The method of aluminothermic self-propagating high-temperature synthesis was used to obtain a composite material based on Nb-Si-C. The study of this system is of interest from the point of view of obtaining high-temperature materials of a new generation for gas turbine engine building, capable of replacing heat-resistant nickel alloys, as well as the potential possibility of forming MAX-phases (phases Mn + 1AXn where n = 1, 2, 3, ...; M is transitional d-metal, A – p-element, X – carbon). The resulting Nb-Si-C composite were studied by X-ray diffraction, scanning electron microscopy, and X-ray spectral microanalysis. It is shown that NbC carbide and silicides γ-Nb5Si3 and NbSi2 are formed in the sample. A detailed analysis of the morphological distribution of the constituent phases has been carried out.

The results of studying a composite material based on Nb-Si-C are presented. The alloy was obtained by aluminothermic self-propagating high-temperature synthesis. To prepare the initial charge a mixture of powders of commercial niobium pentoxide Nb2O5 and commercial silicon carbide SiC was used. The process of niobium oxide Nb2O5 reduction was implemented using an aluminum powder. The study of the Nb-Si-C ternary system is of interest in terms of obtaining high-temperature materials of new generation for gas turbine engine construction capable of replacing heat-resistant nickel alloys, as well as the potential possibility of forming MAX phases (M(n + 1)AX(n) phases, where n = 1, 2, 3, ...; M is the transition d-metal; A is the p-element; and X is carbon). The phase composition of the obtained Nb-Si-C composite was studied by X-ray diffraction. It has been shown that as a result of high-temperature synthesis NbC carbide and gamma-Nb5Si3 and NbSi2 silicides are formed which are competing relative to the Nb2SiC MAX phase. The formation of Nb2SiC under the selected synthesis conditions was not detected. Using scanning electron microscopy and X-ray spectral analysis, the morphological distribution of the phases formed during high-temperature synthesis is analyzed in detail. It is shown that NbC carbide makes up the majority of the volume of the composite and is present in the form of pyramidal-shaped particles with a pyramid face size of similar to 10 mu m. NbSi2 silicides are elongated rod-shaped particles with smooth edges measuring 30 x 80 mu m; Nb5Si3 silicides do not have a specific shape.

In the present work, a mechanism of the destruction of amorphous tantalum oxide Ta2O5 on the tantalum anode surface is suggested based on high absorption properties of tantalum. The data of analysis of the morphological peculiarities of the defective areas of the surface layers show that the destruction of the amorphous film occurs due to the growth of a pyramid-shaped defect; the pyramidal defect is not coherent with amorphous surface Ta2O5, and it is not a product of its crystallization. The nucleation and growth of the “pyramid” occurs due to the directed movement of oxygen during the oxidation of the tantalum surface along the grain boundaries to the region of triple junctions of the tantalum matrix with the subsequent local formation of crystalline Ta2O5. The suggested mechanism offig destruction can be realized when high-purity tantalum powders are used.

Composite materials based on the Ti-Si-C system were obtained by self-propagating high-temperature synthesis with the use of carbon black and graphene as a source of carbon. The comparative investigations of the phase composition and structure of the materials were performed by X-ray diffraction analysis and scanning electron microscopy. It is shown that in the sample obtained with carbon black, similar to 94 wt.% of MAX-phase Ti3SiC2 is formed. When graphene is used, the content of the MAX phase is heterogeneous throughout the sample volume and is in the range of similar to 67 - 90 wt.%.

Structural and phase compositions of castings of chromium cast irons ChKh32 (2–4

The effect of the action of temperature on the structure and phase composition of anFe3C/Fe7C3/P-phase/Cam composite obtained by mechanosynthesis of Fe–75 at % C has been studied by the methods of X-ray diffraction, Mössbauer spectroscopy, thermogravimetric analysis, and differentialscanning calorimetry. It is shown that the structural and phase changes at heating have a multistage character.In the temperature range 315–400°C crystallization of the paramagnetic P-phase with generation of Fe3Cand/or Fe7C3 occur. In the course of heating to higher temperatures, complete decomposition of carbideFe7C3 (in the range 450–550°C) and partial decomposition of Fe3C (at 600°C and above) are observed. Aftercooling from 800–1000°C the mechanocomposite consists of α-Fe, cementite Fe3C, and graphite. The phasetransformations are accompanied by the processes of composite oxidation with the formation of Fe3O4 oxide and its subsequent reduction. The P-phase is a disorded amorphous carbide Fe1 – xCx, which is characterized by magnetic ordering at the temperature of liquid nitrogen.

Методами рентгеноструктурного, металлографического анализов, растровой электронной микроскопии исследованы структурно-фазовый состав отливок хромистых чугунов ЧХ32 (2–4% C; 30% Cr; 2,4% Mn; 0,6% Si), легированных азотом с использованием в качестве лигатуры азотированного хрома. Показано, что азот в полученных чугунах присутствует преимущественно в твердом растворе на основе железа, наряду с хромом, марганцем и углеродом, частично входит в состав карбидной фазы (Fe,Cr)7C3. Легирование азотом чугуна ЧХ32 с содержанием 2% С стабилизирует аустенит, при содержании углерода 3–4% термообработка приводит к формированию аустенитно-мартенситной структуры, с соответствующим повышением твердости материалов. Показано, что добавка азота в количестве 0,2% к чугуну, содержащему 2% углерода, приводит к улучшению его коррозионной стойкости, а повышение углерода в материалах приводит к увеличению их стойкости к абразивному изнашиванию. The structural-phase composition of castings of chromium cast irons ChKh32 (2–4% C; 30% Cr; 2.4% Mn; 0.6% Si) alloyed with nitrogen using nitrided chromium as a ligature has been studied by the methods of X-ray diffraction, metallographic analyses, and scanning electron microscopy. It is shown that nitrogen in the obtained cast irons is present mainly in the iron-based solid solution, along with chromium, manganese and carbon, and is partly included in the composition of the carbide phase (Fe, Cr)7C3. Nitrogen alloying of ChKh32 (2% C) stabilizes austenite. Nitrogen alloying of ChKh32 (3–4% C) followed by heat treatment leads to the formation of an austenitic-martensitic structure with an increase in the hardness of materials. It is shown that the addition of nitrogen in an amount of 0.2% to ChKh32 (2% C) improves its corrosion resistance, an increase in carbon content in these materials increases their resistance to abrasive wear.

The effect of the action of temperature on the structure and phase composition of an Fe3C/Fe7C3/P-phase/Cam composite obtained by mechanosynthesis of Fe–75 at

Structural changes in FTW60 and FTW8000 tantalum capacitor powders after sintering at different temperatures are investigated by X-ray diffraction and scanning electron microscopy. Sintering is carried out in accordance with the current technological process used in the production of oxide–semiconductor capacitors. The anodes obtained as a result of sintering are bulk-porous bodies in the form of a rectangular parallelepiped with a wire lead. On the free surface of the bulk-porous anodes obtained by sintering the FTW60 powder at temperatures of 1800°C and 1920°C, the formation of a wavy relief is observed. No such relief is observed after sintering the FTW800 powder at 1250°C. After analyzing the structural state and the morphology of the initial powders and the anodes obtained as a result of sintering, a phenomenological model of the formation of such a relief is proposed based on collective recrystallization near the free surface during high-temperature exposure.

This paper presents the results of investigating the thermal destruction of fullerite and subsequent carbon ordering with the formation of graphite (graphitation) upon heating the samples to 1700°C. We study C60/70 fullerite powders with a predominant C60 content (C60—82.18%, C70—14.08%) obtained by the electric-arc evaporation of graphite rods with subsequent fullerene extraction from fullerene-containing soot by boiling toluene. Heating is performed in a CO environment. We investigate the changes in the fullerite crystal structure after thermal impact by means of X-ray diffraction, analyze the change in the spatial orientation in the carbon-carbon bonds by Raman spectroscopy, and estimated the particle dispersion by scanning electron microscopy. We show that the structural changes in the C60/70 fullerite upon heating above 950°C are caused by the destruction of its crystal structure with amorphous fullerite-like phase formation. Amorphization is accompanied by an increase in the dispersion of the carbon powders. Subsequent structural ordering is observed upon heating to temperatures above 1500°C and is accompanied by the formation of a graphite-like amorphous phase. Detailed analysis of the fullerite structure after its amorphization is performed on the samples annealed at 1600°C for 0.5–16 h. We find that after heating to 1600°C, the sample consists of, predominantly, turbostratic carbon; we also record α graphite and β graphite with the low degree of ordering. Ordering with graphite formation is shown to be accompanied by carbon-particle coarsening.

The thermal properties of C 60 and C 70 fullerenes and fullerites have been studied by X-ray structural analysis and UV spectroscopy. It was shown that C 70 fullerite is stabler than C 60 fullerite (by ~150°C) due to the different geometry of the fullerenes and stronger intermolecular interactions. The experimental results are given a qualitative and quantitative interpretation based on the calculation of the interaction energy between two fullerene molecules.

X-ray diffraction, scanning electron microscopy, Mossbauer and Raman spectroscopy have been used for studying structural changes arising due to the thermal decomposition of fullerite C-60/70 at heating to 1600 degrees C in CO medium. The scheme of the structural changes of fullerite at the initial graphitization temperatures is given. It is shown that the thermal disordering of fullerite with the formation of a fullerite-like amorphous phase is observed at heating above 900 degrees C. The subsequent ordering of carbon takes place at the temperature of 1600 degrees C with the formation of a graphite-like amorphous phase. The graphitization process proceeds with the formation of turbostratic carbon and crystalline graphite with a low degree of ordering. The nature of the formation of carbon substructures is considered. Carbon structural changes are accompanied by morphological changes. Comparative studies of changes in the structure of soot, graphite and glass-like carbon during heating have been presented. The catalytic effect of Fe on the processes of the disordering of initial fullerite is considered. It is shown that in the Fe presence the temperature of the phase transition 'fullerite-graphite' decreases from 1600 to 760 degrees C. Graphite, formed after the Fe3C decomposition, is characterized by a higher degree of ordering.

In the present paper, X-ray diffraction, scanning electron microscopy, Mossbauer, Raman and X-ray photoelectron spectroscopy and thermal analysis have been used for the comparative studies of composites fullerite-Fe and graphite-Fe containing 75 at% of carbon and obtained by the mechanosynthesis method. The synthesized materials are nanoscale powders. Depending on the time of mechanosynthesis and a used form of carbon, the phase composition of the composites is significantly different. It is shown that compared with graphite, fullerite is characterized by higher stability in the composition of iron-containing mechanocomposites. The sequence of the fullerite structural changes during mechanosynthesis is as follows: fullerite -> amorphous fullerite-like phase -> amorphous carbon. When fullerenes are retained, carbides are not formed in the system fullerite-Fe. After the formation of amorphous carbon, the compared composites fullerite-Fe and graphite-Fe have the same composition (Fe3C, Fe7C3, paramagnetic P-phase and amorphous carbon). The stability of the mechanically synthesized phases at heating has been studied. (c) 2022 Published by Elsevier B.V.

A 3Ti–Si–2C composite material is obtained by self-propagating high-temperature synthesis, in which C60/70 fullerite is used as carbon. Its phase composition and structure are investigated by X-ray diffraction and scanning electron microscopy. It is shown that a composite containing TiC carbide, Ti5Si3Cx carbosilicide, and Ti3SiC2 MAX phase as well as traces of titanium silicide TiSi2 is formed as a result of synthesis. The structure of the samples is inhomogeneous and contains regions that simultaneously consist of several phases.

3Ti–Si–2C composite materials are obtained by self-propagating high-temperature synthesis using various structural forms of carbon (carbon black, carbon nanotubes, fullerite). The phase composition of the samples is studied by X-ray diffraction. The amount of the MAX phase Ti3SiC2 (Mn + 1AXn phase where n = 1, 2, 3, …; M is a transition d metal; A is a p element; and X is carbon) in the obtained materials depends on the form of carbon. The formation of TiC carbide is found in the sample with carbon black, and TiC and Ti5Si3Cx are found in the samples with carbon nanotubes and fullerite. All samples contained traces of the TiSi2 phase. Scanning electron microscopy shows that the structure of the samples with fullerite and nanotubes is inhomogeneous and contains regions simultaneously composed of several phases.