Rare metals — group of relatively new or little-used metals is so named. Scale of their production and scope have not yet been fully mastered. Matter of national importance is use of rare metals in defense industry and in general in national economy.
Open rare metals in earth’s crust are contained in small quantities. However, vanadium, titanium, beryllium, lithium and some others are mined annually in thousands of tons. Sometimes bismuth and tantalum are found in nature in form of nuggets. Rich deposits of tungsten, molybdenum and titanium discovered. Some ores of known deposits contain associated metals like tellurium, indium, selenium, gallium, germanium, which are extracted during ore processing. Minerals of pegmatite veins also contain rare and rare earth metals. Extraction of rare metals is very complex way of processing ore.
Rare metals have unusually valuable and diverse qualities. Radioactive properties of some rare earth metals are most widely used in nuclear energy. Use of even insignificant fractions of rare metals in alloys optimizes their qualities, for example, resistance to cryogenic temperatures, increase heat resistance.
According to scientists, rare metals can change production technologies of modern sectors of economy. When creating modern technology, very high requirements are imposed on alloys used. Rare metals form basis of these alloys. Structure, properties and method of application of some rare metal-based alloys are of exceptional importance for modern technology. Alloys of tungsten with manganese, for example, are of particular interest to researchers. But to obtain these alloys is technologically very difficult, powder metallurgy methods are usually used.
A special class of chemical elements includes metals that melt at high temperatures — at least 1650−1700 °C. Group of refractory metals is led by rare metals. They all have tightly packed atomic structure. High melting point, increased hardness, strength and electrical resistance appear due to close interatomic electronic bonds.
Most important area of application of most refractory metals is aircraft construction, rocket and space technology, metal processing. Refractory alloys as structural material find place in marine shipbuilding, mechanical engineering, in electronic, chemical, electrical, nuclear industries. Particularly valuable structural qualities are hafnium, zirconium, niobium, molybdenum, tantalum. These metals are characterized by relatively low creep resistance at high melting points.
Welding of refractory materials is difficult due to high melting point, high affinity for oxygen, hydrogen, nitrogen. These features contribute to formation of pores in joints, reduce their plastic properties, and lead to formation of grains during heating. Even small amount of gas impurities leads to noticeable changes in properties. Second disadvantage of all refractory metals is need to use various protective coatings against oxidation at high temperatures. In addition, refractory alloys are not sufficiently technological. They are badly deformed, cut, drilled.
Main area is structural elements of nuclear reactors. Refractory alloys can be used in future in space technology with nuclear energy, in electrical and other devices. For example, in capacitors, nuclear-pumped lasers.
Refractory metals and alloys work in vacuum, in neutral and reducing atmospheres, and also in aggressive and liquid metal environments. Oxidative atmosphere at high temperature reduces heat resistance of refractory metals. Therefore, they are used with special protective coatings. Today, all hard alloys used for calibration, stamping or as cutting tools are made on basis of refractory tungsten, titanium, molybdenum, in alloy with nickel or cobalt. High hardness of carbides of refractory metals provides resistance to wear. However, alloys obtained by casting from pure carbides did not give expected superhard qualities due to excessive brittleness. Such carbides are usually included in structure of sintered hard alloys.
Tungsten and molybdenum are most widely used in electric lamp, electric vacuum and radio engineering industries, and are used for alloying tool steel. Vanadium is used in electric vacuum technology and in metallurgy as alloying element. Zirconium and its alloys have been used in nuclear and vacuum industry as degasser, as well as in production of non-ferrous alloys. Tantalum and niobium are used for electrodes in tube rectifiers and powerful generator devices. Tantalum is in demand by chemical industry. Both metals are used as alloying elements for heat-resistant and acid-resistant alloys.
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