A Study On Mechanism Of Self-Propagating High-Temperature Synthesis By Combustion Front Quenching Technique

Because of its unique advantages, the self-propagating high-temperature synthesis (SHS) has become a novel technology of producing compounds and composite materials. TiC is an excellent wear-and heat-resisting material with high hardness and melting point; TiC-Fe is extensively used due to its higher toughness, wear-resisting, and heat-treatability; NiAl is a promising potential low density, high strength, and high temperature material which may take the place of conventional Ti-and Ni-base superalloys; and NiAl-Cu can improve the toughness but maintain the shape-memory effect of NiAl. Therefore, studying the SHS mechanism of these materials is of either scientific significance or engineering applied importance. The phase transition and the microstructural evolution are two keys to the SHS mechanism, and the scanning electron microscope (SEM) observation of the microstructural evolution was used for revealing the SHS mechanism in the present work. A combustion front quenching technique was adopted and developed overall so that more details of the microstructural evolution could be recorded more accurately, giving conclusive and detailed experimental evidences for a deep understanding of the SHS mechanism. The following important results were obtained: A shell-core mechanism and correspondent model of the SHS of TiC were established. A TiC shell was formed by reaction diffusion of C atoms into the Ti particle, and after melting of the Ti core coated with the TiC shell, C atoms dissolved into the molten Ti core by means of diffusion through the TiC shell, and then the TiC grains precipitated. The previously proposed carburization mechanism and dissolution-precipitation mechanism for the SHS of TiC were organically united into the shell-core mechanism, and an experimental phenomenon, the combustion-synthesized TiC was present in the form of slight binding powders, could be explained with this shell-core model. A ternary-reaction-diffusion / dissolution-precipitation mechanism and correspondent model of the SHS of TiC-Fe were proposed. Under a condition of using the coarser Ti and Fe powders, the combustion reaction between Ti and C occurred, respectively, in the Ti and Fe particles, and the former occurred earlier than the latter. The reaction in the Ti particle took place in the solid state by a ternary-reaction-diffusion of Fe, especially C into the Ti particle; because the melting point of the Fe particle rapidly decreased toward the ternary eutectic temperature with a diffusion of C and Ti into the Fe particle, the reaction in the Fe particle took place in the liquid state by a dissolution of C and Ti into the molten Fe liquid as well as a precipitation of TiC particles. A dual-dissolution-precipitation mechanism and correspondent model of the SHS of TiC-Fe were also suggested. In the case of using the finer Ti and the coarser Fe powders, although the combustion reaction mechanism in the Fe particle was not affected, the reaction in the finer Ti particle was changed to a dissolution-precipitation mechanism. In other words, the finer Ti particle melted before the reaction started occurring, C and Fe dissolved into the Ti liquid, and TiC particles precipitated. The melting of the finer Ti particle prior to the reaction was attributed to a significant decrease in both the size-controlled melting point due to a small-size effect of a particle and the composition-controlled melting point due to a rapid diffusion of C and Fe into the finer Ti particle. The role of the Fe addition in the SHS of the TiC-Fe was cleared up. The Fe addition not only served as a diluent and binder, what is more, it played the role of source of the reaction and the role of decreasing the activation energy of the reaction. It not only made the necessary preparations for the combustion reaction of Ti+C by the dissolution of C and Ti into the Fe liquid, but also it provided another source for the precipitation of the TiC particles; and the simultaneous diffusion of C and Fe into the Ti particles, leading to whether a formation of a ternary reaction diffusion layer composed of TiC particles and rich-Ti solid solution or a reaction by the dissolution-precipitation mechanism, changed the movement route of C atoms into the central region of the Ti particle in the absence of the Fe addition, so the activation energy was decreased. These revealed the cause why the ignition temperature of the reaction decreased with an addition of the Fe powder and indicated clearly the process in which the Fe powder became the binder. The effects of the reactant particle size on the characteristics of the SHS of TiC-Fe were investigated and explained satisfyingly with the established mechanisms and models. By comparing the characteristics of the SHS of four kinds of Ti-C-Fe mixtures with a same composition but not same size of Ti and Fe powders, it was found that the finer Ti powder led to the more complete reactions and hence the higher combustion temperatures; the finer Ti powder resulted in the higher reaction velocities, but in the case of using the coarser Ti powder, the finer Fe powder greatly decreased the reaction velocity; and the finer Ti powder made the TiC particles with a greater size, the products with a higher density and the layer-shaped pores parallel to the combustion wavefront. These effects were explained satisfyingly with the proposed mechanisms and models, and hence the availability of the mechanisms and models were confirmed. A dissolution-precipitation mechanism and correspondent model of the SHS of NiAl were established. After melting of the Al particle, the Ni particle dissolved into the Al liquid solution, and the NiAl grains precipitated. This was significantly different from that for a thermal explosion combustion synthesis of NiAl, since all the pre-combustion reactions prior to the melting of Al particle and all the possibly intermediate steps in formation of NiAl which occurred during the thermal explosion combustion synthesis were restrained by a rapid increasing of temperature of the reactants during the SHS. A previously doubtful point, i.e. the reaction velocity was affected by the particle size of both Al and Ni despite the melting of the Al particle before the reaction, was explained reasonably. A dissolution-precipitation-substitution mechanism and correspondent model of the SHS of NiAl-Cu were proposed. It was found that the combustion reaction was initiated by melting of the Al particle, and that during the reaction the Cu particles acted as an intermediate, changing the process of the SHS of NiAl. After melting of the Al particle, the Cu particle more rapidly dissolvedinto the Al liquid because of the lower melting point of Cu than Ni, and a CuAl2 phase precipitated. The CuAl2 phase then transformed into a CuAl phase with melting of the Cu particle. The Cu atoms in the CuAl phase were then substituted by the Ni atoms due to the greater affinity after melting of the Ni particle, finally forming β-NiAl particles containing a small amount of Cu and a Cu binder containing a small amount of Al and Ni.