Investigation of Phase Transformations and Ordering During Combustion Synthesis
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ABSTRACT Two complementary experimental techniques are presented that describe the mechanisms during the combustion synthesis of NiAl and Ti5 Si 3 . The first involves quenching a reacting wedgeshaped sample imbedded in a copper block where the propagating combustion front extinguishes while traveling to the apex. The second technique, time-resolved X-ray diffraction (TRXRD), provides a direct in-situ observation of the sequence of high temperature phase transformations. The information obtained from this investigation will be useful in developing improved process models of combustion synthesis, which can lead to the production of advanced materials with tailored microstructure and properties. INTRODUCTION Combustion synthesis is an attractive technique for the production of a wide variety of advanced materials including ceramics, ceramic-metal composites, intermetallics and nanophase materials. It is based on the principle that once initiated by an external heat source, a highly exothermic reaction can become self-sustaining and yield the final product without requiring additional heat. The reaction can be conducted in two modes. In the self-propagatinghigh-temperature synthesis (SHS) mode, a pressed compact of reactant powders is ignited at one end from which the reaction propagates through the sample in the form of a combustion wave. In the volume combustion mode, the sample is heated uniformly until reaction takes place essentially simultaneously throughout the volume. The heat liberated by the chemical reaction provides uniform heating of each point within the reaction mixture volume, that is clearly an advantage over conventional techniques. The unique processing conditions associated with combustion synthesis include extremely fast heating rates (up to 105-106 K/s), high temperatures (2500-3500 K) and short reaction times (on the order of a few seconds). These conditions have the atSample tractive potential to yield materials with novel structures and properties. In order to utilize this technique effectively, and to control the process, a fundamental knowledge of combustion synthesis mechanisms involving the T.hermocouple evolution of non-equilibrium initial products Wells . toward the final product is of critical importance. This understanding will lead to micro. . ................... scopic descriptions of fundamental phenom-' ena that will allow the design of advanced. materials with tailored properties. This mechanistic understanding is also important for the development of accurate reaction kiFigure 1. Schematic of the combustion wave netic models and ultimately, principles for quenching technique. 593 Mat. Res. Soc. Symp. Proc. Vol. 398 1996 Materials Research Society
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Figure 2. Schematic of the Time-resolved X-ray Diffraction (TRXRD) experimental set-up. scale-up of the process. Combustion synthesis has attracted considerable interest in recent years, and details of the process and its prospects have b
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