Refractory metals

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Li Be   B C N O F Ne
Na Mg   Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba * Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
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  * La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
  ** Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No
  Refractory metals
  Wider definition of refractory metals[1]

Refractory metals are a class of

fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against creep deformation
to very high temperatures.

Definition

Most definitions of the term 'refractory metals' list the extraordinarily high melting point as a key requirement for inclusion. By one definition, a melting point above 4,000 °F (2,200 °C) is necessary to qualify.[2] The five elements niobium, molybdenum, tantalum, tungsten and rhenium are included in all definitions,[3] while the wider definition, including all elements with a melting point above 2,123 K (1,850 °C), includes nine additional elements: titanium, vanadium, chromium, zirconium, hafnium, ruthenium, rhodium, osmium and iridium.[4] Technetium is not included because of its radioactivity, though it would otherwise have qualified under the wider definition.[5]

Properties

Physical

Properties of the refractory metals
Name Niobium Molybdenum Tantalum Tungsten Rhenium
Period 5 5 6 6 6
Group 5 6 5 6 7
Melting point K[prop 1] 2750 2896 3290 3695 3459
Boiling point K[prop 2] 5017 4912 5731 6203 5869
Melting point °C[prop 1] 2477 2623 3017 3422 3186
Boiling point °C[prop 2] 4744 4639 5458 5930 5596
Density g·cm−3[prop 3] 8.57 10.28 16.69 19.25 21.02
Young's modulus GPa 105 329 186 411 463
Vickers hardness
MPa
1320 1530 873 3430 2450
  1. ^ a b Consensus values taken from melting points of the elements with multiple references there.
  2. ^ a b Consensus values taken from boiling points of the elements with multiple references there. Tungsten has a particularly wide band of discrepancy, with two primary sources reporting 5555 °C.
  3. ^ Consensus values taken from densities of the elements with multiple references there.

Refractory metals have high melting points, with tungsten and rhenium the highest of all elements, and the other's melting points only exceeded by

body-centered cubic crystal structure that resists deformation. Moving to the right in the periodic table, more d electrons increase this effect, but as the d subshell fills they are pulled by the higher nuclear charge into the atom's inert core, reducing their ability to delocalize to form bonds with neighbors. These opposing effects result in groups 5 through 7 exhibiting the most refractory properties.[8]

jet engines, or tools used during forging.[9][10]

Chemical

The refractory metals show a wide variety of chemical properties because they are members of three distinct groups in the periodic table. They are easily oxidized, but this reaction is slowed down in the bulk metal by the formation of stable oxide layers on the surface (passivation). Especially the oxide of rhenium is more volatile than the metal, and therefore at high temperature the stabilization against the attack of oxygen is lost, because the oxide layer evaporates. They all are relatively stable against acids.[6]

Applications

Refractory metals, and alloys made from them, are used in

casting. The process of powder metallurgy is used. Powders of the pure metal are compacted, heated using electric current, and further fabricated by cold working with annealing steps. Refractory metals and their alloys can be worked into wire, ingots, rebars, sheets or foil
.

Molybdenum alloys

Molybdenum-based alloys are widely used, because they are cheaper than superior tungsten alloys. The most widely used alloy of molybdenum is the Titanium-Zirconium-Molybdenum alloy TZM, composed of 0.5% titanium and 0.08% of zirconium (with molybdenum being the rest). The alloy exhibits a higher creep resistance and strength at high temperatures, making service temperatures of above 1060 °C possible for the material. The high resistivity of Mo-30W, an alloy of 70% molybdenum and 30% tungsten, against the attack of molten zinc makes it the ideal material for casting zinc. It is also used to construct valves for molten zinc.[11]

Molybdenum is used in mercury wetted reed relays, because molybdenum does not form amalgams and is therefore resistant to corrosion by liquid mercury.[12][13]

Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengthening

coefficient of friction are all properties which make it invaluable as an alloying compound. Its excellent anti-friction properties lead to its incorporation in greases and oils where reliability and performance are critical. Automotive constant-velocity joints use grease containing molybdenum. The compound sticks readily to metal and forms a very hard, friction-resistant coating. Most of the world's molybdenum ore can be found in China, the USA, Chile and Canada.[14][15][16][17]

Tungsten and its alloys

Tungsten was discovered in 1781 by the Swedish chemist, Carl Wilhelm Scheele. Tungsten has the highest melting point of all metals, at 3,410 °C (6,170 °F).

Filament
of a 200 watt incandescent lightbulb highly magnified

Up to 22% Rhenium is alloyed with tungsten to improve its high temperature strength and corrosion resistance. Thorium as an alloying compound is used when electric arcs have to be established. The ignition is easier and the arc burns more stably than without the addition of thorium. For powder metallurgy applications, binders have to be used for the sintering process. For the production of the tungsten heavy alloy, binder mixtures of nickel and iron or nickel and copper are widely used. The tungsten content of the alloy is normally above 90%. The diffusion of the binder elements into the tungsten grains is low even at the sintering temperatures and therefore the interior of the grains are pure tungsten.[18]

Tungsten and its alloys are often used in applications where high temperatures are present but still a high strength is necessary and the high density is not troublesome.

incandescent lighting, but are also common in industrial lighting as electrodes in arc lamps. Lamps get more efficient in the conversion of electric energy to light with higher temperatures and therefore a high melting point is essential for the application as filament in incandescent light.[20] Gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) equipment uses a permanent, non-melting electrode. The high melting point and the wear resistance against the electric arc makes tungsten a suitable material for the electrode.[21][22]

Tungsten's high density and strength are also key properties for its use in weapon

In this applications similar dense materials like the more expensive osmium can also be used.

The most common use for tungsten is as the compound tungsten carbide in drill bits, machining and cutting tools. The largest reserves of tungsten are in China, with deposits in Korea, Bolivia, Australia, and other countries.

It also finds itself serving as a lubricant, antioxidant, in nozzles and bushings, as a protective coating and in many other ways. Tungsten can be found in printing inks, x-ray screens, in the processing of petroleum products, and flame proofing of textiles.

Niobium alloys

Image of the Apollo Service Module with the moon in the background
Apollo CSM with the dark rocket nozzle made from niobium-titanium alloy

Niobium is nearly always found together with tantalum, and was named after Niobe, the daughter of the mythical Greek king Tantalus for whom tantalum was named. Niobium has many uses, some of which it shares with other refractory metals. It is unique in that it can be worked through annealing to achieve a wide range of strength and ductility, and is the least dense of the refractory metals. It can also be found in electrolytic capacitors and in the most practical superconducting alloys. Niobium can be found in aircraft gas turbines, vacuum tubes and nuclear reactors.

An alloy used for

Apollo Service Module. As niobium is oxidized at temperatures above 400 °C, a protective coating is necessary for these applications to prevent the alloy from becoming brittle.[27]

Tantalum and its alloys

Tantalum is one of the most corrosion-resistant substances available.

Many important uses have been found for tantalum owing to this property, particularly in the medical and surgical fields, and also in harsh acidic environments. It is also used to make superior electrolytic capacitors. Tantalum films provide the second most capacitance per volume of any substance after Aerogel,[citation needed] and allow miniaturization of electronic components and circuitry. Many cellular phones and computers contain tantalum capacitors.

Rhenium alloys

Rhenium is the most recently discovered refractory metal. It is found in low concentrations with many other metals, in the ores of other refractory metals,

oxidation. However its rarity makes it the most expensive of the refractory metals.[28]

Advantages and shortfalls

The strength and high-temperature stability of refractory metals make them suitable for hot metalworking applications and for vacuum furnace technology. Many special applications exploit these properties: for example, tungsten lamp filaments operate at temperatures up to 3073 K, and molybdenum furnace windings withstand 2273 K.

However, poor low-temperature fabricability and extreme oxidability at high temperatures are shortcomings of most refractory metals. Interactions with the environment can significantly influence their high-temperature creep strength. Application of these metals requires a protective atmosphere or coating.

The refractory metal alloys of molybdenum, niobium, tantalum, and tungsten have been applied to space nuclear power systems. These systems were designed to operate at temperatures from 1350 K to approximately 1900 K. An environment must not interact with the material in question. Liquid alkali metals as the heat transfer fluids are used as well as the ultra-high vacuum.

The high-temperature

strain
of alloys must be limited for them to be used. The creep strain should not exceed 1–2%. An additional complication in studying creep behavior of the refractory metals is interactions with environment, which can significantly influence the creep behavior.

See also

  • Refractory – heat resistance of nonmetallic materials

References

  1. ^ "International Journal of Refractory Metals and Hard Materials". Elsevier. Retrieved 7 February 2010.
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  4. ^ "Refractory Metal - an overview". ScienceDirect Topics. Elsevier. Retrieved 23 November 2020.
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  14. ^ Magyar, Michael J. "Commodity Summary 2009:Molybdenum" (PDF). United States Geological Survey. Retrieved 1 April 2010.
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  19. ^ National Research Council (U.S.), Panel on Tungsten, Committee on Technical Aspects of Critical and Strategic Material (1973). Trends in Usage of Tungsten: Report. National Academy of Sciences-National Academy of Engineering. pp. 1–3.
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  23. ^ Lanz, W.; Odermatt, W.; Weihrauch3, G. (7–11 May 2001). KINETIC ENERGY PROJECTILES: DEVELOPMENT HISTORY, STATE OF THE ART, TRENDS (PDF). 19th International Symposium of Ballistics. Interlaken, Switzerland.{{cite conference}}: CS1 maint: numeric names: authors list (link)
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  27. ^ a b Hebda, John (2 May 2001). "Niobium alloys and high Temperature Applications" (PDF). Niobium Science & Technology: Proceedings of the International Symposium Niobium 2001 (Orlando, Florida, USA). Companhia Brasileira de Metalurgia e Mineração. Archived from the original (PDF) on 17 December 2008.
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Further reading