A micromechanistic model of microstructure development during the combustion synthesis process
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The influence of the key nucleation and grain growth parameters on (i) the evolution of the microstructure of the product phase (on a microscopic level) and (ii) the combustion synthesis process (on a macroscopic level) were investigated for the combustion synthesis process in the N b - C system. This work is an integral part of the continuing effort1"3 to develop a more complete theoretical model for combustion synthesis processes in general. In particular, the nucleation and growth of the NbC(s) product phase from the supersaturated liquid Nb/C mixture that appears briefly during the combustion synthesis process was treated in a greater detail by using a decidedly more sophisticated treatment of the nucleation and growth process (as developed in the field of rapid solidification and welding). It was shown that the microstructure of the NbC(s) product phase, including the evolution of the grain size and the size distribution, and the development of the grain's morphology, as well as the combustion wave velocity, are significantly influenced by the total number density of the nucleation sites, « max , that are present in the system. The grain size distribution was shown to possess a monosize distribution, since during the combustion synthesis process the rate of increase of the degree of local undercooling was very high so that the nucleation process took place (locally) during a very brief period of time. This work provides a sound basis for developing a better control of the microstructure, and for a better understanding and interpretation of the results of related experimental studies.
I. INTRODUCTION The combustion synthesis [or self-propagating hightemperature synthesis (SHS)] process has been widely studied recently,4"7 particularly as a means to prepare a large number of advanced ceramic,8"12 intermetallic,13"18 and composite materials.19"23 In one embodiment, the process generally starts by preparing a pellet that consists of powdered solid reactants. One end of the pellet is then raised to an elevated temperature by rapidly exposing it to a high-temperature heat source. The reaction will be ignited and proceed in a self-sustaining manner under conditions that favor self-propagation. Since the properties of the product material are ultimately influenced by the microstructure that is formed during the fabrication stage, however, the successful application of the combustion synthesis process to the fabrication of materials with controlled properties and quality will require a quantitative understanding of the influence of each parameter that is important to the process, as well as to the development of the sample's microstructure. A number of empirical studies have been reported in the literature24"33 that are dedicated to revealing the fundamental aspects of the combustion synthesis process. However, due to the complex nature of the combustion synthesis process, these at962
J. Mater. Res., Vol. 10, No. 4, Apr 1995
http://journals.cambridge.org
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tempts have largely been restricted
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