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Studies were carried out on the microstructure and thermal stability of an ultrafine Al/Al2O3 composite with high strength and low density. Transmission electron microscopy indicated that Al2O3 shells remained undeformed below 550 °C, which limited grain growth of Al. Both transmission electron microscopy and x-ray diffraction analysis indicated that no obvious grain growth of Al with time occurred upon annealing at 620 °C. After almost all the alumina shells were destroyed following annealing at 620 °C, the Al2O3 fragments with various morphologies distributed in the material could still limit the migration of Al grain boundaries to increase the thermal stability of the material. After Al melted and resolidified, the grain sizes of Al were still about 200 nm. The bulk composite sample showed good dimensional stability. Even if the Al grains melted, the network of Al2O3 fragments kept the sample from deforming due to the wetting of Al2O3 network with liquid Al.

I. INTRODUCTION

Exposure to oxygen will result in the passivation of Al powders. The formation of oxide overlayer has been investigated by in situ x-ray photoelectron spectroscopy.1 Meanwhile, many studies have been carried out on properties of the bulk material defined as Al/Al oxide composite made by compacting the passivated Al powders. It has been shown that this kind of composite is of low density and high strength.2,3 X-ray analysis showed that grains of Al in the Al/Al2O3 composite did not grow even by annealing at 660 °C, in comparison with the coldcompacted sample.3 The compacted Al/Al2O3 composite did not change shape even up to 1000 °C, which is much higher than the melting point of Al, because the Al2O3 coating remained solid even if the Al cores melted.4 Therefore, the composite is considered as a material with high thermal stability and interesting applications. Until this date, the high thermal stability of the ultrafine Al/ Al2O3 composite was simply attributed to the protection of Al2O3 shells. In fact, the mechanism of thermal stability of the Al/Al2O3 composite has remained obscure. Transmission electron microscopy (TEM) and highresolution electron microscopy (HREM) are useful for studying the microstructure of solid materials,5–7 such as grain size and shape, structure of grain boundaries (GBs), morphology and distribution of the second phase, and dislocations. In addition, visible information is important

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http://journals.cambridge.org

J. Mater. Res., Vol. 18, No. 2, Feb 2003 Downloaded: 18 Jun 2014

in understanding the intrinsic mechanism of both mechanical and thermal stability. To date, there has not been direct evidence on the configuration of the Al2O3 shell after annealing, although TEM and HREM study of the microstructure of such composite could give evidence on the protection of Al2O3 coating and reveal the essence of its thermal stability. Therefore, TEM and HREM studies were carried out on such composites, and it was found that the Al/Al2O3 compos