MICROSTRUCTURE AND MICROCHEMISTRY OF AN AL-ZN-MG-CU ALLOY MATRIX-20 VOL-PERCENT SIC COMPOSITE

The microstructure and microchemistry of an underaged and overaged Al-Zn-Mg-Cu alloy matrix composite reinforced with 20 vol. % SiC was examined using analytical electron microscopy. The presence of MgO particles at the Al/SiC interfaces and Mg2Si in the matrix of the composites suggests that Mg could be depleted in the matrix. The precipitates in the unreinforced control alloys were found to be consistently larger than those of the composites following the same heat treatment conditions, suggesting that the aging kinetics of the unreinforced control alloy are faster than those in the composite. Data on the aging acceleration in other Al-alloy matrix composites has been analyzed to study aging kinetics. Aging acceleration was found to be more pronounced with increasing average absolute values of the atomic size misfit parameters of the constituent elements of the major precipitates. This result suggests that the preferential segregation of solute atoms and easier nucleation of precipitates from segregated solutes is responsible for aging acceleration in other composites. On the other hand, the depletion of Mg atoms in the matrix and the lack of interaction between Zn atoms and dislocations seems to be responsible for the aging deceleration in the reinforced Al-Zn-Mg-Cu alloy composites.