Reactions in Al₂O₃Mg metal matrix composites during prolonged heat treatment at 400, 550 and 600°C

Al₂O₃Mg metal matrix composites were heat treated for up to several hundred hours at 400, 550 and 600 °C. Samples were prepared and studied using optical microscopy, scanning electron microscopy and transmission electron microscopu. At the two higher temperatures extensive reaction between δ-Al₂O₃ fibres and the Mg matrix had occurred after 200-300 h. During this time Mg and Al₂O₃ reacted to form MgO (but not spinel) and Al, which dissolved into the matrix. When the Al solubility in solid Mg was exceeded, liquid started to form. The formation of liquid probably started locally, i.e. long before the bulk matrix composition reached the solubility limit, possibly in metallic films separating growing MgO-grains from shrinking δ-Al₂O₃ grains, thus increasing the reaction rate drastically. At the end of these heat treatments a major part of the matrix was liquid. Upon cooling the liquid formed a eutectic structure consisting of h.c.p.-Mg and the intermetallic γ-phase. H.c.p.-Mg here refers to solid Mg which has the h.c.p.-structure and dissolves Al. In parallel with the reaction between Mg and Al₂O₃, there was a continuous coarsening of MgO and very large faceted grains were found. Below the eutectic temperature (437 °C) in the AlMg system liquid can no longer form and an abrupt decrease in reaction rate is expected. Consequently, no reaction was observed after 740 h at 400 °C. It may thus be concluded that long-term service at elevated temperature will not lead to extensive reaction between matrix and fibres as long as the temptemperature is below 437 °C.