Vol 2019, No 2 (2019)

Some of the reports presented at the Sixteenth International Conference on High Temperature Materials Chemistry (Yekaterinburg, July 2–6, 2018) and related to the topics of journal Rasplavy are published in this special issue.

A new method for synthesizing lithium niobate and lithium tantalate is proposed and tested. It is based on the interaction between lithium, niobium, and tantalum oxides, which are formed during in situ reactions of atmospheric oxygen with homogeneous mixtures of molten lithium, niobium(V), and tantalum(V) chlorides. This method allows us to form single-phase lithium niobate and lithium tantalate powders (LiNbO₃, LiTaO₃) of a homogeneous morphological composition with an average particle size from 200 to 300 nm. The replacement of salt precursors (NbCl₅, TaCl₅) by oxide ones (Nb₂O₅, Ta₂O₅) under the same experimental conditions is shown to increase the average particle size of the synthesized powders and the heterogeneity of their morphological composition and to promote the appearance of by-products.

The effect of temperature in the range from 350 to 650°С on the morphology of L63 brass (37 wt % Zn) during selective anodic dissolution in the eutectic salt melt of lithium, cesium and potassium chlorides is studied. Anodic polarization is accompanied by a change in the state of electrode surface due to the passage of the electronegative alloy component (zinc) into a corrosive medium and to the vacancy-induced rearrangement of the electropositive component (copper). Microscopy, gravimetry, chemical analysis, and hydrostatic weighing are used to estimate the laws of formation of a developed anode surface under galvanostatic conditions of the electrochemical dissolution of the bimetallic alloy. An increase in the temperature leads to a decrease in the selectivity of alloy dissolution, the porosity, and the developed surface of the materials.

The influence of the behavior of adsorbed surface-active hydrogen on the phase formation and structuring in multicomponent iron-based systems, namely, steels, cast iron, and FINEMET alloys, is studied. The diffusion activity of elements with adsorbed hydrogen, the hydrogen penetrability, the types of interatomic bonds during the interaction of elements, and their effect on the functional properties of structural products are analyzed. The purpose of this work is to find the character of interatomic bonds of surface-active hydrogen adsorbed on the elements of multicomponent iron alloys and to reveal the influence of hydrogen on the functional and service properties of metal products during their structuring.

Based on experimental data on density and an ionic bond model, we are the first to perform a molecular dynamics simulation of the multicomponent oxide–oxyfluoride metallurgical slag-forming SiO₂–CaO–Al₂O₃–MgO–CaF₂–Na₂O–K₂O–FeO system, to discuss the obtained results, and to compare them with reported experimental and calculated data. The developed computer model demonstrates a weak temperature dependence of the melt structure under study. The diffusion mobilities of fluorine and alkali metal ions are found to be higher than those of other elements.

The structure and properties of metallic multicomponent Fe–Ni–Co–Cr–Cu_x and Fe–Ni–Co–Cu–Cr_x system alloys (x is the component content, at %), which are the basis of the wide class of high-entropy alloys, are studied. The synthesized alloys are investigated using instrumental analysis methods (DTA, SEM-EPMA, XRD), and the concentration and temperature boundaries of the fcc solid solution have been determined.

Drop calorimetry is used to study the mixing of metallic Cu–Ag, Cu–Sn, and Ag–Sn liquid alloys at 1150°C. The calorimeter sensitivity is calibrated using pure components of the alloys and sapphire standards. The experimental results are described by analytical expressions in terms of quasi-chemical approximation for the subregular solution model. The obtained data are necessary to study the alloy formation in the ternary model Ag–Cu–Sn system.

The structure and the properties of liquid Ni–Al alloys have been studied in a wide temperature range. An analysis of the temperature dependences of the physical and chemical properties of these alloys has discovered a hysteresis phenomenon, i.e., discrepancy between heating and cooling polytherms. This discrepancy is called the temperature hysteresis. A physical model of the Ni–Al alloy structure is proposed on the basis of a quasi-chemical model for a nonequilibrium melt structure. The liquid metal is found to be in a nonequilibrium state after melting. It contains Ni_xAl-type microgroups with a strong Ni–Al bond and fcc-like structure and microgroups with weaker Ni–Ni bonds. The sizes and the life times of these clusters decrease after melting (above the liquidus temperature). The Ni_xAl-type clusters disappear as the melt temperature becomes higher than a critical temperature. The melt transforms from a microinhomogeneous state into an equilibrium state.

The effect of alloy components on the thermodynamic properties of f elements is studied on Zn‑containing alloys. The activity of lanthanum in the La–Zn, La–U–Zn, and La–U–Ga–Zn systems and the activity of uranium in the U–Zn system in the range 573–1073 K are determined. The effect of a second low-melting metal (gallium) and a second f element (uranium) on the activity of lanthanum in zinc-containing alloys is analyzed.

The processes that occur during the reactions of 15 rare-earth element (yttrium, lanthanum, and all lanthanides except for promethium) chlorides with sodium orthophosphate in the melts based on the NaCl–KCl equimolar mixture are studied in an inert atmosphere at 750°C. The phase and granulometric compositions of the rare-earth element phosphates are determined. The influence of the molar ratio of phosphate to rare-earth element on the composition and particle size of the phosphates is studied. The conditions necessary for the quantitative precipitation of both individual rare-earth elements and a mixture of the rare-earth elements from the melt are determined. The mixture of rare-earth elements imitates the content of rare-earth fission products in the spent nuclear fuels to be processed.

The effect of small tin, gallium, and zirconium additions on the magnetic properties of a Co₄₈Fe₂₅B₁₉Si₄Nb₄ alloy at high temperatures is studied. The temperature dependences of the magnetic susceptibility of the compositions under study are measured; the electron characteristics, such as the effective magnetic moment, the density of electron states at the Fermi level, and the paramagnetic Curie temperature, are calculated. Amorphous alloy samples are prepared by melt injection and vacuum suction casting.

The thermodynamic parameters of the heterogeneous Li₂CO₃–Na₂CO₃–K₂CO₃–MgO nanopowder system are detemined by differential scanning calorimetry (DSC). The melting temperature and the reduced enthalpy of melting of the salt phase are found to decrease anomalously as the magnesium oxide content in the composite material increases.

The joint aluminothermic reduction of zirconium and niobium oxides is studied by a thermodynamic simulation of metallothermic reactions, differential thermal analysis, and X-ray diffraction analysis. The research can serve as a scientific basis for the development of promising metal-thermal technologies for the production of rare-metal alloys. Our results can be used to develop advanced metallothermic technologies for making rare-metal alloys.

A mechanism for the appearance of shear rigidity is proposed for disordered systems, the structure of which can be described as a medium with a high density of linear topological defects. This mechanism consists in kink localization in dislocation lines, which limits their mobility. This approach can be used to determine the temperature dependence of plastic deformation, to relate it to the fractal dimension of a set of defects, and to explain the dependence of the glass transition temperature on the cooling rate.

The corrosion behavior of austenitic steels AISI 316L (analog of 03Kh16N15M3), 12Kh18N10T (analog of AISI 321) and 06KhN28MDT and ferritic and ferritic–martensitic steels 12Kh13 (analog of AISI 410), 08Kh17T (analog of AISI 439), and 16Kh12MVSFBR is studied in chloroaluminate melts at a temperature of 550°C. The corrosion rates of these steels in a KCl–AlCl₃ electrolyte are determined. The corrosion damage of the stainless steel surfaces is shown to have a continuous inhomogeneous character and to be characterized by etching off the most electronegative elements, namely, chromium, iron, and manganese.