The effect of reinforcement on the notched and unnotched room temperature tensile properties of Al-4wt%Cu/SiCp MMCs

The tensile and toughness related properties of SiC (3, 17 and 29 mu m) particulate (SiCp) reinforced Al-4wt.%Cu metal matrix composites (MMCs) produced via a powder metallurgy route were studied. The assessment of toughness related properties was carried out by notched tensile testing. The strength of the MMCs increased with decreasing size of the reinforcement. This predominated over any effect on strength of the small variation in 3-dimensional spatial distribution of the reinforcement. Examination of fracture surfaces of failed unnotched tensile specimens indicated that the MMCs with the most inhomogeneous distributions of the reinforcement (those containing 3 mu m SiCp) exhibited a number of anomalies. Strings of large ductile dimples corresponding to ductile failure of thin layers of reinforcement free material, and regions of delamination between matrix rich and reinforcement rich layers of material indicated that local differences in the spatial distribution of the reinforcing particles led to local,variation of the fracture mechanism of those MMCs containing 3 mu m SiCp. Those MMCs also exhibited multiple crack initiation below the final fracture which indicated that the toughness related performance of these composites might be worse than those with larger reinforcing particles. The results of notched tensile testing showed that the best indicator of toughness related properties was the ratio of notched ultimate tensile stress to unnotched proof stress. It was found that the materials became less sensitive to the presence of a notch as the size of the reinforcing particles decreased and as the matrix:reinforcement particle size ratio increased which was contrary to initial expectations. It was concluded that the MMCs containing 3 mu m SiCp exhibited the least resistance to crack initiation, but showed the greatest resistance to crack propagation. (C) 1998 Elsevier Science S.A.