Tantalum Alloys

alloys in which tantalum is the base metal. Tantalum’s crystal structure, atomic dimensions (atomic radius, 1.46 angstroms), and position in the electronegativity scale account for the element’s tendency to form solid solutions and intermetallic compounds with many metals. Tantalum forms continuous series of solid solutions with metals that are isostructural, have approximately the same atomic dimensions, and are close to tantalum in the electronegativity scale. Examples include Nb, W, Mo, V, and β-Ti. A limited number of solid solutions and intermetallic compounds (with Al, Au, Be, Si, and Ni) are formed when there is a wide difference in atomic dimensions and electronegativity. Tantalum forms virtually no solid solutions or compounds with Li, K, Na, Mg, and certain other elements.

Tantalum alloys possess good mechanical properties at normal temperatures; they are also heat-resistant, corrosion-resistant, and more economical than pure tantalum. Tantalum alloys with niobium are very important because their properties are closest to those of tantalum and they can therefore replace tantalum, which is scarce, in many areas of application. Heat-resistant tantalum alloys are of special interest. Tantalum, tungsten, molybdenum, and niobium constitute a special group because alloys based on these metals hold great promise for creating structural materials resistant to high temperatures for aircraft, rockets, and space vehicles.

Tantalum is usually alloyed with W, Mo, V, Nb, Ti, Zr, Hf, Re, Cr, C, and certain other elements. Of the many heat-resistant tantalum alloys, the most important are those made with tungsten. Thus, the tensile strength of a tantalum alloy containing 10 percent W is (in meganewtons/m²) 1,265 at 20°C, a figure much higher than that for pure tantalum, 661 at 980°C, 148 at 1430°C, and 84 at 1650°C (or 126.5, 66.1, 14.8, and 8.4 kilograms-force/mm², respectively). The elongation at these temperatures is (expressed as a percentage) 4.0, 4.2, 17.0, and 33.0. In comparison with tungsten, this alloy shows greater ductility, equal strength, and higher resistance to oxidation at temperatures up to 2800°C. The alloy is used to manufacture parts for combustion chambers, nozzles for jet engines, and the front edges of aircraft tail assemblies. Another tantalum alloy, this one containing 8 percent W and 2 percent Hf, is used for the same purposes; this alloy has a greater specific strength at high temperatures than all other heat-resistant alloys susceptible to deformation. A ductile alloy containing 8 percent W and 2.5 percent Re has been proposed for industrial furnace heaters, heat shields, and components of nuclear propulsion systems of space vehicles.

The electronics industry makes use of tantalum alloys that have high electrical resistance and good thermionic emission properties and contain up to 7.5 percent W. As a rule, tantalum alloys cannot compete with pure tantalum in corrosion resistance; however, corrosion resistance can sometimes be increased through alloying. For example, tantalum alloys containing more than 18 percent W are virtually impervious to corrosion when exposed to 20-percent hydrofluoric acid.

Tantalum alloys holding great promise for use as structural materials resistant to high temperatures include the beryllide, used in the aerospace industry for parts operating at temperatures of approximately 1500°C; the borides, used for coating sheets of tantalum that come in contact with molten uranium and calcium; and the suicides, nitrides, and carbides, used for jackets to insulate fuel elements. The carbide TaC is an important component of several cermet hard alloys. For example, of the 83 tons of tantalum consumed in Japan in 1972, 40 tons were used in producing hard alloys, and of the 600 tons of tantalum consumed in the United States in the following year, 85–90 tons were used in the form of the carbide for producing hard alloys. Niobic ferrotantalum is sometimes used as an admixture for certain steels to prevent intercrystalline corrosion and to improve other qualities, but the use of ferroniobium is preferable in this case because of the scarcity of tantalum. In general, the scarcity and relatively high cost of tantalum prevent its extensive use in the form of tantalum alloys.