Refractory Metals: Tungsten and Tungsten alloys

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Understanding Refractory Metals and Tungsten's Classification

Refractory metals are distinguished by their exceptionally high melting points, significantly exceeding those of common alloying bases such as iron, cobalt, and nickel. These metals demonstrate superior performance at temperatures beyond the capabilities of titanium (melting point 1660°C) and zirconium (1850°C), which primarily serve intermediate-temperature applications. Consequently, chromium with its melting point of 1875°C earns classification as a refractory metal.

The refractory metals group encompasses twelve metals with melting points above 1850°C: tungsten (3410°C), rhenium, osmium, tantalum, molybdenum, iridium, niobium, ruthenium, hafnium, rhodium, vanadium, and chromium. These metals frequently interact with metalloids—elements of small atomic size including hydrogen, oxygen, nitrogen, and carbon—forming interstitial solid solutions or compounds that significantly influence their properties.

Primary Forms and Applications of Tungsten Consumption

Tungsten Carbide Applications

Tungsten carbide represents the dominant consumption category, accounting for approximately 65% of total tungsten usage. This compound combines with cobalt as a binding agent to create cemented carbides, which excel in cutting and wear-resistant applications. These carbides characteristically exhibit high hardness, excellent electrical and thermal conductivity, and remarkable stability under extreme conditions.

The exceptional properties of tungsten carbides enable three principal application categories: structures requiring resistance to chemical reactions, components demanding superior wear resistance, and high-temperature radiant-energy sources. However, the inherent brittleness of carbides prevents their use as single-phase materials in highly stressed structural applications, leading to the development of metal-bonded composites known as cemented carbides or cermets.

Metallic Tungsten and Alloy Applications

Metallic tungsten and tungsten alloy mill products constitute approximately 16% of total consumption. Tungsten and its alloys dominate markets requiring high-density materials, leveraging tungsten's exceptional density of 19.3 g/cm³. Primary applications include kinetic energy penetrators, counterweights, flywheels, and governors, where mass concentration proves critical for performance.

Additional applications encompass radiation shields and x-ray targets, capitalizing on tungsten's high atomic number and density for effective radiation attenuation. Wire-form tungsten finds extensive use in lighting applications, electronic devices, and thermocouples, where its high melting point and electrical properties provide distinct advantages.

Tungsten Chemical Applications

Tungsten-based chemicals represent approximately 3% of total consumption, serving specialized applications including organic dyes, pigment phosphors, catalysts, cathode-ray tubes, and x-ray screens. These applications exploit tungsten's unique chemical properties and its ability to form stable compounds under various conditions.

Manufacturing and Fabrication Processes

Tungsten's extraordinarily high melting point makes it an obvious choice for structural applications exposed to extreme temperatures. Beyond high-temperature applications, tungsten serves lower-temperature uses that benefit from its high elastic modulus, exceptional density, or superior shielding characteristics.

Tungsten and tungsten alloys undergo pressing and sintering processes to form bars, which subsequently undergo fabrication into wrought bar, sheet, or wire products. Many tungsten products require intricate geometries necessitating machining or molding and sintering to near-net shape, as they cannot be fabricated from standard mill products.

Recrystallization Behavior Classifications

Tungsten mill products divide into three distinct groups based on their recrystallization behavior, each exhibiting unique characteristics that influence their applications and processing requirements.

The first group consists of electron beam-melted, zone-refined, or arc-melted unalloyed tungsten, along with other very pure forms of unalloyed tungsten or tungsten alloyed with rhenium or molybdenum. These materials exhibit equiaxed grain structures upon primary recrystallization, with both recrystallization temperature and grain size decreasing as deformation increases.

The second group, comprising commercial grade or undoped powder metallurgy tungsten, demonstrates tungsten's sensitivity to purity levels. While these materials also exhibit equiaxed grain structures, their recrystallization temperatures exceed those of first-group materials. Unlike the first group, these materials do not necessarily show decreases in recrystallization temperature and grain size with increasing deformation. Electron beam-melted tungsten wire may recrystallize at temperatures as low as 900°C (1650°F), while commercially pure undoped tungsten requires temperatures ranging from 1205 to 1400°C (2200 to 2550°F).

The third group encompasses AKS-doped tungsten (aluminum-potassium-silicon doped), doped tungsten alloyed with rhenium, and undoped tungsten alloyed with more than 1% ThO₂. These materials exhibit higher recrystallization temperatures exceeding 1800°C (3270°F) and unique recrystallized grain structures. Heavily drawn wire or rolled sheet develops very long interlocking grains, most readily achieved in AKS-doped tungsten or doped tungsten alloyed with 1 to 5% rhenium.

The potassium dopant distributes along the rolling or drawing direction, volatilizing when heated into linear arrays of submicron-size bubbles. These bubbles pin grain boundaries similarly to dispersed second-phase particles. As deformation increases, the bubble rows become finer and longer, raising the recrystallization temperature and enhancing the interlocking structure.

Commercial Tungsten Alloy Systems

Three tungsten alloys achieve commercial production: tungsten-ThO₂, tungsten-molybdenum, and tungsten-rhenium. The W-ThO₂ alloy contains a dispersed second phase of 1 to 2% thorium, which enhances thermionic electron emission, improving the starting characteristics of gas tungsten arc welding electrodes. This dispersion also increases electron discharge tube efficiency and imparts creep strength to wire at temperatures above half the absolute melting point of tungsten.

Tungsten mill products including sheet, bar, and wire undergo production via powder metallurgy techniques. These products are available in either commercially pure (undoped) tungsten or commercially doped (AKS-doped) tungsten. The additives improve recrystallization and creep properties, proving especially important for incandescent lamp filament applications. Wrought powder metallurgy stock can undergo zone refining by electron beam melting to produce single crystals with higher purity than commercially pure products. Electron beam zone-melted tungsten single crystals serve commercial applications requiring single crystals with very high electrical resistance ratios.

Tungsten Heavy-Metal Alloys (WHAs)

Tungsten Heavy-Metal Alloys represent a specialized category of tungsten-base materials typically containing 90 to 98 weight percent tungsten. Most commercial WHAs feature two-phase structures, with the principal phase being nearly pure tungsten associated with a binder phase containing transition metals plus dissolved tungsten. Consequently, WHAs derive their fundamental properties from the principal tungsten phase, providing both high density and high elastic stiffness that enable most applications for this material family.

Current WHA applications span diverse consumer, industrial, and government sectors, including damping weights for computer disk drive heads, balancing weights for aircraft ailerons, helicopter rotors, and guided missiles, kinetic energy penetrators for armor defeat, fragmentation warheads, radiation shielding components, radioisotope containers, collimation apertures for cancer therapy devices, high-performance lead-free shot for waterfowl hunting, gyroscope components, and weight distribution adjustment in sailboats and race cars.

Applications requiring high gravimetric density for balance weights, inertial masses, or kinetic energy penetrators, or high radiographic density for radiation shielding and collimation, often necessitate large bulk shapes. This requirement eliminates most candidates due to prohibitive costs, typically reducing very dense alloy choices to either tungsten-base or uranium-base materials.

Uranium alloys, like lead, face elimination from increasing numbers of potential applications based on toxicity considerations, with uranium-base materials requiring licensing except for very small quantities. While precious metals possess attractive densities and offer essentially no toxicity, their cost proves prohibitive for all but a few density applications.

WHAs typically consist of 90 to 98 weight percent tungsten combined with various mixtures of nickel, iron, copper, and cobalt. The majority of WHA production falls within the 90 to 95% tungsten range.

Alloy composition selection depends on several considerations, with required density serving as the primary factor. Additional considerations include corrosion resistance, magnetic character, mechanical properties, and post-sinter heat treatment options.

The first developed WHA was a W-Ni-Cu alloy. Alloys of this ternary system still find occasional use today, primarily for applications requiring minimized ferromagnetic character and electrical properties. However, W-Ni-Cu alloys offer inferior corrosion resistance and lower mechanical properties compared to current industry standard W-Ni-Fe alloys.

The majority of current WHA applications are best satisfied with the W-Ni-Fe system. Alloys such as 93W-4.9Ni-2.1Fe and 95W-4Ni-1Fe represent common compositions. Cobalt addition to W-Ni-Fe alloys provides a common approach for slight enhancement of both strength and ductility. Cobalt presence within the alloy provides solid-solution strengthening of the binder and slightly enhanced tungsten-matrix interfacial strength. Cobalt additions of 5 to 15% of the nominal binder weight fraction are most common.

For extremely demanding applications, even higher mechanical properties are obtainable from the W-Ni-Co system with nickel-to-cobalt ratios ranging from 2 to 9. Such alloys require resolution and quenching due to extensive intermetallic formation (Co₃W and others) during cooling from sintering temperatures.

Several special WHAs exist as well. The W-Mo-Ni-Fe quaternary alloy utilizes molybdenum to restrict tungsten dissolution and spheroid growth, resulting in higher strengths but reduced ductility in the as-sintered state.

Various alloy systems remain in development stages for kinetic energy penetrators, intended to provide WHAs that undergo high deformation rate failure by shear localization similar to quenched and aged U-0.75Ti for more efficient armor defeat. These developmental alloys have not yet exhibited property sets of interest for industrial applications.

Mechanical and Physical Properties

Tungsten demonstrates high tensile strength and excellent creep resistance. However, its high density, poor low-temperature ductility, and strong reactivity in air limit its usefulness. Maximum service temperatures for tungsten range from 1925 to 2500°C (3500 to 4500°F), but surface protection is required for air exposure at these temperatures.

Wrought tungsten in the as-cold worked condition exhibits high strength, strongly directional mechanical properties, and some room-temperature toughness. However, recrystallization occurs rapidly above 1370°C (2500°F) and produces grain structures that are crack sensitive at all temperatures.