Production of corundum armored ceramics

The purpose of this study was to develop the compositions and technology of corundum armored ceramics with high ballistic characteristics based on aluminum oxide with an α-Al2O3 content of more than 99 wt.%. The results of the study of the physicochemical processes of obtaining corundum ceramics modified with complex additives consisting of magnesium - aluminosilicate eutectic mixture and oxides of magnesium, yttrium, titanium, zirconium are presented. The introduction of complex additives into the composition of corundum ceramics provides a significant reduction (100-150°C) of the sintering temperature of the product. In this case, additives of yttrium and zirconium oxides contribute to the formation of a uniform-grained microstructure of ceramics, and in the case of adding titanium oxide, a collective recrystallization of corundum grains is noted. The use of small additives (0.3-0.5 wt.%) of magnesium and yttrium oxides together with a eutectic mixture in the compositions of corundum ceramics based on high-quality alumina contributes to the formation of a uniform-grained, dense structure of the material and giving it a high level of physical and mechanical properties as a result of directional the action of each component of the additive on the physicochemical processes of phase formation of the crystalline matrix. A model idea of the mechanism of action of additives on the formation of the microstructure of ceramics during stage-by-stage heating from 1350 to 1650°C in the sintering mode of products is presented. The developed compositions and technologies of corundum armored ceramics have a production focus and implementation in practice.

Effect of carbon nanotubes and oxide nanofibers on mechanical properties of aluminum AD0 after cold working and annealing

Cast aluminium composite materials based on AD0 aluminium, reinforced with carbon nanotubes and aluminium oxide nanofibers are obtained. It is shown that the additives of 0.05 wt % of single wall carbon nanotubes (SWCNT) and aluminum oxide nanofibers (AONF) allow increasing ultimate tensile strength of cast metal by 15 % and 16 % respectively. Strengthening of the metal is maintained after cold deformation and annealing. Aluminum AD0 with additives SWCNT or AONF retains high strength after annealing, while aluminum without additives is significantly weakened, this allows to obtain annealed aluminum wire, with a strength limit of 54 – 69 % and yield strength of 53 – 78 % higher than that of metal without additives. The influence of SWCNT and AONF on the size of aluminum grain AD0 in the cast state, after cold deformation and annealing has been studied. Nano-additives have been found to inhibit the growth of metal grains at the stage of collective recrystallization, as well as to influence the process of primary recrystallization of aluminum.

Features of formation of nanodisperse granular structure during РТ-sintering of hexagonal diamond (lonsdaleite) powder

Results of an electron microscopic investigation of particles of lonsdaleite powder with additives of cubic diamond and polycrystalline specimens based on it at Р = 7,7 GPa in the temperature range 1700―1900 °С are presented. Lonsdaleite particles are characterized by a predominantly ternary texture [1120]l of different degree of perfection. Structural mechanisms of transformations in lonsdaleite particles, which cause the formation on nanograined structure in sintering, have been established. The initial stage is the mechanical dispersion of particles and dispersion as a result of plastic faulting deformation, which promotes their fragmentation without breakdown of continuity. The indicated processes lead to the destruction of texture in particles. Beginning from 1700 °С, the lonsdaleitecubic diamond phase transformation with the orientation ratio (111)dII (001)l occurs. It is realized within rods as elements of the substructure of lonsdaleite. At 1900 °С, the size of detected elements of the structure (grains) is 5―15 nm. Beginning from 2000 °С, the self-association of such grains into aggregates with sizes up to 70 nm and the subsequent process of coalescence of grains in aggregates with the formation of the monocrystalline state occur. The next stage of formation of the granular structure is caused by the formation of grain boundaries and development of collective recrystallization. After sintering at Т = 2100 °С, the grain size in specimens does not exceed 100 nm. It has been established that the transformation in lonsdaleite proceeds by structural mechanisms that are characteristic of wurtzite modifications of boron nitride and silicon carbide (strong disordering in the direction of the basal axis, plastic faulting deformation, and formation of multilayer polytypes during the hexagonal-to-cubic phase transformation). Keywords: lonsdaleite, particles, sintering, fragmentation, grains, coalescence.

Влияние температуры на размер частиц и рекристаллизацию нанопорошков сульфида серебра

The recrystallization of silver sulfide Ag_2S nanoparticles has been studied and the range of the thermal stability of the nanoparticle sizes has been determined. Nanopowders Ag_2S with particle sizes of 45–50 nm were obtained by chemical deposition from aqueous solutions. To study the thermal stability of the Ag_2S nanoparticle sizes, the nanocrystalline powders have been annealed in a vacuum of 0.01 Pa on heating from room temperature to 493 K and in argon at 623 K. Annealing up to a temperature of 453 K leads to insignificant nanoparticle growth and annealing of microstrains, which allows one to consider this temperature range as the region of thermal stability of the silver sulfide nanostate. The temperature range from 450 to 900 K, in which the particle size increases by a factor of 3–6, corresponds to the temperature of collective recrystallization of the silver sulfide nanopowder.

Cu-Ag Solid Solutions Formation and Properties upon Severe Plastic Deformation

A number of nonequilibrium nanocrystalline fcc solid solutions Cu1-x-Agx (x = 0,1; 0,2; ... 0,9; 1,0) were obtained by mechanical alloying of powders by the use of high pressure torsion. Chemical homogeneity, microstructure, mechanical properties and thermostability were studied. The obtained alloys were found to be characterized by a positive excess over Vegard law, ~ 20 nm grain sizes, a microhardeness of 4.5-6 times higher than the Cu and Ag one and brittle type of the fracture surface. In situ shear stress vs. strain measurements and energy-power parameters estimation were performed. A decomposition of nonequilibrium solid solution induced by a thermal influence begins from close to room temperatures and finishes completely at heating up to 500°С accompanied by collective recrystallization development.