiC whisker-reinforced aluminum metal matrix composites (MMCs) has been studied. In this study, composites based on two sources of near randomly oriented SiC whiskers have been fabricated by infiltrating preforms with an Al casting alloy (either A356 Al or A390 Al) under pressure. Squeeze cast Al/SiCw specimens, which contain roughly 17 vol. pct. whiskers, have been examined for their high cycle fatigue strength under fully reversed test conditions at room temperature using a staircase method to determine the mean fatigue strength at 10{dollar}sp7{dollar} cycles. The results show 30 to 40% increases in the fatigue strength of the A356 Al-based composites, when compared to the unreinforced matrix alloy. However, the composites based on the A390 Al show much lower fatigue strengthening, between 3 and 20% depending on the whisker source, despite the fact that they contain an identical whisker reinforcement.; A fractographic analysis of the fatigue crack initiation sites indicates that about 80% of the composite specimens fail as a result of crack initiation within regions which are characterized by low volume fractions of the SiC whiskers. These reinforcement-free regions assume two forms: continuous "veins," which are the more deleterious to fatigue, and discontinuous "unreinforced areas," which are deleterious only in certain shapes and sizes. It is interesting to note that the squeeze casting process successfully inhibited porosity formation to a level that microporosity rarely caused fatigue failure. The fractographic analysis also identifies the importance of primary Si particles in limiting the fatigue strength of the A390 Al-based composites. In addition, a "Mg gettering" effect was observed in comparing the characteristics of the unreinforced regions in the two SiC whisker-reinforced composites. Low levels of Mg in the unreinforced regions results in the low hardness and significantly decrease the fatigue strength.; The role of the unreinforced regions in limiting the high cycle fatigue strength of these composites has been analyzed in terms of the magnitudes of the predicted residual stresses within the unreinforced regions as a function of their shape. Both finite element analysis and an Eshelby-based analysis indicate that the localized stresses within the unreinforced regions appear to be high enough to initiate fatigue cracks, especially if unreinforced areas are elongated and their major axis is aligned to the stress axis.