Influence of heat treatment on mechanical and fracture properties of an alumina microsphere reinforced aluminum matrix composite

The effects of ageing on the mechanical and fracture toughness properties of a 6061 Al reinforced with 20 vol% alumina microsphere (Comral-85) were studied. Fractographic examination combined with X-ray mapping and image analysis were used to investigate the fracture micro-mechanisms. Hardness and tensile strength were found to increase with ageing time reaching a maximum value after approximately 8 hours ageing at 175 °C. Although elongation to failure decreased with ageing time the fracture toughness showed little variation. Specimens for all heat treated conditions (peak-aged, under-aged and over-aged) exhibited ductile fracture consisting of void formation and coalescence. However, the matrix ageing condition and particulate distribution significantly affected the fracture mechanisms. Fracture was characterized by particle cracking and matrix failure in the under-aged condition and by decohesion at the matrix/particle interface in the over-aged condition. Image analysis indicated that regions of particulate clusters were preferential sites for crack initiation and subsequent growth for all heat treated conditions. These results are discussed in terms of the effects on matrix microstructure due to accelerated ageing and the local density of particulates.