Study on Ageing Behaviour of Silicon Nitride Reinforced Al6061 Composites

Aluminium alloy especially 6000 series have the unique property of undergoing ageing or precipitation hardening due to the formation of Mg2Si precipitates in the alloy. The matrix phase of these alloy series when added with reinforcements like Si3N4 undergo enhanced increment in hardness, strength and load bearing capacity due to formation of high dislocation density. In this investigation the ageing behavior of particulate reinforcements of Si3N4 particles in Al6061 metal matrix composite is compared to that of unreinforced Al 6061 base alloy. Both the composite and base alloy were subjected to ageing at solutionizing temperature of 525 °C and ageing temperature of 195 °C. It was noticed that the precipitation kinetics was accelerated in the composites. This is due to the large difference in values the co-efficient of thermal expansion between the matrix (Al) and reinforcement (Si3N4). This creates preferential nucleation sites for precipitation of 2nd phase particles near the reinforcement-matrix interface. In this work, detailed observation of the microstructure of the composites by optical microscopy, scanning electron microscopy has been carried out to support the study.