NUCLEATION PHENOMENON IN SiC PARTICULATE REINFORCED MAGNESIUM COMPOSITE

Metal matrix composite (MMC) technology can improve the service performance of matrix metals and alloys. Most magnesium alloy based MMCs are produced via a casting process, therefore the casting microstructures are of vital importance to their performance in service. The formation of microstructure during solidification of MMCs is mainly influenced by four factors: nucleation (or its absence) on particles; particle pushing by the solidification front; particle settling in the melt; and chemical reaction between particles and matrix. So far many studies on the microstructures of MMCs have focused on identification of the interfacial chemical reaction between reinforcement and matrix [1–3], and there is only a limited and controversial understanding regarding the nucleation phenomena in MMCs. It has been reported [4–6] that primary silicon can nucleate preferentially on the reinforcement particles in hypereutectic Al-Si alloy. However, it is also reported [5] that in hypoeutectic Al-Si alloy, primary a-aluminum phase cannot nucleate on the reinforcement particles and that these particles tend to be pushed into the last freezing interdendritic regions. For magnesium based composites, both particle pushing (or capture) and grain refinement were observed in sand cast SiCp/Mg(AZ91) [7] and SiCp/Mg(ZCM630) composites [8] and permanent mold cast SiCp/Mg(AZ91) composites [9]. However, since it was not clear whether the primary a-Mg phase could nucleate on SiC particles, the grain refinement effect was attributed to a possible growth retardation mechanism. Recently, Luo [10] reported that in a magnesium matrix composite with 10 vol.% SiC, the significant grain refinement (67%) was the combined result of heterogeneous nucleation and restricted grain growth. In the composite with 0.5 vol.% SiC, the moderate grain refinement (37%) was mainly attributed to a heterogeneous nucleation mechanism. The evidence of heterogeneous nucleation of primary magnesium on SiC particles was attributed to the small lattice disregistry (4%) between SiC and magnesium with an orientation relationship of (111)SiC//(0001)Mg. In a word, it can be noted that nucleation phenomenon plays a key role in determining solidification microstructures of MMCs. The primary objective of the present research was to study the interfacial microstructure of SiC particulate reinforced magnesium matrix composite. Great effort was made to identify the orientation relationship between SiC particles and the matrix phase, because this relationship is very important in explaining the process of nucleation on the particles. Pergamon Scripta Materialia, Vol. 41, No. 9, pp. 967–971, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00247-X