Microstructures and age hardenability of Al-5%Si-0.2%Mg based composites reinforced with particulate SiC

Composites of an Al-0.5%Si-0.2%Mg alloy reinforced with particulate SiC have been produced by the compocasting route. The silicon carbide was oxidized at 900 degrees C for 2 h and mixed into the semi-liquid alloy by means of a special stirrer. The slurry was held at 750 degrees C for 5 min and cast into a permanent die preheated to different temperatures and composite ingots containing 9, 13, 17, 22, and 26 vol.% SiC were obtained. The distribution of SiC particles, the porosity content and the dendrite arm spacing of the composites were determined. The phases other than the matrix and the reinforcement were identified to be Mg2Si, MgAl2O4 and FeSi3Al9. It was found that the oxidation of SiC and the presence of magnesium in the matrix alloy rendered easy incorporation without causing any significant magnesium depletion in the matrix. It was found that the composites produced were age hardenable. After prolonged exposure to temperatures above 500 degrees C in the solid state, no formation of additional phases was found to take place in the microstructures. (C) 1997 Elsevier Science Limited