Ivan A. Kovalev

The kinetic dependences of hafnium nitridization at temperatures of 1900, 2200, and 2500°C are established. The choice of temperature is based on the nitridization conditions of the Hf–Ti, Hf–Zr, and Hf–Nb metal pairs. By nitridization of Hf–Ti, Hf–Zr, and Hf–Nb pairs while maintaining the shape of the initial metal billets, binary compact nitrides are synthesized, and the features of the hafnium nitridization process in these bimetallic systems are determined. The sequence of phase and structural transformations proceeding in individual hafnium as part of each pair being nitridizated is established. The formation of hafnium nitride ceramics occurs through the formation of three- and two-layer gradient structures. Ceramics close to the stoichiometric composition of HfN is formed at temperatures of 2200°C and above. The value of the thermo-EMF of the Hf–Ti, Hf–Zr, and Hf–Nb systems is assessed, the character of the dependence of the Seebeck coefficient on the nitridization time is established, and its value is estimated.

By nitriding the Zr–V pair while preserving the shape of the original metallic blanks, binary compact nitrides were synthesized. The kinetic and voltammetric dependencies of the nitriding process were determined. For the individual metals and the weld zone, interaction with nitrogen proceeds via different mechanisms. Pure metals react with nitrogen through the formation of three- and two-layered structures. Nitriding of the weld zone is accompanied by the decomposition of the Zr–V solid solution with segregation of metallic vanadium at grain boundaries as a separate phase. Simultaneously, mutual dissolution occurs between the nitrogen reaction products: nitrogen solid solutions in both metals and their nitrides of varying stoichiometry. The ceramic formation process is accompanied by diffusion of vanadium into the region of its lower concentration and the formation of the intermetallic compound Zr_0.3V_0.6N_0.1, which does not interact with nitrogen at the synthesis temperature. For gradient and ceramic structures (Zr–V)N_x in the temperature range from –195.7 to +550°C, the nature of the dependence of the thermoelectromotive force (thermo-EMF) on nitriding time was established and its magnitude was evaluated. The obtained ceramic and gradient materials can be used as ceramic thermoelectric converters.

The kinetic dependences of hafnium nitridation at temperatures of 1900, 2200 and 2500 °C were established. The choice of temperature is justified by the conditions of nitridation of metal pairs Hf-Ti, Hf-Zr and Hf-Nb. By nitriding Hf-Ti, Hf-Zr and Hf-Nb pairs while preserving the shape of the original metal blanks, binary compact nitrides were synthesized, and the features of the hafnium nitriding process in these bimetallic systems were determined. The sequence of phase and structural transformations occurring in individual hafnium in the composition of each nitriding pair has been established. The formation of hafnium nitride ceramics occurs with the formation of three- and two-layer gradient structures. Ceramics close to the stoichiometric composition of HfN are formed at temperatures of 2200 °C and above. The magnitude of the thermo-EMF of the Hf-Ti, Hf-Zr and Hf-Nb systems was assessed, the nature of the dependence of the Seebeck coefficient on the nitriding time was established and its value was estimated.

Нитридизация металлической пары Ti–V и оценка термо-ЭДС синтезированного керамического образца

Ceramic nitride samples of tailored composition and shape have been prepared by controlled nitridation of Ti–V metal pairs. We have obtained kinetic and current–voltage curves for the interaction of the Ti–V metal pairs with nitrogen. In different parts of the metal pairs, the nitridation process follows different mechanisms. In the case of the pure metals, the formation of nearly stoichiometric ceramics involves the formation of three- and two-layer graded structures. Nitridation of the junction region, containing a Ti–V solid solution, is determined by the chemical affinity of titanium and vanadium for nitrogen. The formation of titanium nitride leads to decomposition of the Ti–V solid solution in the junction and vanadium metal separation on grain boundaries. The rate of vanadium nitridation increases as the titanium content of the solid solution decreases. The thermoelectric voltage of the Ti–V system in the temperature range from –195.7 to +550°C has been evaluated as a function of the degree of nitridation. The thermoelectric voltage and Seebeck coefficient of metal–ceramic and ceramic structures have been determined as functions of temperature. Characteristically, the thermoelectric voltage of all the nitrided pairs rises monotonically over the entire temperature range studied. Nitrided titanium–vanadium pairs with tailored composition can be used as ceramic thermoelectric converters.

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.

—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 % U in the range 3.5–60 min is presented. For each composition, high-temperature nitrogen saturation results in the decomposition of a (Zr,U) solid solution to form ZrN–(ZrN_{1 – n}/U_xE_y/U)–ZrN composite structures (where E = O, N; n, x, y are stoichiometric coefficients). The decomposition of the solid solution gives rise to zirconium nitride and metallic uranium. The latter accumulates impurities, which are present in the initial solid solution, in the central part of the sample. The kinetic curves for a temperature of 1900°C are approximated by an exponential law and correspond to zirconium nitriding. The nitriding rate of the (Zr,U) solid solution increases with the uranium content. To complete the formation of a compact solid solution nitride (Zr,U)N of a stoichiometric composition, the reaction temperature and time should be increased.

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.

Кинетика высокотемпературной нитридизации твердых растворов Zr–Nb

We have demonstrated general kinetic aspects of the formation of nitrides of Zr–Nb alloys (containing 0.1, 2.5, and 5 wt % Nb) at a temperature of 1900°C. The nitridation process has been shown to have a two-stage character, with both stages having an exponential rate law. The reaction rate in the second stage is considerably lower than in the first stage. We have determined the composition of the resulting heterostructures of the form Zr_1–хNb_хN–ZrN_1–n /β-solid solution of zirconium in niobium (Zr_1–хNb_хN) and identified the nitridation sequence of the components of the starting alloy. The first stage of the process is the formation of an α-solid solution of nitrogen in Zr and its conversion into a nonstoichiometric nitride. The kinetic curve of the second stage describes nitridation of the β-Nb phase resulting from the decomposition of the Zr〈Nb〉 solid solution. The duration of the second stage of the process has been shown to be determined by the amount of niobium in the starting solid solution. Experimental evidence is presented that single-stage nitridation of Zr–M alloys can be used for producing single-phase ceramics containing active additions and having the shape of the starting metallic workpiece.