Transmission electron microscopy study of Al/Al 2 O 3 composites fabricated by reactive metal infiltration

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R. E. Loehman and K. G. Ewsuk Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 2 May 1994; accepted 13 January 1995)

The microstructure of A l / a - A l 2 O 3 composites made by infiltrating molten Al into dense mullite preforms has been characterized using transmission electron microscopy. The growth of the A1/A12O3 composites was found to proceed through three stages. Initially, Al infiltrates into a dense mullite preform through grain boundary diffusion, and reacts with mullite at grain boundaries to form a partial reaction zone. Then, a complete reaction takes place in the reaction region between the partial reaction zone and the full reaction zone to convert the dense mullite preform to a composite of a - A l 2 O 3 (matrix) and an Al-Si phase (thin channels). Finally, the reduced Si from the reaction diffuses out of the A1/A12O3 composite through the metal channels, whereas Al from the molten Al pool is continuously drawn to the reaction region until the mullite preform is consumed or the sample is removed from the molten Al pool. Based on the observed microstructure, infiltration mechanisms have been discussed, and a growth model of the composites is proposed in which the process involves repeated nucleation of A12O3 grains and grain growth. I. INTRODUCTION Because of the advanced and tailored properties of metal/ceramic composites, interest in commercial applications of such composites has spawned the development of a variety of techniques for synthesis of the metal/ceramic composites.1"6 Of these techniques, a novel technology based on the directed oxidation of moltens (Lanxide's DIMOX™ Process)37"11 has received considerable attention, primarily due to the low cost, shape and dimensional fidelity, low porosity, and ease of fabrication. This process also offers the ability to produce unreinforced,7 particulate-reinforced,3 and fiber-reinforced8 composites with a wide range of compositions and microstructures. For example, the reaction of a molten aluminum alloy with air has been used to form an Al 2 O 3 /metal composite.37"11 Under the proper conditions, a rapid reaction occurs, and the reaction product grows outward from the original metal surface. The reaction is sustained as the molten metal is wicked through metal channels within the reaction layer by capillarity.7 After solidification, the metal remaining in the channels forms three-dimensional reinforced networks in the ceramic matrix composites.9"11

^Corresponding author. Present address: Environmental and Molecular Sciences Lab, Pacific Northwest Laboratory, P.O. Box 999, Ms K2-12, Richland, Washington 99352. 1216

J. Mater. Res., Vol. 10, No. 5, May 1995

http://journals.cambridge.org

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Recently, a new method for synthesis of metal/ ceramic composites by infiltrating a reactive metal into a dense ceramic preform has been reported by Loehman et al.6 For instance, molten aluminum can infiltrate mullite (Al6Si2O13) at temperatures above 900 °C and convert to an Al 2 O 3 -Al-Si composite based on the oxida

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