Mechanistic processes influencing shock chemistry in powder mixtures of the Ti-Si, Ti-Al, and Ti-B systems
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INTRODUCTION
THE high-pressure, high-loading rates and large amounts of strain, occurring during shock compression of powders (and powder mixtures), generate an unusual combination of structural defects and powder packing characteristics. The chemical reactivity of the constituents can, thus, be significantly enhanced, leading to accelerated mass transport. Under such conditions, powder mixtures can undergo chemical reactions during the shock-compression state,E1-7l via mechanisms different from those requiring conventional formation and growth of nuclei from the liquid or the solid state. HI Many controversies exist~2,4~ about the actual mechanistic processes that lead to occurrence of chemical reaction in the microsecond time scale of shock compression, before unloading of pressure to the ambient. The postulated mechanisms have included both solidstateE2,4,51and liquid-state processes.t6] Past work has suggested that the mechanisms of shockinduced chemical reactions are dominated by powder compression processes occurring during the nanosecond-scale shock rise time and microsecond duration of peak pressure state.t2,3m It has been shown that while thermodynamic properties control the type of products formed, the propensity for shock-initiated reactions is influenced by the characteristics of mechanical mixing of reactants, prior to reaction initiation. Thus, if intimate mixing of reactants is inhibited due to (a) differences in the mechanical (plastic T.E. ROYAL, formerly Graduate Research Assistant, School of Materials Science and Engineering, Georgia Institute of Technology, is Process Engineer, MULTI-ARC, Inc., Duncan, SC 29334. S. NAMJOSHI, Graduate Research Assistant, and N.N. THADHANI, Associate Professor, are with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICALAND MATERIALSTRANSACTIONS A
deformation or fracture) response of the reactantsy ,81 (b) abundance of one reactant species over the other, t91 (c) lack of available void volume for mixing of reactants,tlO~ or (d) altered compressibilities due to premature melting,Vii then reaction initiation during shock compression may not be possible, except under extremely high pressure conditions. These considerations have also been the basis of the "ROLLER" model proposed by Dremin and Breusov t4~ and the conceptual model proposed by Graham,V2] called "CONMAH" (incorporating configurational change, mixing, activation, and heating). Meyers et al. t61 have proposed a concept based on initiation of shock-induced reactions from localized "hotspots" of a critical size, if the energy due to reaction is greater than that dissipated by thermal conduction. Accordingly, they calculated a shock-energy threshold
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