IN-SITU SEM STUDIES ON THE EFFECTS OF PARTICULATE REINFORCEMENT ON FATIGUE-CRACK GROWTH-MECHANISM OF ALUMINUM-BASED METAL-MATRIX COMPOSITE

The effects of particulate reinforcement on the fatigue behaviour a nd fatigue mechanisms of two 6061 aluminium-based metal-matrix composite s (MMCs) in three different heat-treatment conditions were studied in situ with a scanning electron microscope and compared to the unreinfor ced alloy in the as-received condition. It was observed that the; fati gue properties of the MMCs were influenced by the ceramic particles in two ways: firstly the particles increased the fatigue stress intensit y threshold mainly by crack-deflection and crack-closure mechanisms, a nd secondly, the particles raised the fatigue crack growth rates in th e Paris region by providing an easy crack path. The effect of ageing w as small on the fatigue stress intensity threshold of MMCs, but for th e peak-aged MMCs the fatigue crack growth rates in the Paris region we re faster. The mechanism of fatigue crack growth was largely associate d with the matrix/particle interface and the linkage with subcracks in itiated ahead of the main crack at high applied-stress intensity facto rs.