Processing, microstructure, and mechanical behavior of cast magnesium metal matrix composites
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I.
INTRODUCTION
M E T A L matrix composite (MMC) technology improves the wear resistance, elastic modulus, and tensile strength of unreinforced metals and alloys. Among numerous MMC systems under development, composites with aluminum and magnesium matrices and SiC particulate reinforcements are of commercial interests to the automotive and aerospace industries, tl-~3j Presently, an increasingly large proportion of these MMCs is produced by solidification processing, in which the reinforcements are introduced to molten metals using the conventional foundry processes, such as sand casting, permanent mold casting, die casting, and squeeze casting. This is primarily due to the ease of fabrication, low cost of reinforcements (ceramic particulates), and high rate of production obtained in these processes. In the past decade, much of the research has been focused on cast aluminum-based MMCs. L1-71Various aspects, including processing, tl-4~microstructure,[3,4,5] and mechanical behavior,t6.n of cast aluminum MMCs have been extensively studied and relatively well documented. However, the research on the same subjects for similar materials using magnesium matrices has received less attention compared to their aluminum counterparts. It is well known that the SiC reinforcement is thermodynamically unstable in aluminum matrices, causing undesirable chemical reactions and reaction products.tS] The fact that SiC is stable in magnesium matrices makes Mg/SiC a better material choice over A1/SiC. In addition, magnesium, with its higher stiffness-to-weight ratio, and wettability to carbide reinforce-
A. LUO, Group Leader, is with Applied Research and Development, Institute of Magnesium Technology, Inc., Sainte-Foy, Quebec, Canada G1P 4N7. Manuscript submitted August 26, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
ments, offers better matrix materials for the development of lightweight MMCs. Therefore, magnesium-based MMCs are currently being explored for a number of automotive and aerospace applications, such as automotive pulley, t~~ cog-tooth sprockets, oil pump cover,tSl cylinder liner, and aircraft engine casting.t11~ This article presents a liquid mixing and casting process developed at the Institute of Magnesium Technology (ITM), which can be used to produce SiC particulate-reinforced magnesium MMCs via conventional foundry processes. The microstructure, particle/matrix interracial reactions, tensile properties, strain-hardening, and fracture behavior of the cast magnesium matrix composites are studied, and the results are compared with those for the unreinforced magnesium alloy. The process-structure-property relationship and the strengthening mechanisms for the composite materials are also discussed. II.
EXPERIMENTAL
A. Materials Commercially pure magnesium (designated as PMg) and Mg-9 pct AI-1 pct Zn alloy AZ91 were selected as the matrices for the composites. The chemical compositions are shown in Table I. The reinforcement particulates are high purity silicon carbide (98.5 pct r-SiC) with an average diameter of 7
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