A. A. Pankratov

Structural and phase transformations in α + β brasses

The effects of the chemical composition of α + β brasses and heat treatment conditions on the amount, morphology, and chemical composition of the phases forming these brasses have been studied. Apart from the main phases (an α solid solution of alloying elements in copper and a β phase based on the electronic compound CuZn), all brasses under study are shown to contain M5Si3 (M = Fe, Mn, Ni) silicides and particles of free lead (1–2 vol %). The amount and morphology of α and β grains can be controlled by heat treatment, whereas the quantitative characteristics of the silicides are mainly determined by the chemical composition of the alloy. Cu-Zn-Mn-Al-Fe-Si brasses can be additionally hardened due to the formation of ultradispersed Mn5Si3 silicides and martensite upon quenching and due to the partial dissolution of these particles and the formation of a bainite structure upon subsequent tempering at 260–270°C.

High-temperature glassy-ceramic sealants SiO2-Al2O3-BaO-MgO and SiO2-Al2O3-ZrO2-CaO-Na2O for solid oxide electrochemical devices

Glasses of the SiO2–Al2O3–BaO–MgO and SiO2–Al2O3–ZrO2–CaO–Na2O systems were synthesized in the perspective to apply them as sealants in SOFC at operating temperatures of 700–900 °C. Thermal properties of the chosen glass compositions and their compatibility with the SOFC materials (YSZ-electrolyte and alloy-interconnector Crofer22APU, 15×25T) were investigated by means of synchronic thermal analysis and high-temperature dilatometry. The elemental analysis was performed by atomic emission spectroscopy. The average values of the temperature coefficients of the linear extension are 10.0×10−6 °C−1 for glass 45%SiO2– 15%Al2O3–25%BaO–15%MgO and 9.5×10−6 °C−1 for glass 60%SiO2–10%Al2O3–10%ZrO2–5%CaO–15%Na2O. The gluing microstructure in YSZ/glass/Crofer22APU was studied by scanning electron microscopy. The crystallization process of silicate phases was revealed to occur in the SiO2–Al2O3–BaO–MgO glass. The analysis of the crystallization products was performed by Raman spectroscopy and X-ray diffraction. Glassy ceramics was proved to possess better parameters in comparison with amorphous glass to be used as a sealant in electrochemical sensors and oxygen sensors. The SiO2–Al2O3–ZrO2–CaO–Na2O low-temperature amorphous glass can be applied in SOFC.

Structure and phase composition of a V-Al-N master alloy

The structure and phase composition of the V-Al-N master alloys produced using out-of-furnace aluminothermic melting of charges containing nitrided vanadium filings or powdered electrolytic vanadium are studied. It has been found that the nitrogen-containing phase of these alloys is represented by aluminum nitride. The master alloy ingots melted using the nitriding agents are characterized by identical values of the integral parameters of nitride phases. Based on metallography results, certain similarity is found between inclusions and the nitriding agent added to the charge for melting of a V-Al-N master alloy.

Effect of the cooling rate on the phase composition and structure of copper matte converting slags

The effect of the cooling rate on the phase composition and microstructure of copper matte converting slags is studied by X-ray diffraction, combined thermogravimetry and calorimetry, mineragraphy, and electron-probe microanalysis. The compositions of oxide and sulfide phases are determined, and the forms of nonferrous metals in slags cooled at a rate of 0.3 and 900°C/s are revealed. At high cooling rates of the slags, iron silicate glass is shown to form apart from sulfide phases. Repeated heating of the slags leads to the development of devitrification, “cold” crystallization, and melting. A decrease in the cooling rate favors an increase in the grain sizes in oxides (magnetite, iron silicates) and sulfides (bornite-, sphalerite, and galena-based solid solutions).

Extraction of Scandium and Zirconium from Their Oxides during the Electrolysis of Oxide–Fluoride Melts

The main features of scandium and zirconium extraction from their oxides to aluminum during the aluminothermic and electrolytic preparation of Al–Sc and Al–Zr alloys and master alloys in the KF–AlF3, NaF–AlF3, and KF–NaF–AlF3 oxide–fluoride melts with Sc2O3 and ZrO2 additives are studied. The influence of the melt composition and temperature, the synthesis time, the contents of oxides Sc2O3 and ZrO2 in the melts, the mechanical stirring of aluminum, and the cathodic current density on the contents of scandium and zirconium in aluminum and on their extraction from the oxides is determined. The average values of scandium and zirconium extraction are 20–75 and 40–100%, respectively, depending on the synthesis parameters. The electrolytic decomposition of the oxides in the KF–AlF3, NaF–AlF3, and KF–NaF–AlF3 melts results in the enhancement of scandium and zirconium extraction to aluminum. The parameters of the preparation of Al–Sc and Al–Zr alloys and master alloys with the scandium content to 10 wt % and zirconium content to 15 wt % during the electrolysis of oxide–fluoride melts are chosen as a result of the results obtained.

Electrochemical synthesis of tetragonal oxide tungsten bronze nanofilms on platinum

We are the first to synthesize nanofilms of tetragonal oxide tungsten bronze (OTB) on a Pt(110) substrate by the electrolysis of the K2WO4–Na2WO4–WO3 melt at 700 and 750°C. The composition and the morphology of OTB are shown to depend on the deposition potential and the WO3 concentration in the melt. The laws of formation of tetragonal OTB films are discussed. The synthesized OTB samples are found to have a good thermal stability in the temperature range 20–800°C.

Synthesis of new metal-matrix Al–Al2O3–graphene composite materials

The mechanism of formation of ceramic microparticles (alumina) and graphene in a molten aluminum matrix is studied as a function of the morphology and type of precursor particles, the temperature, and the gas atmosphere. The influence of the composition of an aluminum composite material (as a function of the concentration and size of reinforcing particles) on its mechanical and corrosion properties, melting temperature, and thermal conductivity is investigated. Hybrid metallic Al–Al2O3–graphene composite materials with up to 10 wt % alumina microparticles and 0.2 wt % graphene films, which are uniformly distributed over the metal volume and are fully wetted with aluminum, are synthesized during the chemical interaction of a salt solution containing yttria and boron carbide with molten aluminum in air. Simultaneous introduction of alumina and graphene into an aluminum matrix makes it possible to produce hybrid metallic composite materials having a unique combination of the following properties: their thermal conductivity is higher than that of aluminum, their hardness and strength are increased by two times, their relative elongation during tension is increased threefold, and their corrosion resistance is higher than that of initial aluminum by a factor of 2.5–4. We are the first to synthesize an in situ hybrid Al–Al2O3–graphene composite material having a unique combination of some characteristics. This material can be recommended as a promising material for a wide circle of electrical applications, including ultrathin wires, and as a structural material for the aerospace industry, the car industry, and the shipbuilding industry.

Formation of nanocrystalline tetragonal oxide tungsten bronzes on platinum

Cyclic voltammetry is used to study the formation of tetragonal oxide tungsten bronze of the composition KxNayWO3 on a Pt(110) substrate during electrodeposition from a K2WO4–Na2WO4–WO3 melt. The potential ranges in which cathode products of various compositions and morphologies form are found. KxNa(0.66–x)WO3 crystals are shown to form according to the nucleation/growth mechanism. A general scheme is proposed and used to write equations for cathode reactions.

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nbss and γ-Nb5Si3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb3Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.

Influence of the nonequilibrium crystallization on the phase composition of the Al-Mo-Ti alloy

The phase composition, microstructure, and crystal structure of the AMT TU-48-4-366 (Technical Specifications) foundry alloy (which is used as the alloying material when smelting titanium alloys) are investigated by X-ray phase analysis, electron probe microanalysis, and microscopy. Lattice parameters of ɛ, p, and δ phases are calculated and their elemental composition is revealed. No formation of the Mo3Al refractory phase (tm = 2150°C) is observed during the primary crystallization of the Al-Mo-Ti foundry alloy in nonequilibrium conditions. Its presence in the refractory phase in the foundry alloy is caused by secondary crystallization processes, during which an ultradispersed mixture of Mo3Al + Mo3Al8 + TiMoAl6 phases is formed at temperatures 1311 and 1314°C. The ultradispersed silicon-containing σ phase with the Mo2.4Ti2.1Si0.8Al4.7 average composition, which was formed in nonequilibrium crystallization conditions, is revealed. Parameters and interplanar distances of its lattice are determined. It is established that the largest nonuniformity by molybdenum in peritectics of primary crystals occurs at a high crystallization rate, i.e., in the lower part of the Al-Mo-Ti ingot.