The effect of Al2O3 particulates on the precipitation behaviour of 6061 aluminium-matrix composites

The effect of angular and spherical shaped Al2O3 particulates on the precipitation behaviour of 6061 aluminium-matrix composites has been studied using microhardness testing, differential scanning;calorimetry (DSC) and transmission electron microscopy (TEM). The evolution of precipitates at each stage of precipitation corresponding to the DSC-peak was monitored through careful TEM observations. Both the formation and dissolution behaviour of the precipitates in the ageing process in the composites were compared with the unreinforced 6061 matrix alloy. Although an overall accelerated ageing response in the composites was reported,. not all stages in the ageing process were influenced by the addition of particulates. However, the precipitation sequence was not altered by the presence of the particulates. The degree of acceleration and the relative proportion of the phase/phases was found to depend on reinforcement parameters such as size, shape and volume fraction. In the composites, higher dislocation densities were observed in the immediate vicinity of the ceramic particles which may have formed due to the large difference in the coefficients of thermal expansion (CTE) between the ceramic particles and the matrix. These CTE-dislocation effects influence the kinetics of precipitation.