Fatigue behaviour of 6061 aluminium alloy and its composite

Fatigue behaviour of an artificial aged powder metallurgy 6061 aluminium alloy, and a composite made of this alloy with 15% volume fraction of SiCp was investigated. The alloy was subjected to T6 heat treatment, as was the composite material chosen (which incorporated SiC particles of average size 30pm). An extensive experimental programme was carried out in which fatigue lives were determined using load-controlled axial loading of unnotched cylindrical samples, at stress ratios of-1 and 0.1. Tensile properties of both materials were also established. The tensile behaviour of the alloy is in agreement with other published data for this material, and the significant influence of mean stress on fatigue behaviour found, is consistent with that normally observed in metals. The relationship between number of cycles to failure and stress found for the alloy is within the range of scatter usually seen in such tests, and corresponds with available data. However, it has been found that while stiffness of the composite material is higher than that of the matrix, tensile strength and fatigue life are lower in the composite, as is ductility. At each stress ratio, the composite material exhibits shorter fatigue life at low and intermediate stresses, compared with the unreinforced aluminium alloy. While a number of these trends are consistent with expectations for ceramic reinforced metal composites, the relatively low strength of the materials is not. Standard metallographic techniques, as well as fractographic observation under a scanning electron microscope were used to further investigate material behaviour. Particularly large particles of SiC are identified as having contributed to fracture behaviour, along with poor interfacial properties