Recrystallization microstructure in cold-rolled aluminum composites reinforced by silicon carbide whiskers
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I.
INTRODUCTION
M E T A L matrix composite development has been primarily concerned with continuous filament composites. However, high cost limits their use to specific applications. Therefore, many investigations have concentrated on metal matrix composites with discontinuous fibers which are in the shape of chopped whiskers, particles, or platelets. In particular, silicon carbide-reinforced aluminum matrix composites are especially attractive, because they can be shaped and machined by utilizing conventional metal fabrication techniques. Most of these materials are fabricated either by molten metal infiltration or by powder metallurgy. Secondary fabrication can involve conventional metal working processes such as extrusion, cold rolling, or drawing. The fabrication method and mechanical properties of silicon carbide-reinforced aluminum composites have been well investigated, tl-9j However, the effect of thermomechanical processing on the microstructure has only been investigated to a limited extent. I6,8,91 The present study was carried out specifically to study the effect of cold rolling and recrystallization on the microstructural development. As one of the three major composite fabrication routes (the others being fusion metallurgy and diffusion bonding), a powder metallurgical technique was used in the present study to prepare silicon carbide whisker-reinforced aluminum composites. The content of silicon carbide was chosen as low as 2 vol pct in order to facilitate the microstructural studies and to allow a comparison with previous results obtained for alloys containing a small fraction of relatively large intermetallic particles. [l~ As a consequence of the production route, an oxide contamination is frequently introduced in aluminum Y.L. LIU, Research Scientist, N. HANSEN, Department Head, and D. JUUL JENSEN, Research Scientist, are with the Metallurgy Department, Rise National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark. Manuscript submitted August 17, 1988. METALLURGICAL TRANSACTIONS A
composites, tSj This oxide phase can influence the recrystallization behavior of the material in an uncertain way.r81 The technique of manufacturing used in the present study overcame this uncertainty by mixing silicon carbide whiskers with atomized aluminum powder which contained a well-defined amount of aluminum oxide (present as surface oxide). Therefore, the size and distribution of the aluminum-oxide particles and their effect on recrystallization were well k n o w n . D2"16'191 The relative size of atomized aluminum powder and silicon carbide whiskers was optimized so that a uniform mixture could be obtained by powder blending. II.
EXPERIMENTAL
A. Manufacturing of Materials The atomized aluminum powder (mean particle diameter 6.4 p m ) used as composite matrix had the chemical composition given in Table I. SiC whiskers made by Tokai Carbon Co. Ltd., Japan, were used. They consisted of/3-type (cube) SiC single crystals 0.1 to 1.0/~m in diameter and 30 to 100/zm in length. The SiC particulate content was les
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