Tungsten and Tungsten Alloys

Abstract

Tungsten, discovered in 1755 and produced as a metal in 1783, is notable for having a very low vapor pressure, the highest melting point of any metal, and the highest tensile strength of any metal above 1650°C. It is used in the manufacture of high speed steels and as a metal filament in electric lamps, where it is fabricated by powder metallurgy using the Coolidge process. The largest application is in the manufacture of cemented carbide cutting tools. Tungsten is rated as a strategic material in the U.S. economy. Tungsten occurs in a number of mineral forms, the most important being scheelite and wolframite. The beneficiation and hydrometallurgy are described, together with the principal intermediate compounds used to make the metal powder by hydrogen reduction. Technically it is possible to process compacted powder by arc or electron‐beam melting, but commercially the power metallurgy process of compaction, sintering, and working is used exclusively. The largest reserves of ores occur in China, which has, in more recent years, become the principal source of supply not only for ores but for metal powder and intermediates at prices that have caused most U.S. mines to be closed. About 65% of tungsten usage is as tungsten carbide in cutting and mining tools. The instability of the market as well as economic and environmental factors have favored development of recycling processes. Used cutting tools can be treated with molten zinc and both tungsten carbide and cobalt binder are recovered as mixed powders after vacuum‐distillation of the zinc. It is estimated that 25% of cutting tools are recycled in this way in the United States. Tungsten alloyed with nickel and iron is used for a range of high density compositions that find applications as counterweights in aircraft, as armor‐piercing penetrators, and for collimators and shielding in x‐ray machines. These alloys are characterized by high density and high modulus, which make them suitable for use in critical dynamic structures. Tungsten forms only a few alloys; tungsten–rhenium is important for thermocouples and as a tough lamp filament alloy. Insoluble tungsten has relatively low toxicity, but aerosols with cobalt and with impurities found in mining can have some effect and require controls of exposure level.

Effect of dissolved tungsten on the deformation of 70Ni-30Fe alloys

“…Though not resolvable in the traces provided in Figure 3, all alloys consistently exhibited serrated flow behavior (ϮϷ2 MPa) Standard deviations for the analyzed compositional data are indicated. The melting temperature as reported by Spencer and Mullendore [5] is indicated.…”

“…As shown, a series of instantaneous strain-rate changes cause concomitant increases in flow stress. The resulting change in flow stress with a given change in strain rate is used to calculate the strainrate-sensitivity, m. heat treatment at 1400 °C intersects the experimental conditions reported in the literature by Spencer and Mullendore, [5] who investigated the role of matrix composition (i.e., Ni:Fe ratio) on melting temperature, and on the 1400 °C solubility of tungsten. These researchers report a tungsten solubility of approximately 23.5 pct at 1400 °C, essentially identical (within reasonable statistical bounds) to the 23 wt pct value determined in the present work.…”

“…[1][2][3][4][5][6][7][8][9][10] With regard to the Ni-Fe-W system, alloys with Ni:Fe composition ratios of approximately 7:3 (by weight) have emerged as baseline alloys for study, presumably due to this composition's proximity to a freezing point minimum, and as a composition where the formation of certain cooling-rate-sensitive, undesirable intermetallic phases, which adversely affect mechanical behavior, can be avoided. [3,4] While other studies have suggested that other compositions may offer a superior combination of properties, they nonetheless make their comparisons to the properties that have been established for the 7:3 composition.…”

“…[3,4] While other studies have suggested that other compositions may offer a superior combination of properties, they nonetheless make their comparisons to the properties that have been established for the 7:3 composition. [5][6][7][8][9][10] It is generally accepted that the ductility, toughness, and impact resistance values that are measured for the tungsten heavy alloys rely in great part on the physical metallurgy of the matrix phase, that is, on the microstructural details of the continuous Ni-Fe-W solid solution that envelopes the tungsten particles. However, most of our current understanding in this regard has been gleaned from data obtained from full composite samples, i.e., from alloys that typically contain tungsten concentrations of 80 pct or more.…”

“…However, most of our current understanding in this regard has been gleaned from data obtained from full composite samples, i.e., from alloys that typically contain tungsten concentrations of 80 pct or more. [1][2][3][4][5][6][7][8][9][10] For example, Ekbom et al [6,7] have shown that strain partitioning to the Ni-Fe matrix occurs early in the deformation of these alloys, leading them to infer that the ductile matrix experiences increased rates of work hardening. Coates and Ramesh [8] likewise measured the hardness of individual phases within a composite Ni-Fe alloy containing 90 wt pct W following high-strain-rate deformation, and have inferred that the matrix phase likely exhibits a higher strain-rate sensitivity that the tungsten phase in these alloys.…”