2982
Ind. Eng. Chem. Res. 1996, 35, 2982-2985
Gravity-Induced Microstructural Nonuniformities during Combustion Synthesis of Intermetallic-Ceramic Composite Materials Hu-Chun Yi, Arvind Varma,* Alexander S. Rogachev, and Paul J. McGinn Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46656
Combustion synthesis of intermetallic-ceramic composite materials usually involves high combustion temperature as well as high temperature gradient. Gravity may play an important role in such systems when the liquid phase is present, and the intermetallic and ceramic phases have different densities. The combustion characteristics and microstructure may both be affected by gravity. Combustion synthesis of (1 - x)Ni3Al + xTiB2 composites, with the weight fraction x varying from 0.05 to 0.8, was carried out under normal gravity conditions. Both the combustion temperature and wave propagation velocity increased, and the propagation mode changed from unstable (x e 0.2) to stable (x g 0.4), as the TiB2 content increased. The combustion temperatures were higher than the melting point of Ni3Al for samples with x g 0.4, resulting in a composite material consisting of ceramic TiB2 particles dispersed in a Ni3Al matrix. Owing to buoyancy of TiB2 particles in the denser molten Ni3Al phase, gravity was found to affect the microstructure of the composite, yielding a nonuniform distribution of phases. The phase separation distance calculated by using Stokes’ law compared well with measurements. 1. Introduction
Table 1. Characteristics of the Reactant Powders
Intermetallic matrix composites (IMC’s) exhibit superior mechanical and heat resistance properties which make them excellent candidates for high-temperature applications. Recently, it has been demonstrated that combustion synthesis is a promising technique to produce these materials (Yi and Petric, 1994; Lebrat et al., 1994). In this method, chemical reactions between reactant powders are initiated at one end and propagate through the reaction mixture in a self-sustained manner. Many high-temperature materials have been synthesized using this technique. The combustion synthesis process and its prospects have been discussed in several recent review articles (Munir and AnselmiTamburini, 1989; Holt and Dunmead, 1991; Varma and Lebrat, 1992; Merzhanov, 1994). Combustion synthesis is mainly characterized by four parameters: combustion temperature (Tc), combustion wave velocity, mode of propagation (stable or unstable), and the product microstructure. These are influenced by many factors including particle sizes of the reactants, pellet density, composition, thermal history, and phase transitions (e.g. melting). Stable combustion usually occurs in highly exothermic systems, while compounds with relatively low heats of formation, such as intermetallics, can exhibit unstable combustion modes (Maksimov et al., 1981; Zhang and Munir, 1992; Wenning et al., 1994). Previous studies indicate that combustion and structure formation mechanisms involve several stages including melting of reactants and products, spreading of the melt, droplet coalescence, diffusion and convection, buoyancy of solid particles, and densification of the liquid product. Most of these processes are affected by gravity. Although there have been a number of publications examining the role of gravity in materials processing (Ostrach, 1977; Froyen and Deruyttere, 1984; Laxmanan et al., 1987; Sharma et al., 1994), there is only limited work dealing with the effect of gravity on * Tel: 219-631-6491. Fax: 219-631-8366. E-mail: avarma@ darwin.cc.nd.edu.
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powder
size, µm
purity, %
vendor
Ni Al Ti B (amorphous)
25-44
