A theoretical and experimental study of aluminum alloy 6061-SiC metal matrix composite to identify the operative mechanism for accelerated aging

Abstract

Abstract Accelerated aging in metal matrix composites (MMCs) can be attributed to an increased dislocation density in the vicinity of the reinforcements or to the matrix residual stress field near reinforcements. Both mechanisms aid the diffusion of solute atoms, thereby leading to more rapid precipatation. In this work, the precipitation behavior of aluminum 6061 alloy reinforced with 10 vol.% SiC whiskers of variable aspect ratio was studied experimentally. The results were compared with the precipitation behavior of a control aluminum alloy 6061 in the unstrained and plastically strained conditions. It was found that the strained control alloy, with approximately the same expended plastic work as the composite, showed a similar β′ precipitation rate and activation energy as the composite. On the contrary, the unstrained alloy had a much higher activation energy for precipitation. A theoretical model was developed to predict the rate of precipitation in the residual stress field of the matrix. This rate was compared with the rate of precipitation on a regular edge dislocation array. It was found that, for realistic values of fiber radii and dislocation densities (about 0.25–1 μm and 1013–1014m−2 respectively), both mechanisms give comparable precipitation rates. However, solute atoms flowing towards the matrix-fiber interface under the influence of the residual stress field on encountering matrix dislocations are trapped, thereby lowering the activation energy to that of precipitation on dislocations. It was concluded that, for MMCs with large fibers and high dislocation densities, dislocation generation is the principal contibutor to accelerated aging while, in MMCs with small fibers and low dislocation densities, the residual stress mechanism predominates. For intermediate fiber radii and dislocation densities, both mechanisms could be important although, in real MMCs, dislocations seem to play the dominant role.