Microstructural characterization of self-propagating high-temperature synthesis/ dynamically compacted and hot-pressed t
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INTRODUCTION
COMBUSTION synthesis, reaction synthesis, or selfpropagating high-temperature synthesis (SHS) are methods of producing ceramics and intermetallics that are receiving a great deal of attention. An early example of this synthesis method is the thermite reaction used to join train rails. In the thermite reaction, a n Fe203 q- A1 mixture reacts exothermally, producing AI:O 3 and Fe as a weld metal. Extensive investigations in the Soviet Union by Merzhanov and c o - w o r k e r s [1'2"3] have yielded over 300 compounds produced by this method, and a number of industrial processes have resulted. This collection of work inspired a considerable amount of interest within the United States, and Frankhouser et a l . ' s monograph t41 provides a thorough analysis of the early Soviet work. There are currently a number of on-going efforts in the United States, Japan, and Western Europe; a compre-
KENNETH S. VECCHIO, Assistant Professor, JERRY C. LaSALVIA, Graduate Student, and MARC A. MEYERS, Professor, are with the Department of AMES, University of California, San Diego, LaJolla, CA 92093. GEORGE T. GRAY III, Staff Member, is with Los Alamos National Laboratory, Los Alamos, NM 87545. 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
hensive review of this work has been provided by Munir and Anselmi-Tomburini. tSJ As part of a program intended to synthesize ceramics by combustion synthesis followed by consolidation, titanium carbide disks were produced by a novel method of combining SHS and dynamic forging, which is described in greater detail by LaSalvia et a1.[61 and Meyer et al. tTJ T h e use of dynamic consolidation in a high-speed forging machine after the conclusion of the combustion reaction (5 to 15 seconds) enabled the collapse of the porosity of the as-synthesized product while it was still hot and ductile. Earlier work by Niiler et al. N had shown that full densification could be accomplished by SHS followed by explosive compaction. The densification process is required because combustion synthesis produces a product with approximately 50 pct porosity. Figure 1 shows a polished section of titanium carbide produced by SHS without subsequent compaction, demonstrating the porous nature of the SHS reaction product; the dark areas in the micrograph represent the voids. There are three sources of porosity: (a) the initial porosity of the powder mixture prior to the reaction; (b) the shrinkage associated with the reaction ( i . e . , products that have a higher specific volume than the starting reactants); and (c) the formation of gases in conjunction with the reaction, and their subsequent expansion, entrapment, or escape. Indeed, one often observes a considerable increase in the dimensions of the SHS product compared with the reactants. VOLUME 23A, JANUARY 1992--87
Fig. 1 --Optical micrograph
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