refractory metal[ri′frak·trē ¦med·əl]
according to the classification adopted in engineering, any of a group of metals whose melting points exceed 1650–1700°C. The refractory metals (Table 1) include titanium, Ti, zirconium, Zr, and hafnium, Hf (Group IV of the periodic system), vanadium, V, niobium, Nb, and tantalum, Ta (Group V), chromium, Cr, molybdenum, Mo, and tungsten, W (Group VI), and rhenium, Re (Group VII). With the exception of Cr, all these elements are rare metals, and Re is a disseminated rare metal. Although the platinum metals and thorium also have high melting points, they are not classified as refractory metals.
Refractory metals, which have similar electronic structures, are transition elements with incomplete d subshells (see Table 1). Not only the outer s electrons but also the d electrons participate in the interatomic bonds of refractory metals, which accounts for the great strength of these bonds and, hence, the metals’ high melting point, mechanical strength, hardness, and electrical resistance. Refractory metals have similar chemical properties. The variable valence of the metals gives rise to different types of chemical compounds; the metals form metallic refractory solid compounds.
|Table 1. Refractory metals|
|Symbol||Atomic number||Outer electron subshells||Melting point (°C)|
Refractory metals are not encountered in nature in the free state; in minerals, they often act as isomorphous substituents of one another. Hf occurs in isomorphous association with Zr, and Ta occurs in this way with Nb, as does W with Mo. The separation of these pairs, an extremely difficult problem in chemical engineering, is usually effected through extraction or sorption from solutions or through the rectification of chlorides.
Physical and chemical properties. The crystal lattices of the metals of Group IV and of Re are hexagonal; those of Ti above 882°C, Zr above 862°C, and Hf above 1310°C and of the remaining metals are body-centered cubic. Ti, V, and Zr are relatively light metals, while Re and W—the most refractory of all metals—are exceeded in density only by Os, Ir, and Pt. Pure, annealed refractory metals are ductile and susceptible to both hot and cold pressure treatment, especially the metals of Group IV and V. The retention of the desirable mechanical properties of refractory metals and of alloys based on the metals at very high temperatures is an important characteristic. In this respect, the metals may be regarded as heat-resistant structural materials. However, the mechanical properties of refractory metals depend to a great extent on purity, degree of deformation, and heat-treatment conditions. Thus, Cr and its alloys lose their ductility even with low contents of certain impurities; Re, which has a high modulus of elasticity, undergoes considerable strain hardening, and thus annealing is necessary even with a slight amount of deformation.
Impurities of carbon have a pronounced effect on the properties of refractory metals (except Re). The properties of the metals in Groups IV and V are extremely sensitive to hydrogen impurities, and impurities of nitrogen and oxygen have a strong effect on the properties of all the metals. The impurities act to embrittle the metals. Refractory metals typically are resistant to air and many corrosive media at room temperature and slightly elevated temperatures and extremely reactive at high temperatures. At such temperatures, the metals must be kept in a vacuum or in an atmosphere of gases that are inert with respect to the metals. The reactivity of the metals of Groups IV and V is greatest when heated; hydrogen also reacts with these metals. At 400°–900°C, it is absorbed by these metals with the formation of brittle hydrides; when the hydrides are heated in a vacuum at 700°–1000°C, the hydrogen is liberated. This property is used when converting the massive metals into powders by hydrogena-tion (and embrittlement) of the metals and by pulverization and dehydrogenation. Refractory metals of Group VI and Re are chemically less reactive, with the reactivity of the Group VI metals decreasing according to their order of appearance in the group. These metals do not react with hydrogen; in addition, Re does not react with nitrogen. Mo begins to react with nitrogen only at temperatures above 1500°C, and W reacts with nitrogen only above 2000°C. Refractory metals form alloys with many metals.
Production. Approximately 80–85 percent of the V, Nb, and Mo produced (USA, 1973) and a significant portion of the other refractory metals, with the exception of Hf, Ta, and Re, are obtained from ore concentrates or industrially produced oxides by aluminothermic or silicothermic methods in the form of ferroalloys used in alloying steel. Here, molybdenum concentrates are subjected to prior roasting. Pure refractory metals are produced from ore concentrates by a complex three-stage process: breaking up the concentrate, separating and purifying the chemical compounds, and reducing and refining the metal. Production of massive Nb, Ta, Mo, and W and of these metals’ alloys is based on powder metallurgy, which also figures in the production of other refractory metals. Arc, electron-beam, and plasma melting are finding increasing use in the metallurgy of all the refractory metals. Refractory metals and alloys of especially high purity are produced as single crystals through electron-beam and plasma zone melting. Semifinished shapes of refractory metals in the form of sheets, foil, wire, and tubes are produced by ordinary pressure treatment methods combined with intermediate heat treatment.
Use. The enormous importance of refractory metals and of the metals’ alloys and compounds derives from their exceedingly desirable properties. In addition, the metals typically offer combinations of properties. They are used most commonly in heat-resistant alloys in nuclear engineering, high-temperature technology, and the aerospace industry. Parts made from alloys of refractory metals are usually protected with heat-resistant coatings.
Refractory metals and their alloys are also used as structural materials in machine building, shipbuilding, and electrical engineering and in the chemical, electronics, and nuclear power industries. Oxides and many other chemical compounds of refractory metals have found a variety of uses.
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O. P. KOLCHIN