Mechanistic Studies of Combustion Synthesis

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In this program, two techniques will be used to study the fundamentals of materials synthesis via SHS processing; the first involves quenching pellets during the reaction, while the second involves study of powder particles on a heated surface of a substrate. The first procedure will be carried out by reacting a wedge-shaped sample imbedded in a high thermal conductivity block, resulting in extinguishment of a combustion wave propagating through the sample towards the apex of the wedge. After quenching, the sample will be cut and polished for metallography study and a layer-by-layer X- ray diffraction analysis will be performmed to quantitate variation of phase content as a function of position. SEM/EDXS studies combined with quantitative optical microscopy will be used to observe changes in local composition, grain size, and pore distributions. Effects of sample preparation conditions, such as powder particle sizes and green density, will be studied. These experiments will provide information on evolution of mixture micro-structure as the reaction front propagates, yielding insight into mechanisms which can be used to control the process and tailor the product properties. In the second procedure, a narrow size cut of reactant powder (lower-melting component) will be dispersed on a foil of the other reactant, which will be electrically heated at rates typically encountered in SHS processing. Upon heating, the powder will melt and spread on the foli surface; the dynamics of spreading (important in SHS processing) will be studied at various heating rates and particle sizes. Upon further heating, reactions will occur; these will be quenched by current shutoff, and the resulting samples analyzed with SEM/EDXS, delineating reaction and microstructure evolution. Initially, attention will be focussed on Ni3Al synthesis, with later extension to other intermetallics. The synthesis of advanced materials by combustion reactions is an attractive alternative to other methods of materials preparation due to simplicity, relatively low energy requirements, high product purity potential, and (probably most important) opportunity for formation of metastable products resulting from the combination of rapid cooling and high reaction rates. In combustion synthesis, highly exothermic self-sustaining reactions in a powder bed yield final products; in one variant of this procedure, self- propagating high-temperature synthesis (SHS), reaction initiated at one end of the powder sample self-propagates through it as a combustion wave, while in another (thermal explosion mode) the sample is heated uniformly until reaction is initiated throughout the sample. One very important task in further development of combustion synthesis processing is establishment of control of product properties via control of microstructure; attainment of such control depends on fundamental understanding of chemical and structural transfor- mations taking place in and behind the propagating combustion front.