The work is devoted to solving actual scientific and technical problems - the development of the physical fundamentals of structure and property control of the directionally reinforced composite materials based on oxygen-free refractory compounds, as well as the creation of ceramic composite materials isotropic at the macro level and anisotropic at the micro level.
In order to control geometrical characteristics of the microstructure of composite materials during the crystallization of eutectic alloy melts, the activation energies of nucleation and growth of the reinforcing phase were calculated. It was concluded that the constitutional component of supercooling has the greatest influence on the nucleation and growth processes for crystalline phases in eutectic alloys.
In order to investigate the effect of constitutional supercooling on the nucleation and growth of diboride inclusions during directional solidification of eutectic alloys of the LaB6-MeIVB2 system, the mechanical vibrations were imposed during the growth process of the crystal during directional solidification. It was defined that the microstructure of composites obtained with the imposition of mechanical vibrations is more uniform, and has a more uniform size distribution of fibers.This contributes to an increase in compressive strength up to 2 times compared with the eutectic alloys grown without the imposition of mechanical vibrations.
In order to reduce the amount of constitutional component of the minimum supercooling for the nucleation and growth of fibers, the doping with silicon, carbon and aluminum additives of mixtures of eutectic alloys of the LaB6-MeIVB2 and B4C-MeIVB2 systems during directional solidification was carried out. It was shown that an increase in the amount of aluminum and silicon admixture to 2-3 vol.% leads to a decrease in transverse size and distance between the diboride inclusions. At the same time, increasing the amount of carbon impurities on the contrary leads to an increase in diameter and a decrease in number of diboride inclusions. It was established that the doped directionally solidified eutectic LaB6-TiB2 and B4C-TiB2 alloys have 1,5-2 times higher bending strength.
The interdependence between structure and mechanical properties in the range of temperatures from 20 to 2000 oC of quasiternary directionally solidified eutectic alloys of B4C-TiB2-SiC та B4C-TiB2-NbB2 systems were found. It is shown that the introduction of additional structural components into the composites allows to increase their bending strength to 370 MPa at 2000 oC, which is more than 0,9 of their melting temperature.
In order to eliminate the main disadvantage of the directionally solidified eutectics – anisotropy due to the nature of microstructure of the composites, the orientation of the fibers of one of the components along the direction of crystallization and limitation on the shape and size of the parts, the basics of technology for manufacturing of powders from reinforced composite materials of B4C-TiB2 and LaB6-TiB2 systems by mechanical grinding and centrifugal plasma spraying and their consolidation were developed. The use of such powders can increase the high-temperature bending strength of the spark plasma sintered B4C-based composites at 1600 °C to 407,2 MPa.
By the method of X-ray diffraction analysis it was found that the nature of the phase components primarily affects the level of residual stresses in the matrix phase in the ceramic composite. It was found that the increase in strength of the directionally solidified eutectic composites caused by the increase in plasticity of the phase components with testing temperature and by the mechanism of grain boundary (on the fiber-matrix interface) strengthening. It was shown the realization of strain hardening mechanism in monocrystalline fibers of transition metals and contributes to increasing the strength of the composite.
Key words: ceramic composites, directional crystallization, directionally solidified eutectic alloys, boron carbide, lanthanum hexaboride, refractory borides, reinforcement mechanisms, zone melting, spark-plasma sintering, doping, mechanical vibrations, multiphase eutectics.
