Morphology and growth mechanism of alumina whiskers in aluminum-based metal matrix composites
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Morphology and growth mechanism of alumina whiskers in aluminum-based metal matrix composites Y-F. Li, C-D. Qin, and D.H.L. Ng Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China (Received 1 September 1998; accepted 2 April 1999)
A process has been invented for growing alumina (Al2O3) whiskers in the matrix of the aluminum-based metal matrix composite by the internal oxidation reaction between aluminum (Al) and molybdenum oxide (MoO3) at 850 °C. The whiskers were formed during the firing stage and distributed uniformly throughout the metal matrix. The fractional volume of the whiskers was about 30%, and their average size was 1 m in diameter and 10 m in length. The growth mechanism of the Al2O3 whiskers was studied. It was found that the nutrient for the growth of the whiskers was supplied from the molten Al at the base of the whiskers where they reacted with MoO3 forming the Al2O3 whiskers and the Al–Mo intermetallic phases.
I. INTRODUCTION
The existing aluminum-based metal matrix composites (MMC) are mainly reinforced by ceramics such as Al2O31, SiC2, TiC3, and TiB24 in the forms of fibers, particles, or whiskers. The technique to manufacture these types of ceramic-reinforced metal matrix composites usually requires either high-temperature mixing of molten metal with the “already prepared” ceramic reinforcements,5 or forced infiltration of molten metal into the porous ceramic perform.6,7 Other techniques that produce ceramic reinforced Al matrix are the exothermic dispersion (XD) method8 and the self-propagating synthesis reaction,9 which involve the firing of a mixture of Al and other ceramic powders. In our work, we had fabricated a new type of Al-based metal matrix composite; the reinforcements were Al2O3 ceramic whiskers and Al–Mo intermetallic phases, which were formed when certain conditions were satisfied during sintering. This paper reports the process of making this type of metal matrix composite. We describe the growth mechanism of these whiskers during firing and the necessary conditions for their formation. II. GROWTH MECHANISM OF WHISKERS
Two different models for growth of whiskers in a material matrix have been proposed—namely, the tip growth theory and the base growth theory. The first theory suggests that whiskers can grow by extending their tips, and there are two ways to achieve the tip growth. One of them is called the vapor liquid solid (VLS) mechanism, which was proposed by Wagner and Ellis for the growth of Si whiskers10 and by Milewski et al. for the growth of SiC whiskers.11 In this VLS J. Mater. Res., Vol. 14, No. 7, Jul 1999
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process, the growth of the whisker is initiated by the condensation of droplets of material at the tip of the whisker. A whisker that grows in this way would have a round top. The other type of tip growth was proposed by Seitz.12 This process involves the condensation of droplets on the screw dislocation of the matrix, and the w
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