Vol 7, No 2 (2016)

A comparative analysis of structural changes in the high-strength aluminum 7475 alloy subjected to equal channel angular pressing (ECAP) and multidirectional forging (MDF) at a temperature of 400°C (~0.75 T_m) is performed. It is shown that both ECAP and MDF of the present alloy led to significant grain refinement. The main mechanism of structural changes is mainly related to the formation of deformation bands such as geometrically necessary boundaries and microshear bands, which propagate in various directions and fragment initial grains, thus developing in “continuous” dynamic recrystallization. Comparison between ECAP and MDF performed to roughly the same strain values reveals the formation of similar micro-structures, which are characterized by close values of the average misorientation angles and the fractions of high-angle boundaries. It is also found that, irrespective of the deformation scheme, the grain size developed in both ECAP and MDF obeys a common dependence (unique function) on Zener–Hollomon parameter Z. Microstructure evolution in the highly alloyed aluminum alloy during high-temperature severe plastic deformation and the effect of severe plastic deformation techniques on grain refinement are discussed in detail.

Composite materials based on the Al₂O₃–MgO system reinforced with multiwalled carbon nanotubes (MWCNTs) were fabricated by vacuum sintering and hot pressing. The ceramic matrices were represented by corundum Al₂O₃ ceramic with the addition of 0.25 wt % MgO and magnesium aluminate spinel MgAl₂O₄. The prepared composites based on corundum and magnesium aluminate spinel containing MWCNTs were investigated by optical microscopy and scanning electron microscopy, and their microhardness and fracture toughness were studied. The samples are a densely sintered matrix with the average crystallite size of 3–5 μm with MWCNTs distributed uniformly on grain boundaries forming a reticular carcass. The samples of the Al₂O₃–MgO composite with 8 vol % MWCNTs demonstrated the flexural strength and fracture toughness two times higher as compared to the initial ceramic material (Al₂O₃ + 0.25% MgO).

Optical and structural homogeneity and photorefractive properties of LiNbO₃:Zn crystals (0.03 to 4.5 mol % Zn) were investigated by the methods of laser conoscopy, photoinduced light scattering, and Raman spectroscopy. It is established that the photorefractive effect nonmonotonically depends on the concentration of Zn²⁺ and is almost absent in LiNbO₃:Zn crystals containing 0.05 and 4.5 mol % Zn. The suppression of photorefraction is explained by a decrease in the number of defects with the electrons localized on them due to the replacement of the NbLi and VLi defects by Zn²⁺ cations, by the appearance of shallow traps near the bottom of the conduction zone, and by the radiative recombination of laser-induced photo-electrons. The conoscopic pattern of a single-axis crystal of high optical quality was observed in a LiNbO₃:Zn crystal containing 4.5 mol % Zn upon irradiation by the laser with a power of 90 mW. This pattern is of much better quality than the one observed with a laser radiation power of 1 mW, which is explained by curing of the defects by the laser radiation. An area of highly ordered structure with high alternation ordering of main and doped cations and vacancies along the polar axis in the cation sublattice is detected.

The processes of changing the charge state of metal–oxide–semiconductor (MOS) structures that contain a multilayer gate dielectric on the basis of a thermal SiO₂ film doped with phosphorus under the influence of high-field electron injection are investigated. It is determined that a negative charge accumulating in a thin phosphosilicate glass (PSG) film of MOS structures containing a SiO₂–PSG two-layer gate dielectric under high-field tunneling injection of electrons can be used for the correction of the threshold voltage, for the enhancement of the charge stability, and for an increase in the breakdown voltage of devices with MOS structure. It is shown that the density of electron traps increases with an increase in the thickness of the PSG film containing them, while their capture cross section remains unchanged. The method for the modification of the electrophysical characteristics of MOS structures by the high-field tunneling injection of electrons into a dielectric under a controlled current load is proposed. The method allows one to control changes in the parameters of MOS structures directly in the process of their modification. It is established that a MOS structure with a high thermal field stability can be obtained by its annealing at 200°C after the modification of its charge state by the electron injection.

The origin of the main defect-impurity complexes induced in silicon crystals with different contents of oxygen and carbon impurity atoms by electron irradiation in the temperature range 30–600°C has been investigated by means of IR absorption spectroscopy. The efficiencies of the formation of various optically active centers as a function of temperature of irradiation are obtained. The radiation-enhanced formation of a complex consisting of a substitutional carbon and oxygen dimer (T_irrad = 450°C) is revealed in carbon-containing Si. After irradiation at T_irrad = 500°C, vacancy–oxygen trimer–carbon centers, which give rise to vibrational absorption bands at 902, 956, and 1025 cm₋₁, are detected for the first time.

The effect of electron irradiation with E = 2 MeV and a dose of 1.9 × 10¹⁷ el/cm² of annealed and pristine carbon nanotubes (CNTs) on the electrical conductivity σ(ρ), thermoelectric emf (α), and elastic characteristics during deformation and relaxation during unloading is studied. The annealing of CNTs at 800°C removes the topological defects (the Stone–Wales ones, vacancies, and adatoms) which distort the CNT shape. As is found, after electron irradiation with small doses (0.63 × 10¹⁷ el/cm²), the defects accumulate, while at high doses (1.9 × 10¹⁷ el/cm²) they are healed. The growth and radiation defects lead separately to a topological disorder, reducing the σ(ρ) parameter and increasing α. However, upon irradiation and during the interaction between the defects of different genesis, their healing becomes more effective in comparison with annealing, and the mechanism of effective healing can be associated with the small CNT sizes, rapid migration of the sputtered atoms between the graphene layers, and their recombination with vacancies.

The influence of pulsed laser irradiation on mechanical properties of annealed and cold-worked pure aluminum is studied. Mechanical tests are performed by means of a miniaturized disk bend test technique using specimens with the diameter of 3 mm. It is established that laser irradiation does not influence the mechanical properties of annealed aluminum but leads to softening of cold-worked material. After 100 laser pulses, nearly complete recovery of aluminum strength occurs to the value characteristic of the annealed state. The plasticity of cold-worked aluminum is lower than that of annealed material and does not vary upon laser irradiation. Thermal and shock-wave mechanisms of laser impact are estimated, and it follows that, under the given parameters of irradiation, the main contribution to the variation of properties is that of shock-wave mechanism related to generation and propagation of acoustic waves which stimulate athermal annealing and recovery of dislocation structure.

Scaffolds made of recombinant spidroin and fibroin of Bombix mori silk were produced by the salt leaching technique. The regenerative properties of scaffold were evaluated in experiments with rats by implantation into bone wounds. According to the X-ray tomography data, the use of both types of biocompatible materials provides the restoration of the integrity of a bone. By analyzing the dynamics of regeneration, it was found that the use of spidroin leads to more rapid regeneration of bone tissue in the defect area as compared to silk fibroin.

An affine sorbent of the “core–shell” type was prepared on the basis of fly ash ferrospheres with the aluminosilicate glass phase content of 41 wt % as a magnetic core. The synthesis of the functional coating included four steps: the steam treatment of the fraction, the synthesis of a mesoporous silica coating on the ferrospheres surface, the surface activation by boiling in an alkaline medium, and the subsequent immobilization of Ni²⁺ ions on the surface of modified ferrospheres by impregnation. The modified ferrospheres were characterized by the methods of simultaneous thermal analysis, infrared spectroscopy, X-ray diffraction, and low-temperature nitrogen adsorption. It was found that the obtained sorbent was characterized by an acceptable strength of binding of the functional shell and the sorption capacity with respect to the green fluorescent protein of up to 6.7 mg/cm³. The stability of the sorbent in repeated use was studied. It was established that the sorption capacity after the seventh cycle was stabilized at the level of 70% of the initial capacity.

Structural examinations and strength tests of powder composites are carried out. The powder composites are produced by hot consolidation of aluminum, titanium, and silicon powder blends. The temperature and deformation modes of the hot consolidation to get the maximum bending strength and plasticity are determined. It is shown that the bending strength and plasticity of the hot compacted powder composites depend basically on the mechanical properties of a metal matrix being formed by joining of the adjacent aluminum particles during hot deformation processing.

The electrical properties of contacts of second-generation (2G) GdBCO superconducting tape conductors, which are soldered by Rose’s and PbSn solders, are investigated in an external magnetic field up to 5 T at a liquid nitrogen boiling temperature. The joint resistance at T = 77 K in the case of PbSn solder is approximately half that in the case of Rose’s alloy. The rise in the contact magnetoresistance with the field is weak and independent of the orientation of the magnetic field, and it is saturated in fields on the order of 3 T for both solders. The mechanical tensile strength of initial tape conductors and contacts is measured at room temperature. The ultimate strength of contacts produced by the PbSn solder is more than twice that for the contact made by Rose’s alloy. In the latter case, the ultimate tensile strength is lower than the critical stress of superconductor degradation.

The process of the autoclave leaching of boehmite-kaolinite bauxite of Severoonezhsk deposits by hydrochloric acid is researched. The effect of temperature, process time, and concentration of hydrochloric acid necessary for the aluminum transition into solution are investigated. The rate constants and apparent activation energy of the process are determined. The mechanism of bauxite dissolution in hydrochloric acid is proposed. It is shown that the kaolinite dissolution is the limiting stage of the process. The results obtained make it possible to synthesize a new generation of mixed coagulants based on aluminum hydroxychloride and iron.

The possibility of obtaining new relatively inexpensive electrode materials to provide enhanced efficiency of hydrogen evolution reaction (HER) in an aqueous acid solution was investigated. For this purpose, the surface properties of cathodes made of microcrystalline graphite were modified by pulsed laser deposition of thin films of WSe_x. The structure, morphology, and chemical composition of the thin film coatings were varied by changing the deposition conditions and subsequent heat treatment. The compact and dense structure of the film in an amorphous and crystalline state did not result in a marked positive impact on the character of the HER process, which was investigated in 0.5 M H₂SO₄ solution at room temperature. Formation of thin layers consisting of nanocrystalline “petals” WSe₂ caused an increase in cathodic current by more than 6 times (at a voltage of–150 mV), and the Tafel slope of the voltage vs. current curve was reduced by about 80 mV/dec. The conditions were determined to produce on the surface of the graphite cathode a high density of new catalytically active sites that formed on edges of molecular planes forming a layered structure characteristic of WSe₂ nanocrystals.

The obtaining of double layer cylindrical blanks with a combination of layers of a heat-resistant niobium alloy N65V2MTs + a high-strength steel OKhN3M is studied. A cladding technique with the N65V2MTs alloy inside of a cylindrical surface of the steel OKhN3M is developed and the prototypes with 100% integrity of the layers on the entire length of the blanks are produced. A microanalysis of the welding zone of the layers shows the need for optimization of the explosive welding to reduce the thickness of the transition zone and to create a more even waveform of the welding zone.

Three batches of finely divided Nb₃Al intermetallic powders obtained by calcium hydride reduction were studied for the contents of light elements (H, O, N, C, and S). The oxygen-containing phases present in the Nb₃Al powder samples were determined. The oxygen contained in the powder was qualitatively and quantitatively divided into oxygen as a part of adsorbed water, oxygen as a part of surface organic compounds, and oxygen as a part of niobium oxides (NbO_x) and aluminum oxides (Al₂O₃).