As is known, the Cu–C system is a low-soluble interstitial solution (one carbon atom per 104–105 Cu atoms at 700–800°С) that can be prepared by thermic fusion, rapid quenching, and shock waves [1, 2]. In order to decrease thermal expansion (αCu = 16.8 × 10–6 K), Cu-based composites can be armored by ceramics [3]. Non-equilibrium composites Cu1 – x–Сx (carbon, x = 0.05, 0.10, 0.20, 0.50) can be prepared by mechanical synthesis from Cu powder and graphite [4].

The high-temperature treatment of powder mixtures of Ni-C, Al-C and Ni-Al-C (5% wt C (soot)) has been carried out up to the melting temperature of the metals. It was found that melt particles coagulate with the formation of spherical particles of Ni, Al, and the intermetallic compound NiAl, respectively. Ni particles are characterized by the almost perfect spherical shape and a multilayer graphite coating. NiAl particles have a thin graphene (graphite) coating.

Self-propagating high-temperature synthesis has been performed in a heterogeneous Ni–Al–Ti–B model system consisting of Al + Ni composite particles in the form of granules produced by mechanical activation and a Ti + 2B mixture of titanium and boron powders. Two main chemical reactions occurred in the combustion wave front: between aluminum and nickel in the composite particles and between titanium and boron in the mixture around the composite particles. The combustion process resulted in the formation of a titanium diboride matrix in which small pores were filled with molten nickel aluminides, and the composite particles gave way to pores reproducing their shape. The porous metal–ceramic synthesis product had a structure of composite material, with interpenetrating diboride and intermetallic skeletons.

The combustion of highly exothermic multicomponent mixtures of Co3O4/Cr2O3/Nb2O5/Al with additives of MoO3, WO3, and carbon (graphite) under overload up to 200 $$g$$  has been studied. It has been shown that the introduction of carbon into the initial mixture has a marked effect on the combustion, formation of chemical composition, and structure of combustion products. When the weight percentage of carbon in the initial mixture increases from 0 to 3.9%, the burning rate decreases by more than half and the rate of dispersion of combustion products and the mass loss increase markedly. Under the action of overload, the two-phase melt of combustion products is stratified into two layers, which crystallize upon cooling. The lower metal layer contains Co, Nb, Cr, W, Mo, C, and impurity aluminum, and the upper layer contains mainly Al2O3. With an increase in the carbon content above 4.0%, the separation of the metal and oxide phases ceases, and with a further increase, the flammability limit is reached. With an increase in the carbon content in the mixture from 0 to 3.9%, its concentration in the cast composite material reaches 5.4%, the Al content is about 4.0% and the content of Co, Nb, Cr, W, and Mo changes slightly. The combustion slag contains reducing metal oxide (Al2O3) and impurity Cr2O3 dissolved in it.

A two-phase powder alloy based on substitutional solid solutions with bcc and fcc lattices has been obtained by the high-intensity mechanical treatment (HMT) of a multicomponent powder mixture Fe + Cr + Co + Ni + Ti. Sections of the samples and particles of the resulting mixtures are studied on an ultra-high resolution scanning electron microscope (SEM) by scanning electron microscopy. X-ray diffraction patterns of the mixtures are recorded on a DRON 3 diffractometer using Fe K α and Cu K α radiation. It is found that after 10 min of HMT one intense superposition reflex remains on the XRD pattern, the angular position of which corresponds to reflections of 111 and 110 phases with fcc and bcc lattices, respectively. Spark plasma sintering (SPS) at temperatures of 800 and 1000°C from the mixture after 90 min of HMT resulted in samples of a compact high-entropy material. Their specific electrical resistance and density, as well as the dependence of these characteristics (properties) on the sintering temperature, have been determined. It is shown that, in the process of SPS of the powder alloy, the enrichment of the substitutional solid solution with the bcc lattice with titanium probably occurs.

This paper investigates the features of the high-temperature interaction of carbon fibers with a nickel melt under the action of a short electric pulse and the formation of a multilayer graphite coating on the nickel surface. The saturation of a nickel droplet with carbon is achieved by diffusion on contact with carbon fibers. A multilayer graphite coating with a thickness of about 3 μm is formed on the nickel surface due to the segregation of carbon atoms dissolved in a molten nickel droplet.

A powder of uniform high entropy alloy AlCoCrFeNi was produced by relatively short-term (90 min) mechanical alloying in a planetary mill followed by annealing at 873, 1073 and 1273 K for 5.5 h. Results of high-temperature in situ XRD analyses demonstrated the occurrence of specific crystal lattice transformations taking place in major fcc and bcc phases during annealing and cooling down. Shrinkage of atomic structures was detected for all phases after annealing at any temperature, while precipitation of the sigma-phase occurred only at 1073 K. A drift of the (111) peak relative to the (200) peak of the fcc phase at 1273 K allowed us to assume the occurrence of weak martensitic transformations: a high-temperature cubic phase turns into slightly tetragonal phase (a/c = 1.00426) upon cooling down to room temperature. This transformation takes place without mechanical stress or deformation. Despite structural transformations, the major high-entropy phases remained after annealing. (C) 2020 Elsevier B.V. All rights reserved.

The use of nanopowders implies the ability to safely store them without changes in their physicochemical parameters ensured by passivation. In this work, we have determined the thermal stability range in air for compact samples of different diameters prepared from pyrophoric nickel nanopowders and then passivated. The results demonstrate that compact samples prepared from nickel nanopowder can be safely stored in air at temperatures of up to about 200°C without additional oxidation, which points to their high thermal stability. The lack of a noticeable heat release during slow heating to 200°C and the presence of only Ni in both the passivated and heat-treated samples according to X-ray diffraction data suggest that the oxide layer is very thin or that there are noncrystalline oxide phases. The observed uniform oxygen and nickel distributions over fracture surfaces of samples after heating suggests that their interaction with air during both the passivation process and heating to temperatures on the order of 200°C is a bulk process.

In this paper, we study compact samples of pyrophoric nickel powders with the average particle size of 85 nm, obtained by the chemical-metallurgical method. For the first time, it is experimentally shown that it is possible to passivate compact samples with a diameter of 3 mm from pyrophoric nickel powders with nanosized particles in air. For a relative density of 0.4 to 0.5, the passivation time is only 3–5 s. According to the X-ray phase analysis data, only the Ni phase is observed in passivated samples. It is found that passivated samples retain their thermal stability in air upon slow (<10 deg/s) heating to ~200°C, which is an important parameter for fire safety when handling nanopowders. The electron microscopic analysis of the passivated samples did not reveal traces of sintering of nickel nanoparticles, including after checking for thermal stability. The uniform distribution of oxygen over the passivated samples according to the data of energy dispersive analysis (the standard deviation is 0.9 at %) indicates the volumetric nature of the interaction of the samples with air during passivation. For the obtained passivated samples, the critical heating conditions were determined, under which self-ignition occurs, which is in agreement with N.N. Semyonov’s classical theory of thermal explosion.

— We have studied the structuring of combustion products in the Ti–Al system upon interaction with carbon fibers during self-propagating high-temperature synthesis. The combustion products have been characterized by X-ray diffraction and scanning electron microscopy in combination with X-ray microanalysis using an energy dispersive detector. The results demonstrate the formation of a thin (~200–300 nm) TiC carbide layer on the surface of the graphite fibers. This layer is covered with a layer of nanolaminate grains of the Ti 3 AlC 2 MAX phase. The present results can be useful in the fabrication of composite materials based on titanium aluminides reinforced with titanium carbide-coated carbon fibers.

It was experimentally shown for the first time that compact samples of pyrophoric nickel nanopowders can be passivated in air for 3–5 s. The passivation of the samples was confirmed by the absence of noticeable heat release during slow heating to 200°C, an insignificant oxygen content according to the data of X-ray powder diffraction analysis and scanning electron microscopy, and the retention of the chemical activity of the samples. The uniform distribution of oxygen throughout the sample suggests the volumetric nature of their interaction with air during the passivation. The critical conditions for heating of passivated samples under which ignition occurs were determined.

Pyrophoric nickel nanopowders obtained by a chemical-metallurgical method were used in investigations. The average nanoparticle size was 67 nm. Compact samples with diameters of 5, 7 and 10 mm of different densities were made of nonpassivated nickel nanopowder in a glove box in argon atmosphere. In preliminary experiments, in which closed weighing bottles with samples made of nickel pyrophoric nanopowder after extraction from the box and until extraction of samples from the weighing bottles were in an argon atmosphere, the optimal value of the relative density 0.2 of samples was determined, at which they maintained pyrophoric properties, since they self-ignition occurred at heating to similar to 550 - 600 degrees C. Then, for the samples with the densities <0.2, the time spent with the closed weighing-bottle in the air as well as the relative weight gain of the samples sufficient for their passivation were determined. The amount of adsorbed monoatomic oxygen layers and the active surface fraction of compact nickel pyrophoric nanopowder samples were calculated based on the obtained relative weight gain information and literature data. It was established that during the exposure of the weighing boxes to the air the passivation of the samples with the preservation of their high chemical activity occurred, since when the oxidation reaction was initiated by a high-temperature source, a combustion wave propagated within the sample with the velocity about 0.3 mm/s. Analysis of the fracture of passivated samples showed no sintering of nickel nanoparticles; EDA showed an almost uniform distribution of oxygen over the cross section of all passivated samples, which makes it possible to conclude that passivation has superficial nature.

The composition and crystal structure of compounds produced by self-propagating high-temperature synthesis (SHS) from the 5Ta–2Ni–3Al (at %) powder mixture followed by vacuum remelting at 3000°C are studied. The SHS product contains the following phases: TaNiAl (Laves τ1 phase), NiAl, Ni2Al3, and Ta. Its microstructure includes Ta85Ni7Al8, Ta52Ni20Al28, and Ta53Ni25Al22 ternary phases according to elemental analysis data. Reflections belonging to no known ternary phases in the Ta–Ni–Al system under consideration are revealed in the X-ray diffraction pattern of the remelted material. Based on the homological approach, it is found that these reflections belong to three phases with the structural types W6Fe7 ($$R\bar {3}m$$), Ti2Ni ($$Fd\bar {3}m$$), and Ta3Al ($${{P{{4}_{2}}} \mathord{\left/ {\vphantom {{P{{4}_{2}}} {mnm}}} \right. \kern-0em} {mnm}}$$). They are identified as reflections of three compounds, Ta6.5Ni6.5, Ti2Ni, and Ta2.84Al0.91, with unit-cell parameters differing from these for the same compounds with the conservation of the structural type. An increase in the unit-cell parameters of all revealed phases is noted when compared with known binary intermetallic compounds. This can be associated with the presence of Al atoms in the crystal lattice from the Ta6.5Ni6.5 phase and Al and Ta atoms in the phase with the Ti2Ni structural type. Phases Ta6.5Ni6.5 and Ti2Ni phases are identified as Ta6Ni6Al and Ta2Ni0.5Al0.5 by X-ray structural analysis and crystal-chemical modeling, and their structural type, composition, and unit-cell parameters are determined. The structure and composition are refined by the full-profile analysis, and the unit-cell parameters of the phases and their quantitative ratio in the material are determined. The phase composition of the material is as follows, wt %: 47 Ta6Ni6Al, 16 Ta2Ni0.5Al0.5, and 37 Ta3Al.

Nanocrystalline powder of the CoCrFeNiAl high-entropy alloy was produced by high-energy ball milling (HEBM) and consolidated by spark plasma sintering (SPS). Microstructure and crystal structure transformations occurring in the course of HEBM and SPS processes were explored by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Rays Diffraction (XRD) methods. Synthesized materials showed a microhardness of 4000–6000 MPa and electrical resistivity of 0.2 mΩ⋅cm at room temperature.

Ceramic-metal composites TiC-High-Entropy Alloy (HEA) CoCrFeNiMe (Me = Mn, Ti or Al) were first produced by combustion synthesis method. Self-sustained synthesis occurs due to heat release from exothermic reaction Ti + C = TiC; the binder content was varied between zero and 40-50 wt.%. The combustion velocity and temperature gradually decreased with increasing binder content. Resultant materials consist of TiC grains and two-phase (fcc and bcc) binder. Vickers microhardness (100 g) of compacted cermet materials with 30 wt. % of binder was in the range of 10-17 GPa and increased with increasing bcc to fcc ratio. Based on experimental results and thermodynamic calculations, the mechanism of microstructure formation in TiC-HEA cermets was suggested.