REACTIONS IN AL2O3-MG METAL-MATRIX COMPOSITES DURING PROLONGED HEAT-TREATMENT AT 400-DEGREES-C, 550-DEGREES-C AND 600-DEGREES-C

Al2O3-Mg metal matrix composites were heat treated for up to several h undred hours at 400, 550 and 600-degrees-C. Samples were prepared and studied using optical microscopy, scanning electron microscopy and tra nsmission electron microscopy. At the two higher temperatures extensiv e reaction between delta-Al2O3 fibres and the Mg matrix had occurred a fter 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 solu bility 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 s eparating growing MgO-grains from shrinking delta-Al2O3 grains, thus i ncreasing the reaction rate drastically. At the end of these heat trea tments a major part of the matrix was liquid. Upon cooling the liquid formed a eutectic structure consisting of h.c.p.-Mg and the intermetal lic gamma-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 fa ceted grains were found. Below the eutectic temperature (437-degrees-C ) in the Al-Mg 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-degrees-C. It may thus be concluded that long-term service at elevated temperature will not lead to extensive reaction be tween matrix and fibres as long as the temperature is below 437-degree s-C.