Modeling the reaction synthesis of shock-densified titanium-silicon powder mixture compacts
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I. INTRODUCTION
REACTION synthesis processes involving highly exothermic self-sustained combustion-type chemical reactions (occurring in mixtures of elemental or thermite-type powder mixtures) result in the formation of compounds of high purity, fine grain size, and even metastable phases.[1–4] However, the large volume change accompanying product formation, the violent gas expulsion, and shrinkage occurring during solidification from the melt can lead to considerable retained porosity in the reaction product. Reaction synthesis followed immediately by dynamic-densification, while the product is hot and plastic, has been applied with some success[5–8] to form bulk materials. However, the brittle nature of ceramics and intermetallics can lead to their cracking due to thermal stresses generated during cooling of the densified reaction products through the ductile-to-brittle transition temperature. An alternative, which combines the benefits of reaction synthesis and dynamic densification, is to use the latter process to first densify powder mixtures into greendensity compacts for subsequent reaction synthesis. Dynamic densification, employing shock compression of powders, produces a dense-packed highly activated state of mixture constituents. The plastic flow, dispersion, and mixing of reactants, intimate contacts between cleansed surfaces of abraded powder particles, and grain size reduction via fracture and/or subgrain formation[9,10] resulting from shock compression can significantly enhance the chemical reactivity of the reactants.[11] One can advantageously use this highly activated dense-packed state of powders to impose S.A. NAMJOSHI, Graduate Student, and N.N. THADHANI, Professor, are with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. Manuscript submitted September 16, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
thermodynamic and kinetic limitations on the combustion reaction and, at the same time, accelerate the kinetics of solid-state reactions. Thus, dynamic densification offers the opportunity to precondition the material and control the postshock reaction synthesis process thereby avoiding problems inherent to self-sustaining combustion reactions and forming dense compounds with refined microstructures. Figure 1 schematically illustrates three situations showing fraction reacted as a function of temperature corresponding to the behavior that may be observed during reaction synthesis of highly exothermic elemental powder mixtures. Curve 1 shows the case corresponding to where no reaction product is formed until the melting of one of the constituents (or the eutectic), upon which a combustion-type reaction almost instantaneously converts all reactants to products. Such a reaction behavior is typical of loosely packed powder mixtures of highly reactive systems, in which the rate of heat dissipation is much lower than the rate of heat generation. Curve 2 shows an initial constant-rate increase in reaction fraction due to a solid-state reactio
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