Thermodynamic analysis of the reaction of titanium with boron carbide in a self-propagating high-temperature synthesis regime

Adiabatic temperatures, thermal characteristics, and the concentration of reaction products in the system Ti−B₄C containing 1–99 mass % B₄C are determined by thermodynamic analysis using the ASTRA package of programs. In order to prepare alloys of titanium with titanium boride and titanium carbide in a self-propagating high-temperature synthesis regime a mixture with 10–13 mass % B₄C is recommended, and for alloys of boron carbide with titanium carbide a mixture containing 53–70 mass % B₄C is recommended. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 72–76, March–April, 1999.     

Granulation in the powder technology of self-propagating high-temperature synthesis

Granulation, which is a very simple procedure for the formation of an artificial structure of a powder medium, is widespread in conventional powder metallurgy. However, it is very rarely used to implement self-propagating high-temperature synthesis (SHS). It is shown by the examples of various SHS processes (with directed gas filtration, with a reduction stage, in an aluminum melt, and the combustion of a thermite mixture) that charge granulation can substantially affect both the parameters of the SHS process and the properties of its product.     

Application of titanium to self-propagating high-temperature synthesis

An overview of the application of various kinds of titanium to self-propagating high-temperature synthesis (SHS) is presented. Kinds of Ti include powders of various origins, sponge, dioxide (rutile concentrate and pigment powder), and scrap (namely, chips and scale). Using the combustion of solid-solid and solid-gas systems (titanium-carbon and titanium-nitrogen) as an example, the influence of general titanium characteristics on the properties of the final SHS product is shown. The principles are substantiated for selecting titanium for SHS which are defined by the following factors: chemical and granulometric compositions, the morphology of the particle surface, fire-and-explosion safety, and the cost and availability of one or another kind of titanium. SHS-based methods for the production of Ti powder, namely, the magnesiothermic reduction of dioxide with the subsequent acid enrichment and SHS hydrogenation of the sponge with the following dehydrogenation, are briefly considered. A short list of areas for the use of some titanium-based SHS products is given.     

Deoxidation of the tantalum powder produced by self-propagating high-temperature synthesis

The possibility of removal of oxygen and magnesium from the products of the magnesium reduction of tantalum pentoxide under self-propagating high-temperature synthesis conditions that contain 30 wt % Mg₄Ta₂O₉ is studied. Additional reduction of this material can decrease the magnesium content to <0.01%. The oxygen content in the fabricated tantalum powder does not exceed its amount in the surface oxide (3 × 10^?3 g/m²). The specific surface area of the powder is an order of magnitude higher than that of the initial material, which can result from the formation of a tantalum powder with a specific surface area >30 m²/g during the reduction of Mg₄Ta₂O₉.     

Magnesium-thermic reduction of tantalum oxide by self-propagating high-temperature synthesis

The magnesium-thermic reduction of tantalum pentoxide is studied during self-propagating high-temperature synthesis, i.e., under the conditions of manufacturing solid products in an autowave combustion mode. The effects of the charge density, the charge temperature, the particle size of reagents, and thermal ballast on the combustion rate and the maximum process temperature are investigated. Depending on the charge parameters, the combustion rate changes from 1.6 to 13 mm/s and the measured combustion temperatures varies in the range 1100–1885°C.     

Production and use of ferrosilicotitanium produced by self-propagating high-temperature synthesis

The production of silicotitanium alloys by self-propagating high-temperature synthesis in the titanium-ferrosilicon and ferrotitanium-silicon systems is investigated. On the basis of the results, a production technology is developed for a new material: ferrosilicotitanium (titanium ferrosilicide) produced by self-propagating high-temperature synthesis, for use in economical titanium alloying of steel. Industrial tests at OAO MMK show that the assimilation of titanium from titanium ferrosilicide is greater (by a factor of 1.5) and more stable than from ferrotitanium.     

Reaction of boron-containing materials with titanium during self-propagating high-temperature synthesis

We have studied the contact reaction of titanium, chromium, and tungsten borides with titanium. We have established that the materials react according to an exothermic solid-phase reaction mechanism with formation of titanium monoboride and diboride. The reaction is limited by diffusion of boron atoms through the layer of titanium monoboride formed. This makes it possible to successfully use reaction to effectively control the process for obtaining composite materials in the Ti-B₄C system under selfpropagating high-temperature synthesis conditions. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 67–72, March–April, 2006.     

Production of cast higher chromium carbide using self-propagating high-temperature synthesis

The mechanisms of self-propagating high-temperature synthesis as they apply to cast chromium carbide have been examined. It is demonstrated that by optimizing the initial-mixture composition, it is possible to synthesize cast higher chromium carbide Cr₃C₂ with a 1.5–3 wt. % aluminum addition. Treating the final product with a hydrochloric acid solution enables the aluminum content to be reduced to 0.2 wt. %.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 57–60, November, 1992.     

The use of self-propagating high-temperature synthesis and argon-arc overlaying when strengthening the surface of products made of titanium alloys

A new method for the deposition of protective coatings (wear-resistant overlaying) that are 2 to 10 mm thick and more on the surface of titanium articles is suggested, developed, approved, and patented. The method, called “SHS-arc overlaying,” combines the exothermic synthesis of adding material (AM) and argon-arc overlaying, which proceed simultaneously with the deposition of the coating. AMs made of exothermic mixtures are developed. Using this new method, protective composite coatings for titanium articles are obtained. The physicochemical, microstructural, and exploitation properties of the overlaying are investigated. Its operational characteristics exceed the characteristics of standard samples by a factor of 1.5–2.0. This new technology can be recommended for the deposition of protective coatings on products of wide indentation made of titanium alloys.     

Synthesis of zirconium diboride-alumina composite by the self-propagating,high-temperature synthesis process

Synthesis of a ZrB₂-Al₂O₃ in-situ composite powder by the self-propagating, high-temperature synthesis (SHS) process using oxides as raw materials has been investigated. The combustion velocity was found to be about 5 mm/s. The combustion temperature varied in the range of 2173 to 2223 K, respectively. The SHS reaction was found to be mediated through melting of aluminum and, thereby, the SHS reaction. The ignition temperature is estimated to be about 1503 K by thermal analysis. Dendrite structures were also observed during SHS processing of the composite.     

基于自蔓延高温合成(SHS)的涂层技术

综述了几种利用自蔓延高温合成(SHS)反应制备涂层的技术,包括反应热喷涂、SHS离心铸造涂层、SHS熔铸涂层以及反应铸渗涂层.介绍了它们的工艺过程、工艺特点、适应的反应体系以及涂层性能.这些基于SHS反应,用于制备耐磨耐蚀陶瓷或金属陶瓷涂层的涂层技术,充分利用SHS技术本身具有的节能、工艺简单、产物纯度高等优点,显示出了很强的竞争优势.     

Surface of powders of silicon nitride produced by self-propagating high-temperature synthesis

Conclusions The surface of a starting silicon powder used in the production of silicon nitride by the SHS method is covered with an oxide film whose thickness varies, depending on the method of comminution and time of storage of the powder, in the range 1–12 nm. During SHS the Si + SiO₂ reaction results in the formation of gaseous SiO, which becomes adsorbed on the cold walls of the reactor and then experiences disproportionation. This lowers the oxygen content of the resultant silicon nitride, but the overall purification effect achieved is small. After comminution, a 1- to 2-mm-thick layer of powdered silicon nitride produced by SHS consists of a silicon oxynitride with an oxygen content of 5–30 wt.%. Milling conditions do not significantly affect the concentration of oxygen inside the nitride particles, which does not exceed 0.4%. To obtain silicon nitride of low oxygen content, it is necessary to employ starting reactants of high purity.Translated from Poroshkovaya Metallurgiya, No. 1(253), pp. 48–54, January, 1984.     

Surface of particles of TiNx-base powders produced by self-propagating high-temperature synthesis

Conclusions The surface of a powder produced by the nitration of titanium by the SHS method is covered with a titanium oxynitride film of 8- and 15-Å thickness. For a specimen of low total nitrogen content the [N/Ti]_at ratio is much higher in the surface layer than in whole volume of the specimen.Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 1–5, November, 1979.