Morphology of second-phase precipitates in carbon-fiber- and graphite-fiber-reinforced magnesium-based metal-matrix composites

Magnesium-matrix composites reinforced with carbon fibers and graphite fibers have been examined. The surfaces of carbon and graphite fibers, the morphology of the precipitate and the interfacial microstructure have been studied. rt is found that the precipitates in the two composites are different and the two corresponding interfacial microstructures possess different characteristics although these two composites were fabricated by the same vacuum-pressure infiltration casting method without any extra heat treatment. In composites reinforced with carbon fibers, there is an interface layer composed of some individual magnesium grains, subgrains and a number of Mg17Al12 particles. These particles precipitate discontinuously at the interface. In the composite reinforced with graphite fibers, no distinct interface layer is observed, but massive, continuous precipitates are distributed in the matrix. Only a few of the precipitated particles are scattered at the interface. Moreover, the precipitated particles are usually co-formed with inclusions. It is thought that the difference in the precipitate morphologies in the two composites comes from the surface condition of the fibers. Carbon fiber has a greater propensity to adsorb the alloying elements in the matrix and provides more nucleation sites, so that discontinuous precipitation occurs easily. Graphite fiber induces continuous precipitation because of its low surface energy. (C) 1997 Elsevier Science Limited.