Specific features of formation of bulk porous systems in the self-propagating high-temperature synthesis. Borides

The effect of ultrasound oscillations (USO) on the combustion velocity and temperature, phase composition and microstructure of compounds produced by the self-propagating high-temperature synthesis (SHS) in the Ti–B system is studied using the earlier developed experimental setup. It is shown that the effect of powerful ultrasound on SHS is connected with the thermal factor, viz. the enhancement of heat removal from the specimen surface due to forced convection, and the physical (non-thermal) factor that consists in the influence of USO on the melt spreading and crystallization of different phases in the SHS wave. It is demonstrated that for multiphase system Ti–B an increase of the boron content in the initial charge leads to grain refinement of the synthesized product and the imposition of USO on the SHS results in a change in the grain morphology: their shape becomes closer to equiaxial. In composition Ti+1.5B, under the action of USO cubic-shape product grains are observed on the inner surface of pores, and in composition Ti+2.0B the synthesized TiB2 grains become more vividly facetted. As a result of imposition of ultrasound on the synthesis process, grain refinement is observed for all the compositions along with the formation of a large number of borides and their redistribution in the volume. The use of ultrasound brings about changes in the quantitative phase composition of the synthesis products and the ratio between the orthorhombic and cubic modifications of phase TiB. The research has shown that the presence of pores in the initial charge plays an important role in the structure formation of the final product, and hence it appears impossible to obtain an equilibrium material by the SHS method. It is found that an optimal USO amplitude exists at which it is possible to obtain uniform fine-grained structure of the material. This permits controlling the structure formation at SHS.