Refractory metals are a class ofmetals that are extraordinarily resistant toheat andwear. The expression is mostly used in the context ofmaterials science,metallurgy andengineering. The definitions of which elements belong to this group differ. The most common definition includes five elements: two of thefifth period (niobium andmolybdenum) and three of thesixth period (tantalum,tungsten, andrhenium). They all share some properties, including a melting point above 2000 °C and highhardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points makepowder metallurgy the method of choice forfabricating 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 their high melting points, refractory metals are stable againstcreep deformation to very high temperatures.

Most definitions of the term "refractory metals" list an 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, which includesiridium,osmium,niobium,molybdenum,tantalum,tungsten,rhenium,rhodium,ruthenium andhafnium.^[2] The five elementsniobium,molybdenum,tantalum,tungsten andrhenium are included in all definitions,^[3] while the widest definition includes all elements with a melting point above 2,123 K (1,850 °C), such astitanium,vanadium,zirconium, andchromium.^[4]Technetium is not included because of its radioactivity, though it would otherwise have qualified under the widest definition.^[5]

Refractory metals have high melting points, with tungsten and rhenium the highest of all elements, and the others' melting points exceeded only byosmium andiridium, and the sublimation ofcarbon. These high melting points define most of their applications. All the metals arebody-centered cubic except rhenium which ishexagonal close-packed. The physical properties of the refractory elements vary significantly because they are members of differentgroups of the periodic table.^[6][7] The hardness, high melting and boiling points, and highenthalpies of atomization of these metals arise from the partial occupation of the outerd subshell, allowing the d electrons to participate in metallic bonding. This gives stiff, highly stable bonds to neighboring atoms and abody-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 inertcore, 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]

Creep resistance is a key property of the refractory metals. In metals, the starting of creep correlates with the melting point of the material; the creep in aluminium alloys starts at 200 °C, while for refractory metals temperatures above 1500 °C are necessary. This resistance against deformation at high temperatures makes the refractory metals suitable against strong forces at high temperature, for example injet engines, or tools used duringforging.^[9][10]

The refractory metals show a wide variety of chemical properties because they are members of three distinct groups in theperiodic 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]

Refractory metals, and alloys made from them, are used inlighting, tools,lubricants,nuclear reactioncontrol rods, ascatalysts, and for theirchemical or electrical properties. Because of their highmelting points, refractory metal components are never fabricated bycasting. The process ofpowder 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 intowire,ingots,rebars,sheets, orfoil.

Molybdenum-based alloys are widely used, because they are cheaper than superior tungsten alloys. The most widely used alloy of molybdenum is theTitanium-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 moltenzinc makes it the ideal material for casting zinc. It is also used to construct valves for molten zinc.^[11]

Molybdenum is used inmercury-wetted reed relays, because molybdenum does not formamalgams and is therefore resistant to corrosion by liquidmercury.^[12][13]

Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengtheningalloy ofsteel.Structural tubing andpiping often contains molybdenum, as do manystainless steels. Its strength at high temperatures, resistance to wear, and lowcoefficient of friction are all properties which make it invaluable as an alloying compound. Its excellent anti-friction properties lead to its incorporation ingreases andoils where reliability and performance are critical. Automotiveconstant-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 molybdenumore can be found in China, theUSA, Chile, andCanada.^{[14][15][16][17]}

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

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: 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 tungsten heavy alloys, binder mixtures ofnickel andiron or nickel andcopper 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 thesintering 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 a high strength is still necessary and the high density is not troublesome.^[19] Tungsten-wire filaments provide the vast majority of householdincandescent 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, so 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-meltingelectrode. 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 weaponprojectiles, for example as an alternative todepleted uranium for tank gun rounds.^[23] Its high melting point makes tungsten a good material for applications likerocket nozzles, for example in theUGM-27 Polaris.^[24] Some of the applications of tungsten are not related to its refractory properties but simply to its density. For example, it is used in balance weights for planes and helicopters or for heads ofgolf clubs.^[25][26] In these applications, similar dense materials like the more expensiveosmium can also be used.

The most common use for tungsten is as the compoundtungsten carbide indrill bits, machining and cutting tools. The largest reserves of tungsten are inChina, with deposits inKorea,Bolivia,Australia, and other countries.

It also finds itself serving as alubricant,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 ofpetroleum products, and flame proofing oftextiles.

Niobium is nearly always found together with tantalum, and was named afterNiobe, the daughter of themythicalGreek kingTantalus 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 andductility, and is the least dense of the refractory metals. It can also be found inelectrolytic capacitors and in the most practicalsuperconducting alloys. Niobium can be found inaircraftgas turbines,vacuum tubes, andnuclear reactors.

An alloy used forliquid rocket thruster nozzles, such as in the main engine of theApollo Lunar Modules, is C103, which consists of 89% niobium, 10% hafnium and 1% titanium.^[27] Anotherniobium alloy was used for the nozzle of theApollo 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 is one of the mostcorrosion-resistant substances available. Many important uses have been found for tantalum owing to this property, particularly in themedical andsurgical fields, and also in harshacidic environments. It is also used to make superior electrolytic capacitors. Tantalum films provide the second mostcapacitance per volume of any substance afterAerogel,^{[citation needed]} and allowminiaturization ofelectronic components andcircuitry. Manycellular phones andcomputers contain tantalum capacitors.

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,platinum orcopper ores. It is useful as an alloy to other refractory metals, where it addsductility andtensile strength. Rhenium alloys are used in electronic components,gyroscopes andnuclear reactors. Rhenium finds its most important use as a catalyst. It is used as a catalyst in reactions such asalkylation,dealkylation,hydrogenation andoxidation. However, its rarity makes it the most expensive of the refractory metals.^[28]

The strength and high-temperature stability of refractory metals make them suitable for hotmetalworking applications and forvacuum 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 extremeoxidability 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. Liquidalkali metals as the heat-transfer fluids are used as well as theultra-high vacuum.

The high-temperaturecreepstrain 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.

• Refractory – heat resistance of nonmetallic materials

• Refractory metals
• Metals
• Metallurgy
• Refractory materials

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