Tungsten Alloys

alloys based on tungsten. Metals (Mo, Re, Cu, Ni, Ag, and others), oxides (ThO₂), and carbides (TaC) are alloyed with tungsten to improve its heat resistance, plasticity (at temperatures up to 500° C), and workability, as well as to obtain the necessary set of physical properties. Tungsten alloys are made by methods of powder metallurgy or by melting the components in arc or electron-beam furnaces. Cermet tungsten alloys are mainly used in industry. Depending on their structure, three groups of tungsten alloys are distinguished: alloys that are solid solutions, pseudoalloys with compounds (artificially disperse systems), and pseudoalloys with metals.

The most important tungsten alloys with a single-phase solid-solution structure are those of tungsten with molybdenum (Mo; up to 50 percent) and rhenium (Re; up to 30 percent). The addition of Mo raises the heat resistance and electrical resistance of the alloys; in addition, W-Mo alloys have a coefficient of thermal expansion that is approximately equal to that of various kinds of refractory glass. These alloys are more easily worked than pure tungsten. Tungsten alloys with 20-50 percent Mo are used in vacuum electrical devices for the construction of heaters and screens. Rhenium in a solid solution based on W substantially increases low-temperature plasticity and, correspondingly, workability. Tungsten alloys containing 20-28 percent Re have maximum plasticity. If the Re content is further increased, plasticity begins to drop again because of separation of the excess σ phase. In addition to having improved plasticity, W-Re alloys are characterized by great heat resistance and high thermal electromotive force when used in a couple with W or with themselves. Despite the scarcity and high cost of Re, in the 1950’s these alloys began to be used in vacuum-electrical devices (alloys with 5-30 percent Re) and as thermocouple materials intended for operation at temperatures up to 2500° C.

Artificial disperse systems based on W and containing 0.5-2 percent ThO₂ and 0.3-0.5 percent TaC are characterized by record high recrystallization temperatures (up to 2000° C) and heat-resistance indexes (two to three times greater than for unalloyed W at 2200° C). In addition, ThO₂ improves the alloys’ emission characteristics. These alloys are used in vacuum electrical devices, as well as for making certain parts of rocket motors and airplane engines.

The pseudoalloys of W with Cu and Ag, which are insoluble in it (they are added separately or together in amounts of 5-40 percent), have a heterogeneous structure, consisting of grains of W surrounded by layers of Cu and Ag or of their alloy. These materials combine the great hardness, heat resistance, wear resistance, and resistance to electrical erosion characteristic of W with the good electrical and heat conductivity of Cu and Ag. Electrical contacts are made from these tungsten alloys. Tungsten impregnated with Ag and Cu is also used in other areas of technology—for example, as a material for the nozzles of uncooled rocket motors. The so-called heavy alloys of W, with 3-10 percent Ni and 2-5 per-cent Cu, have a structure close to that of the pseudoalloys of W with Ag and Cu. The density of these alloys reaches 18 g/cm³ after sintering of pressed billets. The heavy alloys are used as protective materials against gamma radiation in radiotherapy and in making containers for storing radioactive preparations. The high density of the heavy alloys makes possible their use in other fields—for making gyroscope rotors, airplane counterweights, and so on.

Because of technical difficulties, smelted tungsten alloys intended for the manufacture of large-size semifinished products and articles operating at temperatures above 1500° C have not yet been produced on an industrial scale.

The smelted tungsten alloys that are being developed and put into use are solid solutions that are additionally strengthened by a small quantity of disperse particles of carbides (less frequently oxides and borides). The metals added are Mo, Ta, Re, Zr, Nb, and Ti. The first three are used in quantities of a few percent, and even dozens of percent; the rest are used in quantities of tenths of a percent. The minimum necessary low-temperature plasticity is the basis for choosing the maximum amount of the various alloying elements. Promising tungsten alloys, combining high heat resistance with satisfactory low-temperature plasticity, are W (1 10)Re (1 10)Ta, W 25Mo 0.1 H- 0.15Zr 0.05C, and W 0.05 2Nb 0.001 4- 0.02C (additive con-tent in percent). The binary alloy of W with 15 percent Mo is designed for making jet engine blades.