Mechanistic and processing studies in combustion synthesis of niobium aluminides
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Combustion synthesis of NbAl3 and Nb2Al was studied using the volume combustion mode. The effects of heating rate and green density were examined for NbAl3 synthesis. The effect of green density was also investigated for Nb 2 Al. Greater reaction completion was achieved at higher heating rates and green densities. In both NbAl3 and Nb2Al samples, the reaction was initiated above the melting point of Al. Quenching (Nb+3A1) samples pressed at relatively high and low densities below the ignition temperature, and results of a particle-foil experiment, identified the spreading characteristic of molten Al over Nb, providing mechanistic details about niobium aluminide product formation.
I. INTRODUCTION Intermetallic aluminide compounds are promising candidates for structural applications owing to their attractive properties at elevated temperatures. These intermetallic materials have high-temperature strength, high melting points, relatively low density, and exhibit resistance to oxidation and corrosion.' Despite their potential for use as high temperature structural materials, one drawback of ordered intermetallics is embrittlement. However, ductility can be improved by alloying or by adding reinforcements, resulting in substantial improvement in mechanical and metallurgical properties.2 Aluminides are usually prepared by powder processing, with or without hot isostatic pressing, plasma spray, and arc melting and casting, which can be followed by either high-temperature extrusion or alternate cold rolling and recrystallization treatments.3 Although these techniques are successful in the synthesis of intermetallics, they require several processing steps and complex equipment. An alternative method of producing intermetallics is by combustion synthesis. In this method, the exothermic reaction between powder reactants is initiated by an external energy source (e.g., heated coil, laser beam) and becomes self-sustaining to yield the final product progressively without requiring additional heat. This process has advantages over conventional methods due to its simplicity and relatively low energy requirements. Combustion synthesis can be conducted in two modes: the self-propagating high-temperature synthesis (SHS) mode and the volume combustion mode. In the SHS mode, the reaction is initiated at one end of the sample,
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Permanent address: Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka, 142 432, Russia. b) Author to whom correspondence should be addressed.
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J. Mater. Res., Vol. 10, No. 9, Sep 1995
http://journals.cambridge.org
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and a combustion wave self-propagates through the mixture of reactants. In the volume combustion mode, the sample is heated uniformly in a controlled manner until reaction takes place essentially simultaneously throughout the entire sample volume. Combustion synthesis has attracted considerable interest in recent years, and details of this process and its prospects have been discussed in several review articles.4"7 In order to control
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