The change of the eutectic microstructure upon heat treatment from the F condition to the T6 condition is clearly visible in Fig.4. The fibrous silicon is completely spheroidized[12] in both the 11% SiC MMC and the 50% SiC MMC (including the 27% SiC MMC that is not shown). It is relatively difficult to distinguish the spheroidized silicon in the eutectic from the small SiC particles also in the eutectic in the 50 % SiC MMC. Once again a large number of small SiC particles are visible in the proeutectic α (Al) globules. The matrix of the Duralcan MMC is a heat treatable Al-Si-Mg alloy. Precipitation strengthening by Mg2Si precipitates was achieved through solution heat treatment, quenching and subsequent artificial aging. This process increase strengths above the original F condition properties[12]. In the current work hardness is the only measure of such an increase in strength. Table 2 indicates the hardness of each SiC MMC with different volume fraction in the F condition while Table 3 indicates the hardness of those in T6 condition.