Vol 10, No 3 (2019)

The damage and structural state of the surface layer of Al–Li–Mg samples composed of Al–5% Mg–2% Li (wt %) under pulsed action of power streams of high-temperature deuterium plasma and high-energy deuterium ions in the Plasma Focus (PF) device have been investigated. The radiation power density was q ~ 10⁶ W/cm²; the pulse duration was 50–100 ns. Pulsed thermal heating and rapid cooling is established to lead to the melting and solidification of a thin surface layer of the alloy for several tens of nanoseconds. At the same time, in the superheated surface layer of the alloy, microcavities of a spherical shape are formed which is associated with intense evaporation of lithium into micropores within the heated layer. Thermal stresses caused by abrupt heating, melting, and cooling of a thin surface layer of metal result in formation of microcracks in the near-surface zone of the samples. The evaporation by the power electron beam of the elements of the anode material of the PF device (copper and tungsten) and their subsequent deposition onto the irradiated surface of the investigated samples in the form of droplets of submicron size are noted. It is shown that the thermal and radiation-stimulated processes generated in the alloy under the action of pulsed energy fluxes in the implemented irradiation regime lead to the redistribution of elements in the surface layer of the aluminum solution, contributing to an increase in magnesium content and the formation of magnesium oxide on the surface.

The competition of intermediate phases at the initial stages of reactive diffusion is analyzed in the context of two fundamental ideas that were put forward together with K.P. Gurov in 1981–1990. The first deals with the kinetic suppression of certain phases during the nucleation stage by the adjacent phase layers. The second idea considers the thermodynamic suppression of the intermediate phase nucleation due to the dependence of the nucleation barrier on the local concentration gradient in the initial phases. The critical concentration gradient makes the nucleation impossible. The experimental data are shown to justify the theoretical suppositions.

X-ray diffraction analysis, optical metallography, and transmission electron microscopy demonstrated that irradiation with high-current pulsed electron beams in the melting mode leads to formation of residual compressive stresses and a finely dispersed globular-lamellar microstructure (the α, α', and α'' plates being oriented parallel or nearly parallel to the surface) in the near-surface layer of samples of VT6 and VT8 alloys up to 20 μm thick. These phenomena are expected to increase the fatigue strength of the material during bending tests. Meanwhile, residual tensile stresses are formed on the surface of samples of VT9 alloy at any energy density of electron beams; vacuum annealing is required to remove these stresses.

The method of spectral pyrometry is used for determining the dynamics of heating of Si, Nb, and Cu samples by continuous radiation of a Nd:YAG laser (λ = 1.064 μm, power up to 30 W, power density of (0.6–1.1) × 10³ W/cm²). The thermal radiation spectra were recorded by a small-sized spectrometer operating in the visible region with a frequency of 33–15 Hz in the temperature range above 900 K. The surface temperature of the samples was determined by processing 200–300 spectra with the special program Spectral Pyrometry and the heating dynamics was calculated on the basis of the obtained data. For an electrotechnical steel sample, both the dynamics of heating of the material under laser action and its cooling after the termination of the laser radiation were determined.

A probe study of glow discharge plasma is carried out according to the technique of a single cylindrical Langmuir probe at a pressure of 60 Pa in a gas mixture of 70% Ar + 30% N₂. The distribution of density of charged particles in a cavity formed by a technological screen to create a hollow cathode effect and by the treated surface of the material is obtained. It is established that the hollow cathode effect leads to increasing ion concentration near the cathode surface during ion nitriding in a glow discharge. The maximum ion concentration in the center of a cathode cavity is n_i = 3.5 × 10¹⁶ m^–3. Effects of the local ion nitriding in the plasma of glow discharge with a hollow cathode as well as the duration of the plasma treatment process on the microhardness and structure of diffusion layer of martensitic structural steel at the temperature of 550°C are studied. The process of diffusion saturation for martensitic steel local ion nitriding is found to be described by an inverse parabolic law. It is proved that smoothing of microhardness distribution through the hardened case rises with the rise of duration of nitriding in plasma of glow discharge with hollow cathode. Three zones are formed in the steel surface as a result of the local ion nitriding: plasma modified zone, transition zone, and unmodified zone. The thickness of the diffusion layer in the zone modified with plasma of glow discharge with hollow cathode effect is two times greater than that in the zone modified without hollow cathode effect.

A technique for measuring the internal stresses in polymer films deposited on a glass substrate in gas discharge plasma is developed. Using this technique, the dependence of internal stress in thin films of polymethylmethacrylate and polystyrene on the deposition time and the current density of the barrier discharge at atmospheric pressure is studied. It is found that, when forming thin polymer coatings from monomers with high polymerization efficiency, the average value of internal stresses is higher than that for coatings obtained from monomers having low polymerization efficiency in barrier discharge plasma at atmospheric pressure. This can be explained by an increase in the specific number of cross-links in the first type of films under the action of ultraviolet radiation from the plasma. The quasi-optimal deposition time of the polymer film coating from styrene is found in which the value of the internal stress is minimal at a film thickness acceptable for a number of practical applications.

The effects of the conditions of a raw material introduction as well as mixing of it with a high-temperature gas flow under the plasma chemical synthesis of superdispersed powder of W–C have been studied. In the case of incomplete raw material evaporation and its fast quenching, preferred cubic tungsten carbide β-WC formation is found to occur, as well as nano- and microparticles with a complicated phase composition. In the case of complete raw material evaporation, the products of reducing synthesis are nanopowder of tungsten or semicarbide W₂C depending on the amount of hydrocarbon introduced. There is a second stage of the process of low-temperature synthesis which leads to a single-phase of tungsten carbide α-WC formation. Complete evaporation of raw material must be ensured in the plasma chemical process for obtaining of α-WC powder with a maximum specific surface area.

The effect of low-energy pulsed laser irradiation (laser LRS-150A, λ = 1.06 μm) on the surface of Ta–10% W coating produced by ion-plasma deposition in vacuum was investigated. The coating thickness was ~250 μm. With the help of the metallography method, it was shown that the coating average grain size was ~25 μm. The laser pulse energy was 30 J, pulse duration τ = 14 × 10^–3 s, and the interval between laser pulses was 7.5 s. The laser beam was focused on the sample surface into a spot with a diameter of 5 mm. The microrelief of the coating surface was investigated with the use of a TESCAN VEGA II scanning electron microscope. X-ray microanalysis in the scanning line of the surface and cross section of the coating and the substrate was carried out on an INCA Energy 250 energy dispersive spectrometer. It was shown that, after a multicycle laser irradiation (200 pulses), numerous rounded protrusions appeared on the surface of the coating. The diameter of the protrusions ranged from 25 to 225 μm and their maximum height was 30 μm. In the irradiation zone, redistribution of W occurred. The concentration of W in the region of the protrusions decreased substantially and increased in the ranges between the protrusions. It was concluded that the appearance of protrusions on the surface of irradiated surface could be due to the simultaneous occurrence of several interdependent processes, such as the loss of stability of the coating with the formation of alternating areas of tensile and compressive stresses, diffusion of tungsten into the regions of compressive stresses, and relaxation of internal stresses by grain-boundary sliding with extrusion of grains and their conglomerates on the surface.

A method for the formation of composite electrolytic coatings in the process of electrodeposition in an electrolyte with ultradisperse diamond (UDD) particles in the programmable pulsed current regime under simultaneous laser irradiation has been developed. This method provides the layer-by-layer deposition of gradient nickel coatings 15–20 µm in thickness with a variable content of UDD particles along their depth. The amount of UDD particles is minimal (0.10–0.13 at %) in the layers deposited in the initial period of this process, whereas the concentration of UDD particles in the layers formed at the later process stages grows to 0.19–0.26 at %. The concentration gradient of UDD particles along the depth improves the adhesion properties and wear resistance of coatings and provides a 16% decrease in the consumption of UDD particles with a local increase in the concentration of UDD particles in a coating to 0.32 at %.

The possibility of formation of high-quality MAO-coatings on structural carbon steels by complex treatment including a liquid aluminizing and microarc oxidation in solutions with finely dispersed corundum powder has been shown. It is ascertained that a metal Al coatings are formed on the steel substrates during aluminizing. The outer layers of these coatings are composed of Al and the inner layers consist of Al and Fe. MAO-coatings are formed as a result of oxidation of aluminum coatings and introduction of microparticles of corundum from the solution into growing alumina layer. Ultrasound treatment is found to strengthen the process of introduction of corundum microparticles in alumina. It is shown that the MAO process must be completed before the aluminum oxidation begins in the inner layers of the metal Al coating, since otherwise Fe can fall into the MAO-coating, which leads to a change in the structure and degradation of physicomechanical properties of MAO-coatings. The conditions for obtaining thicker MAO-coatings with increased hardness, wear resistance, and also higher electrical resistivity and breakdown voltage are determined.

Thermal processes of modifying the surface layer of a metal in a moving substrate with continuous induction heating of a rectangular substrate region by a high-frequency electromagnetic field have been studied using numerical simulation. The distribution of electromagnetic energy over the substrate surface is considered uniform, and over its volume, it is described by empirical formulas. It is assumed that, during the melting of the metal, the nanosized particles of the refractory compound inside of it are evenly distributed over the volume of the liquid metal and act as crystallization centers during its cooling. The boundary of the melting region is determined in the Stefan approximation, and the boundaries of the solid phase growth region are computed within the framework of the Kolmogorov theory of the metal crystallization. In the quasi-stationary distribution of the temperature field approximation, the size of the melting and crystallization zones was estimated and the growth kinetics of the solid phase was calculated. The characteristics of induction heating that make it possible to reduce the surface treatment time were determined. Calculated estimates of the specific power of the electromagnetic field were obtained that ensures that the melting region remains unchanged at the substrate velocity in the range of 1–3 cm/s.

One of the main disadvantages of polymers widely used for the manufacture of implants, including prosthetic vascular grafts, as well as resorbable matrices for tissue engineering and regenerative medicine, is the hydrophobicity of their surface. The purpose of this work is to carry out comparative analysis of methods for surface modification of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) specimens aimed at regulating the hydrophilic, morphological, and biological properties of their surface. It was found that, when various hydrophilic compounds are incorporated into the volume of the polymer at a concentration of 10–50 wt %, the greatest hydrophilization of the surface of PHBV specimens is observed when using Pluronic^® F-68 at a concentration of 20 wt % (contact angle θ = 53 ± 9°). The treatment of specimens with 1 N NaOH for 6 h at 37°C made it possible to achieve a high degree of surface hydrophilization (θ = 36 ± 3°) with a concentration of negatively charged acid groups of 2.9 ± 0.1 nM/cm² and without visual signs of destruction. The incorporation of hydrophilic compounds into the volume of film specimens allows achieving significant hydrophilization of their surface but is poorly effective in terms of improving their matrix properties. Alkaline hydrolysis makes it possible to hydrophilize the surface of the specimen much more efficiently by generating a negative charge during treatment, which has a positive effect on the adhesion and proliferation of human adipose tissue-derived mesenchymal stromal cells.

A method for production of Sn–Sb–Cu alloy (babbitt) based composite coatings reinforced with micron-sized SiC and B₄C particles on a steel substrate has been developed. A combined action of uniformly distributed high-modulus reinforcements and plastic matrix increases the strength and durability, reduces the wear rate, and significantly improves the antifriction properties of the coating. The surfacing rods based on B83 babbit containing reinforcing particles for arc welding of coatings on steel substrate were produced by the extrusion method. These rods were characterized by lack of porosity and discontinuities in the internal volume as well as by defect-free surface. The density and hardness of the rods are comparable with the same characteristics of the cast babbitt. Arc welding of coatings was performed in the AC mode with a non-consumable tungsten electrode in an argon atmosphere. Welding current I = 160–170 А, arc voltage U = 16–18 V, welding rate V_w = 4.2 m/h, and protective gas (Ar) flow rate V_Ar = 12–14 L/min. The structure and tribological properties of the composite rods and coatings deposited were investigated with the help of electron and optical microscopy methods and under dry sliding friction tribological tests. The size of SnSb intermetallic compounds in the rods after extrusion and in the coatings deposited was determined. It was found that the size of intermetallic phases decreased by a factor of two after introduction of SiC and B₄C particles into the babbitt alloy. The results of wear tests show that reinforcing and modifying the structure of the surfacing rods by introduction of high-strength micron- and submicron-sized ceramic SiC and B₄C particles improves the antifriction properties of surfaced composite coatings. The friction coefficient of the composite coating was the same throughout the load interval, while for the unmodified babbit coating its value was by two times higher than that for the composite coating under a load up to 39 N and 20% higher under loads greater than 39 N. The coatings with high-strength SiC and B₄C particles have a low coefficient of friction (0.4) and a high coefficient of the friction process stability (0.8).

Mechanoluminescence of a composite material on the basis of polymethyl methacrylate and finely dispersed powder of SrAl₂O₄:(Eu²⁺,Dy³⁺) phosphor was investigated. The mechanoluminescent (sensory) layer sensitive to mechanical action was formed directly in the surface layer of the polymethyl methacrylate that is transparent in the visible region of the spectrum. The kinetics of mechanoluminescence and the photoluminescence spectra of the SrAl₂O₄:(Eu²⁺, Dy³⁺) phosphor microparticles in the polymethyl methacrylate matrix were studied. Photoluminescence was excited by nanosecond laser pulses with a wavelength of λ = 355 nm, and mechanoluminescence was initiated by an acoustic wave generated by short laser pulses, shock mechanical action with hammer device, or dynamic pressure of the stylus sliding along the surface of the sensory layer.

The frequency dependences in the range of 25–10⁶ Hz of dielectric properties (real and imaginary parts of complex dielectric permittivity and electric modulus) of composite materials based on high-density polyethylene and dispersed filler TiO₂ are studied. It is shown that the real part of the complex dielectric permittivity ε of composites increases with decreasing frequency. The magnitude of the effect increases with an increase in the TiO₂ content in the composite of more than 20%. At high frequencies >2 × 10³ Hz, the value of ε of composites is practically independent of frequency. The frequency dependences of the electric modulus M—the inverse complex dielectric permittivity—are determined. In composites with a TiO₂ content of up to 20%, the imaginary part of the electric modulus M' decreases with increasing frequency, and in composites containing from 20 to 50% TiO₂, the opposite effect of decreasing M at low frequencies is observed. On the frequency dependences of all dielectric characteristics, there are no peaks indicative of possible mechanisms of dielectric relaxation associated with molecular mobility.

FeNi alloy nanoparticles as a part of metal-carbon nanocomposites were obtained. The synthesis was carried out under conditions of IR annealing of precursors based on joint solution of polyvinyl alcohol (PVA) and hydrates of iron and nickel nitrates. The features of formation of FeNi nanoparticles as a function of the synthesis conditions, as well as the effect of the synthesis temperature and the structure and composition of nanocomposites on their magnetic properties, were studied. It has been established that FeNi alloy nanoparticles can be formed in two ways simultaneously: through the reduction of nanoparticles of nickel-substituted magnetite (Ni, Fe) Fe₂O₄ and the reduction of Fe₂O₃ to iron, which dissolves in nickel or Ni₃Fe. The average size of the alloy nanoparticles increases from 6 to 14 nm, the size distribution range widens, and its maximum shifts from 3–5 to 9–11 nm as the synthesis temperature increases to 700°C. The growth of nanoparticles occurs owing to agglomeration processes caused by structural changes in the nanocomposite matrix during IR heating. It has been shown that the materials obtained at temperatures above 400°C exhibit ferromagnetic properties and at T = 700°C exhibit properties characteristic of nanosized particles of a solid solution of FeNi. The growth of nanoparticles as the synthesis temperature rises and reduction of iron oxides leads to the increase in saturation magnetization. Changes in the coercive force are determined by an increase in the average size of FeNi nanoparticles with increasing synthesis temperature. So for nanocomposites synthesized at 600°C, the coercive force is maximal, and then a secondary decrease occurs owing to the formation of larger nanoparticles.

The feasibility of obtaining a self-lubricating composite material of the Al–Sn system with the alloyed matrix by the liquid-phase sintering of compacts obtained from the mixture of powders of the eutectic Al–12Si alloy and tin powders was investigated. The porosity of the raw compacts was ~10%; the strength of the sintered Al–12Si–xSn composites was significantly lower as compared with the additive value. The porosity of compacts decreased and the strength reached the theoretical value after additional hot densification of the sintered samples at 200 and 250°C (lower and higher than the tin melting point). The plasticity of composites under compression after densification increased as well and showed its maximum in the composite with 10–20% Sn. Analysis of the compression curves showed that, for the sintered composites, the long linear stage of plastic flow with a low coefficient of strain hardening is a characteristic feature. With the increase in the tin content, the stage of the linear flow shortens and with the 40% Sn concentration disappears. The transcrystalline localized flow and material cracking is observed before fracture.

The present work shows the ability to synthesize bimetal Janus nanoparticles of immiscible iron and lead metals via the electric explosion of two conductors. The electric characteristics of the explosion of Fe and Pb conductors are studied, and the dispersion/phase composition of nanoparticles, as well as the elemental distribution in nanoparticles, is comprehensively analyzed. Finally, the mechanism of the formation of Fe–Pb Janus nanoparticles under the electric explosion of conductors is proposed as well.

A method for obtaining the quasi-one-dimensional distribution of tritium in zirconium alloys was developed. Various conditions of saturation of samples with tritium were analyzed and distributions of tritium concentration corresponding to the saturation conditions were measured. An algorithm for restoring of the diffusion coefficient of hydrogen in zirconium alloys by the radioluminography method was developed. The temperature dependence of the hydrogen diffusion in E110 alloy was constructed.

The machinability of 0.2% Sn free-cutting steel produced by vacuum melting and hot shaping under pressure using the method of hammer forging has been studied. The distribution of elements (Sn, S, Mn) in the steel has been investigated by secondary ion mass spectrometry (SIMS). It has been established that the doping with tin appreciably increases the machinability index of the free-cutting steel to k_v = 1.72. The effect of S and Mn segregation with the formation of manganese sulfide MnS precipitates has been revealed. Tin in the form of fine particles is uniformly distributed in crystalline grains and along their boundaries and segregates on MnS particles. In the process of cutting, low-melting tin particles are melted to initiate the Rehbinder effect (liquid-metal embrittlement), thus embrittling the steel. Moreover, tin has a lubricating effect at the point of contact between the cutter and the machined surface. All these effects improve the machinability of tin-bearing free-cutting steel.

Results of multiyear research of zirconium materials from nuclear reactors show that the hydrogen content measured by a high-temperature extraction method does not always correlate with the number and size of zirconium hydrides measured by metallography. Additional examinations found that hydrogen is contained not only in the metal but also in the oxide film of samples under examination. This means that the level of hydrogenation of zirconium materials under irradiation should be estimated by the hydrogen content in metal only to allow for a correct comparison of the results of gas analysis and metallographic investigations. In this connection, the hydrogen content was measured separately in the metal and in the oxide film. A device to mechanically remove sediments and oxide film from samples was developed. However, when removing the oxide mechanically, a top layer of metal is also removed, as a rule, which leads to an uncontrolled increase in the measurement error because of a nonuniform distribution of hydrides in the metal. The assessment of the contribution of oxide hydrogen using a curve “total hydrogen content vs. oxide film thickness” showed no distinct dependence. Moreover, it was stated that the results of the total hydrogen content measurements by the high-temperature extraction method can be affected by the elementary composition of sediments and oxide films on the surfaces of spent items. The performed research resulted in the development of a technique to measure the hydrogen mass fraction by high-temperature extraction in an inert gas flow separately in the metal and oxide film without any preliminary mechanical removal of the oxide. This will allow examining the dependence between the hydrogen content in the oxide film and its thickness, as well as estimating a possible correlation between the hydrogen mass fraction in the metal and the quantitative characteristics of hydrides measured by a metallographic image.

In this work, we carried out metallographic studies of the lateral and longitudinal sections of the fuel element claddings used in the fuel assemblies of VVER-1000 reactors. The average fuel burnup in these fuel elements during one and six annual campaigns was ~20 and ~70 MWd/kg U, respectively. The effect of long-term thermal tests under simulated dry storage conditions of fuel elements on the hydride morphology in the lateral and longitudinal sections of E110 zirconium alloy claddings was experimentally studied. The test fuel elements were kept in helium at 380°C for 468 days under stationary conditions and from 20 to 380°C (48 cycles, holding time from 1 to 10 days at 380°C) for 427 days during thermal cycling. Metallographic studies of the microstructure of the lateral and longitudinal sections of fuel element claddings were carried out in order to determine the effect of fuel burnup and thermal testing modes on the morphology of zirconium hydrides in the fuel elements from the regions corresponding to the middle of the fuel rod and the gas collector location. It was established that the specific length of hydrides does not exceed 4.5 × 10^–3 μm^–1 and correlates with a low hydrogen content in the claddings. Thermal tests did not change the hydrogen content in the claddings, but caused an increase in the specific length of hydrides in the lateral sections of the claddings by 1.5–1.9 times in fuel elements with low burnup. In the longitudinal sections of the claddings of these fuel elements, as well as in the fuel elements with a high level of fuel burnup, this coagulation of hydrides was not observed. Thermal tests under stationary conditions and in thermal cycling mode did not lead to a significant change in the hydride orientation, which was predominantly tangential with a high fraction of the chaotic component in the lateral section of the claddings, whereas the axial orientation of hydrides prevailed in the longitudinal section.