Effect of fibre-matrix-binder interactions on the matrix composition and age-hardening behaviour of 6061-based MMCs

Abstract

Reactions between magnesium, alumina fibre and silica binder, during the manufacture of 6061 metal matrix composite (MMC) by the pressure infiltration technique, have been investigated for their effect on the structure, composition and age-hardening response of the MMC with increasing infiltration distance. The structure and composition were examined using optical and scanning electron microscopy, and electron probe microanalysis. The age-hardening behaviour, of both the MMC and unreinforced alloy, was determined using hardness measurements. There was a progressive depletion of magnesium in the MMC with increasing infiltration distance, which was particularly marked when the silica binder content exceeded 1 wt % (in a 20% V _f preform). This has been explained in terms of a reaction which results in the formation of an oxide at the fibre/matrix interface and a release of silicon into the matrix. The depletion of magnesium was associated with a reduction in the age-hardening response of the MMC, consistent with predicted behaviour based on the Al-Mg₂Si pseudo-binary phase diagram. In spite of these effects, the overall ageing behaviour of the MMC was enhanced compared with the unreinforced alloy, showing both higher peak-aged hardnesses and enhanced ageing kinetics, particularly at lower ageing temperatures.