Refractory metals
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'''Refractory metals''' are a class of [[metal]]s extraordinarily resistant to [[heat]] and [[wear]]. Refractory metals are said to be poorly resistant to oxidation and corrosion<ref>UFL EMA 6625: Advanced Metal Processing, Summer 2008, Lecture 09</ref> These properties make them useful in many applications. Household [[Incandescent light bulb|incandescent bulbs]] contain [[refractory]] metals in their tungsten [[Electrical filament|filaments]], and nearly all manufactured goods, particularly those containing metal or electronics, contain or were produced using refractory metals.
The five refractory metals are:
* [[Tungsten]] (W)
* [[Molybdenum]] (Mo)
* [[Niobium]] (Nb)
* [[Tantalum]] (Ta)
* [[Rhenium]] (Re)
[[Calcium]] (Ca) and [[Magnesium]] (Mg) are considered refractory metals, too (as shown from the results of [[SEM-EDX]] analyses conducted on particles from biomass combustion).
Refractory metals are used in [[lighting]], tools, [[lubricant]]s, [[nuclear reaction]] [[control rods]], as [[catalyst]]s, and for their [[chemical]] or electrical properties. Because of their high [[melting point]], refractory metal components are never [[fabrication (metal)|fabricated]] by [[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 can be worked into [[wire]], [[ingots]], [[Pole|bars]], [[Sheet metal|sheets]] or [[foil (chemistry)|foil]].
Tungsten was discovered in [[1781]] by the [[Sweden|Swedish]] chemist, [[Karl Wilhelm Scheele]]. Tungsten is both the most abundant of the refractory metals, and has the highest [[melting point]] of all metals, at 3,410C(6,170F). Tungsten wire filaments provide the vast majority of household incandescent lighting, but are also common in industrial lighting as electrodes in arc lamps. TIG-welding (Tungsten Inert Gas welding) or GTAW-welding (Gas Tungsten Arc welding) equipment uses a permanent, non-melting tungsten [[electrode]]. The most common use for tungsten is as the compound [[tungsten carbide]] in [[drill bits]], machining and cutting tools. It also finds itself serving as a [[lubrication|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, [[photographic chemicals]]{{Dubious|date=April 2008}}, in the processing of [[petroleum]] products, and flame proofing of [[textiles]]. Tungsten is also used by virtue of its strength and [[density]], in applications ranging from weights in [[helicopter]] rotors and weapon [[projectiles]] to the heads of [[Golf club (equipment)|golf clubs]]. The largest reserves of tungsten are in [[China]], with deposits in [[Korea]], [[Bolivia]], [[Australia]], and other countries.
Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengthening [[alloy]] of [[steel]]. [[Hollow structural section|Structural tubing]] and [[piping]] often contains molybdenum, as do many [[stainless steel]]s. Its strength at high temperatures, resistance to [[wear]] and low [[coefficient of friction]] are all properties which make it invaluable as an alloying compound. Its excellent anti-[[friction]] properties lead to its incorporation in [[Grease (lubricant)|greases]] and [[oils]] where reliability and performance are critical. Automotive [[constant-velocity joint]]s 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 the [[USA]] and [[Canada]].
Niobium is nearly always found together with tantalum, and was named after [[Niobe]], the daughter of the [[Greek mythology|mythical]] [[Greece|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 [[Elasticity (physics)|elasticity]], and is the least [[density|dense]] of the refractory metals. It can also be found in [[electrolytic capacitor]]s and in the most practical [[superconducting]] alloys. Niobium can be found in [[aircraft]] [[gas turbine]]s, [[vacuum tube]]s and [[nuclear reactor]]s.
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 [[medicine|medical]] and [[surgery|surgical]] fields, and also in harsh [[acid]]ic environments. It is also used to make superior electrolytic capacitors. Tantalum films provide the most [[capacitance]] per volume of any substance, and allow [[miniaturization]] of [[electronic components]] and [[electrical network|circuitry]]. [[Cellular phone]]s and [[computer]]s contain tantalum capacitors.
Rhenium is the most recently discovered refractory metal. It is found in low concentrations with many other metals, in the [[ore]]s of other refractory metals, [[platinum]] or [[copper]] ores. It is useful as an alloy to other refractory metals, where it adds [[ductility]] and [[tensile strength]]. Rhenium alloys are being found in electronic components, [[gyroscope]]s and nuclear reactors. Rhenium finds its most important use as a [[catalyst]]. It is used as a catalyst in reactions such as [[alkylation]], [[dealkylation]], [[hydrogenation]] and [[oxidation]]. However its rarity makes it the most expensive of the refractory metals.
==The creep behavior of refractory metals==
Refractory metals and alloys attract the attention of investigators because of their remarkable properties and on account of promising practical prospects.
Refractory metals are characterized by their extremely high melting points, which range well above those of [[iron]] and [[nickel]].
When the refractory metals are considered to be those metals melting at temperatures above 2123 K, twelve metals constitute
this group: tungsten (the melting point 3683 K), [[rhenium]], [[osmium]], [[tantalum]], [[molybdenum]], [[iridium]], [[niobium]], [[ruthenium]], [[hafnium]], [[zirconium]], [[vanadium]], and [[chromium]].
Physical properties of refractory metals, such as molybdenum, tantalum and tungsten, their strength, and high-temperature
stability make them suitable material 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 to 2273 K.
However, a poor low-temperature [[fabrication (metal)|fabricability]] and an extreme [[oxidation|oxidability]] at high-temperatures are shortcomings of the most refractory metals. Interactions with environment can significantly influence on their high-temperature [[Creep (deformation)|creep]] strength. Application of these metals requires a protective atmosphere or a coating.
The refractory metal alloys of molybdenum, niobium, tantalum, and tungsten have been applied for the 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 ultrahigh vacuum.
The high-temperature creep [[Strain (materials science)|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.
==References==
{{cite book | author=Levitin, Valim | title=High Temperature Strain of Metals and Alloys: Physical Fundamentals | publisher=WILEY-VCH | year=2006 | editor= | id=ISBN 978-3-527-31338-9}}
{{cite article | author=Brunner, T. | title=Chemical and structural analyses of aerosol and fly-ash particles from fixed-bed biomass combustion plants by elecron microscopy | year=2000 | editor=James&James Ltd., London | id=ISBN 1-902916-15-8}}
== See also ==
* [[Refractory ceramic]]
[[Category:Metals]]
[[Category:Refractory materials]]
[[de:Refraktärmetalle]]