Shock-induced chemical reactions and synthesis of nickel aluminides
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INTRODUCTION
THE combustion of metal-metal and metal-ceramic powder mixtures has been applied to synthesize materials that have unique properties. This materials processing method is more commonly known as self-propagating high-temperature synthesis (SHS). fl'21 In the SHS process, the reaction is self-sustained if the heat released from the reaction between the constituent elements is dissipated at a rate slower than the release rate. The reaction is initiated either by locally igniting a green compact of a powder mixture using a nichrome wire, electrical match, or even laser pulse (combustion mode) or by uniformly heating the green compact in a furnace (thermal explosion mode), t3,41 Typically, the reaction starts at temperatures near or equal to melting (or eutectic) temperature of the reactant having the lower melting point. [4] Chemical reactions can also be initiated by shock compression of powder mixtures, t5,61 This type of a dynamic processing technique, referred to as shock-induced reaction synthesis ( S R S ) , [7] utilizes the simultaneous application of very high pressure and temperature generated during the passage of shock waves through a powder mixture. Rapid increases in pressure and temperature induce extensive plastic deformation, turbulent flow and mixing of powders of different species, and rubbing and fracturing of powder particles. The resulting densification and generation of intimate contacts between cleansed surfaces of dissimilar powder particles can lead to reaction initiation at temperatures often lower than those required for SHS reactions, t81 The fundamental processes that control shock-induced chemical reactions leading to the synthesis of comI. SONG, Graduate Research Assistant, and N.N. THADHANI, Associate Research Professor, are with the Center for Explosives Technology Research, New Mexico Institute of Mining and Technology, Socorro, NM 87801. This paper is based on a presentation made in the symposium "Reaction Synthesis of Materials" presented during the TMS Annual Meeting, New Orleans, LA, February 17-21, 1991, under the auspices of the TMS Powder Metallurgy Committee. METALLURGICAL TRANSACTIONS A
pounds are not completely understood. Because of ~he exothermic nature of these reactions, the reaction temperature often exceeds the product melt temperature. Thus, once the reaction is initiated, the product exhibits a microstructure typical of melting and rapid solidification and not that of the high-pressure shock state. Unlike the SHS process, shock-induced chemical reactions occur in time durations of a few microseconds, which limits time-resolved monitoring of the events OCCUTTing during the reaction process. The dynamic nature of shock-induced chemical reactions is, in some respects, analogous to the detonation phenomenon in explosives, t9j However, detonation of explosives is accompanied by a large buildup in pressure (due to generation of gaseous reaction products), which can be instrumented using various types of pressure-sensing probes. Shock-induced chemical react
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