— An oxidative constructing technique is used to obtain bulky samples of compact metal-ceramic composite materials with a gradient structure consisting of a nitride ceramic shell in the form of a subsurface layer of ZrN and a core of a solid solution of nitrogen in zirconium. The synthesized composites exhibit enhanced strength properties compared to the original metal blanks. The ZrN–Zr composite samples are characterized by plasticity under compression conditions and are brittle under bending conditions. Compared to conventional techniques, the applied technique of oxidative constructing makes it possible to rather efficiently and economically obtain gradient metal-ceramic samples of products with dimensions and shapes that closely repeat those of an original metal blank.

The kinetic patterns of nitride formation have been established and the sequence of structural transformations characterizing high-temperature (at 1900 °C) nitridation of Zr-U alloys containing 2 and 5 wt.% U in the range from 3.5 to 60 minutes is presented. During high-temperature saturation with nitrogen for each composition, the solid solution (Zr,U) decomposes with the formation of composite structures ZrN-(ZrN1-n/UxEy/U)-ZrN (where E is O, N; n, x, y are stoichiometric coefficients ). During the decomposition of the solid solution, zirconium nitride is formed and a phase of metallic uranium is released, which accumulates impurities contained in the initial solid solution in the central part of the sample. Kinetic curves for a temperature of 1900 °C are approximated by an exponential law and correspond to the nitridation of zirconium. The nitridation rate of the (Zr,U) solid solution increases with increasing uranium content. To complete the formation process of a compact nitride solid solution of (Zr,U)N of stoichiometric composition, it is necessary to increase the temperature and increase the reaction duration.

Определены кинетические закономерности образования нитридов сплавов Zr–Nb (содержание Nb 0.1, 2.5 и 5 мас. %) при температуре 1900°C. Процесс азотирования характеризуется двухстадийностью, где обе стадии описываются экспоненциальным законом. Скорость химической реакции на второй стадии значительно меньше, чем на первой. Охарактеризован состав формирующихся гетероструктур Zr 1– х Nb х N–ZrN 1– n /β-твердый раствор циркония в ниобии–Zr 1– х Nb х N, установлена последовательность нитридизации компонентов исходного сплава. На первой стадии процесса происходят образование α-твердого раствора азота в Zr и его переход в нестехиометрический нитрид. Кинетическая зависимость на второй стадии описывает нитридизацию фазы β-Nb, образовавшейся при распаде твердого раствора Zr〈Nb〉. Показано, что продолжительность второй стадии процесса определяется количеством ниобия в исходном твердом растворе. Экспериментально подтверждена возможность создания однофазной керамики с активными добавками нитридизацией сплавов Zr–М в одностадийном процессе с сохранением исходной формы металлической заготовки.

We have demonstrated general kinetic aspects of the formation of nitrides of Zr–Nb alloys (containing 0.1, 2.5, and 5 wt

Abstract—The kinetic laws of nitride formation are revealed. The sequence of structural transformations characterizing high-temperature (at 1900°C) nitriding of Zr–U alloys with 2 and 5 wt

We describe a sequence of structural transformations characterizing high-temperature nitridation of zirconium–niobium alloys containing 0.1–10 wt % niobium. High-temperature saturation of solid solutions of niobium in zirconium with nitrogen is accompanied by decomposition of the Zr〈Nb〉 solid solution and the formation of Zr1 – хNbхN–(ZrN1 – n/β-solid solution of Zr in Nb)–Zr1 – хNbхN composite structures. During nitridation of the heterostructures, zirconium nitride reacts with β-niobium, which is the final step of the nitridation of the parent Zr〈Nb〉 solid solution. Characteristically, the ceramics thus prepared have near-surface porosity reproducing the surface porosity of the as-rolled material.

Ceramics based on hafnium carbide of a given shape have been synthesized by direct carbidization of rolled hafnium in a hydrocarbon atmosphere. Excess carbon resulting from high-temperature pyrolysis of hydrocarbons forms on the surface of ceramic hafnium carbide an easily detachable layer consisting of graphite with an admixture of amorphous carbon. The final formation of ceramics occurs at a temperature of 2400 °C in an atmosphere of an inert gas — argon. The phase composition and structure of the synthesized ceramics are characterized.

Zr1 – хUхN nitrides have been synthesized via nitridation of solid solutions of uranium in zirconium. High-temperature saturation of the solid solutions with nitrogen has been shown to yield Zr1 – хUхN ceramics uranium-enriched in their central part. We have presented the concept of a new type of fuel, based on Zr1 – хUхN, for high-temperature gas-cooled reactors. Nuclear calculations of a reactor core demonstrate the feasibility of reaching the critical mass at uranium contents of 10 and 20 wt %. The core size of a conceptual reactor has been determined. We have performed thermophysical calculations of the reactor core and demonstrated the feasibility of obtaining high-grade heat.

— A thermodynamic model is proposed for condensed phases in the ternary system U–Zr–N in the range 298–2800 K. The model is based on previously reported models of the constituent binaries U–Zr, U–N, and Zr–N and available data on the properties of the pseudobinary system UN–ZrN. It allows one to calculate phase equilibria involving liquid phase. The model is used to calculate a number of isobaric–isothermal sections in the U–Zr–N system.

— We have studied structural and phase transformations in the surface layer of compact titanium nitride-based ceramics during high-temperature annealing in vacuum, nitrogen, and air. The formation of an oxide phase during heat treatment of TiN x ceramics in air is controlled by bulk oxygen diffusion from the gas phase across an initially forming dense rutile TiO 2 layer. Annealing in vacuum or nitrogen in the range 400–800°C changes neither the phase composition nor the grain structure of the ceramics.

This paper presents dilatometric analysis data for the sintering of off-the-shelf molybdenum disilicide (MoSi2) powder prepared by magnesiothermic synthesis. We have obtained continuous shrinkage curves for MoSi2 powder compacts with an initial relative density of 70% at different heating rates: 5, 10, 20, and 30°C/min. From a quantitative analysis of densification curves for the compacts, the activation energy for the initial stage of sintering has been determined to be Q = 695 kJ/mol. It has been shown that the dominant process in the initial stage of MoSi2 powder sintering is volume diffusion from grain boundaries and surfaces.

High temperature oxidation of iron and nickel alloy blanks is characterized by the formation of bilayer ceramic and cermet structures, holding the original shape of the metal blank. The composition of the resulting ceramic depends on the temperature and synthesis time and the ratio of components with different sensitivity to oxygen in the initial alloy. Dwell at a low temperature leads to stabilization of the composition of the synthesized ceramic within the outer layer owing to the concentration leveling of the spinel phase. The outer and inner layers are characterized by different morphology of the chip surface and different porosity. The inner polycrystalline layer is porous; the outer layer of the sample is monolithic. The structure of interphase boundaries of the heterophasic sample ensures the integrity of the material and provides high adhesion properties of different phases to each other. The resulting oxidative constructed composites of ferrous alloys with nickel are promising for testing as inert anodes.

Compact vanadium nitride in the form of a finished product is prepared using the oxidative constructing approach. The samples are subjected to resistive heating in a nitrogen gas environment. A complex of physical studies of the prepared material is carried out.

Oxidative construction of thin-walled ceramics (OCTC) is used to obtain compact niobium nitrides with the given phase composition in the form of finished products. The change in the sample phase composition is studied at different temperature modes.