Enhanced solid-state reaction kinetics of shock-compressed titanium and carbon powder mixtures
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Enhanced solid-state reaction kinetics of shock-compressed titanium and carbon powder mixtures Jong-Heon Leea) and Naresh N. Thadhani School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Received 22 December 1997; accepted 28 May 1998)
The effect of shock compression on the solid-state chemical reactivity of titanium and carbon powder mixtures was investigated with the objective of forming net-shaped TiC ceramics with a fine-grain microstructure. The combination of defect states and intimate interparticle contacts introduced during shock compression results in significant enhancement of the otherwise sluggish solid-state diffusion of Ti and C through the TiCx boundary layer. The apparent activation energy for TiCx formation was determined using solid-state reaction kinetics models, and was found to be reduced by four-to-six times that of diffusion of Ti into TiCx and two-to-three times that of diffusion of C in TiCx . As a result, net-shaped sections of shock-densified compacts (,85% dense) were reaction synthesized via solid-state diffusion, producing microstructures with grain size ,6 mm and microhardness of ,2000 kgymm 2 , in contrast to statically pressed powder compacts which reacted by a combustion process resulting in a highly porous product.
I. INTRODUCTION
Titanium carbide, TiC, is a unique ceramic having a high melting temperature (3067 ±C), high hardness (3200 kgymm2 for single crystal), and electrical resistivity (0.3–0.8 V ? cm) higher than that of typical metals. However, commercially available TiC (sintered or hotpressed) has low fracture toughness (KIC 3.4 MPa). Current applications of TiC are limited to its use as a cermet in cutting tools, a dispersant in composites, and thin and thick coatings. Many structural applications of TiC can be made possible if its properties are improved, which can be accomplished by grain size refinement using alternative processing methods, or combining TiC with various second-phase additives. Self-propagating high temperature (combustion) synthesis,1–4 in situ reaction synthesis,5–7 liquid infiltration,5,8 and dynamic densification9–11 techniques are some of the alternative processes which have been studied for this purpose. Combustion synthesis of materials, based on selfsustaining chemical reactions occurring between constituents of a powder mixture having large negative heat of mixing (DHR ), has been the most commonly investigated new process for TiC synthesis.1–4,12,13 Starting with a Ti 1 C powder mixture, a self-sustaining chemical reaction is initiated with the onset of melting of Ti.2,14 The melted Ti quickly spreads across the C particle surfaces due to capillary forces. Subsequent reaction occurs via a “dissolution-precipitation” mechanism or via “solid-state
a)
Present address: High Energy Rate Laboratory, Department of Mechanical Engineering & Materials Science, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860, Japan.
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