Formation of Al 3 Ni Nanofibers in an Al-Based Metal Matrix Composite Fabricated by Reaction Sintering
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We developed an Al2O3 and Al3Ni intermetallic reinforced Al-based metal matrix composite by sintering an Al-20 wt% NiO powder compact at 1000 °C. Differential thermal analysis showed that a two-step reaction took place between Al and NiO in the temperature range 600–650 °C. The initial stage was a solid-state reaction, but it was soon paused by the newly formed Al2O3 and Al3Ni phases that separated the Al and NiO grains. A liquid–solid reaction was later resumed as the temperature was increased above the eutectic temperature of Al–Al3Ni at 640 °C when molten Al–Ni appeared. When the molten sample was quenched to room temperature, a two-phase Al–Al3Ni eutectic was found in the sample. In comparison with the air-quenched and the oil-quenched samples, the sizes of proeutectic Al3Ni grains and Al–Al3Ni eutectic phases in the salt-solution-quenched sample were much finer. The eutectic contained Al3Ni nanofibers with a diameter of approximately 50 nm. A reaction model of Al and NiO was proposed. A thermodynamic model was also proposed to describe the formation of the composite. I. INTRODUCTION
Aluminum-based metal matrix composites (Al-MMCs) have widely been used in structural applications due to their superior specific properties.1,2 The technique to manufacture Al-MMCs usually requires either high temperature mixing of molten Al with the “already prepared” reinforcing agents prior to solidification,3,4 or forcedinfiltration of molten Al into porous ceramic preform.5,6 Recently, more and more Al-MMC products are fabricated by the in situ methods, in which the reinforcements are synthesized through chemical reactions during fabrication.3 Reaction sintering of Al and metal oxide powders is one of the in situ techniques. Typically, Al2O3 and Al-based intermetallics are in situ formed in the reactions, and they are used to reinforce the Al matrix of the composite. Metal oxides used in the fabrication of this type include MoO3,7 CuO,8 TiO2,9 and Fe2O3.10 Nickel oxide has also been used11–14 and its corresponding Al-MMC contains Al–Ni-based intermetallics, which have relatively low densities, excellent oxidation resistance, and high strength even at elevated temperature.11 In the earlier works of producing Al–NiO products, high NiO content green samples were often prepared, and AlNi or AlNi3 were the commonly in situ–formed phases in the sintered products,12–14 whereas the Al3Ni phase was rarely made. In this work, we intend to produce the Alrich Al–NiO product that contains the lightweight Al3Ni intermetallic. We also expect that unique microstructure
DOI: 10.1557/JMR.2004.0154 J. Mater. Res., Vol. 19, No. 4, Apr 2004
http://journals.cambridge.org
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can be found in this Al-rich MMC when the sample phase is cooled rapidly after sintering. Currently, nanostructured materials are the focus of many research works because of their outstanding specific properties when compared with those of their coarse-grained counterparts.15–17 A conventional way of producing composite that contains nanometer-sized (nm
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