Refractory Metals: Niobium, Molybdenum and Rhenium

The refractory metals are conveniently described as those which, first of all, melt at temperatures well above the melting points of the common alloying bases, iron, cobalt, and nickel. Second, it seems appropriate to consider the refractory metals as those which have higher melting points than do titanium and zirconium, which are used chiefly at intermediate temperatures. On the other hand, chromium is usually classed as a refractory metal.

The refractory metals include niobium (also known as columbium), tantalum, molybdenum, tungsten, and rhenium. With the exception of two of the platinum-group metals, osmium and iridium, they have the highest melting temperatures and lowest vapor pressures of all metals. The refractory metals are readily degraded by oxidizing environments at moderately low temperatures, a property that restricted the applicability of the metals in low-temperature or nonoxidizing high-temperature environments. Protective coating systems have been developed, mostly for niobium alloys, to permit their use in high-temperature oxidizing aerospace applications.

Refractory metals at one time were limited to use in tamp filaments, electron tube grids, heating elements, and electrical contacts; however, they have since found widespread application in the aerospace, electronics, nuclear and high-energy physics, and chemical process industries.

The refractory metals are extracted from ore concentrates, processed into intermediate chemicals and then reduced to metal. The refractory metals, except for niobium, are produced exclusively as metal powders, which are consolidated by sintering and/or melting. They can be formed, machined, and joined by conventional methods. They are ductile in the pure state and have high interstitial solubilities for carbon, nitrogen, oxygen and hydrogen. Because of the high solubilities in niobium and tantalum, these embrittling contaminants normally do not present problems in fabrication. The process for niobium differs only in that the metal is most commonly reduced by aluminothermic reduction of oxide.

Niobium

Niobium and its alloys exhibit properties that provide technological capabilities unique among refractory metals, although it is the least refractory of these metals. Its cryogenic ductility and ease of fabrication are excellent, being superior to those of molybdenum and tungsten but not as good as those of tantalum. Niobium oxidizes noncatastrophically; it is superior to molybdenum and tungsten and about equal to tantalum in this characteristic. It is in abundant supply; the estimated free-world reserves of niobium are much greater than those of tantalum and tungsten and probably are even superior to those of molybdenum.

Niobium is a ductile and soft metal at elevated temperatures. Strength can be improved by alloying to make it competitive with molybdenum (and molybdenum alloys), its closest rival for use at temperatures through at least 2500°F. The advantages of niobium alloys may dictate their ultimate use in preference to other refractory metals to temperatures as high as 3300°F. Lack of oxidation resistance is a major barrier to the use of niobium alloys in structural applications at elevated temperatures.

Niobium Alloys. Commercial application of niobium is dominated by its use as an alloying element in steels. Almost 75% of all niobium metal is used as minor alloying additions in low-alloy steel. Another 20 to 25% is used as alloy additions in nickel-base superalloys and heat-resisting steels. Only 1 to 2% of all niobium is used in the form of niobium-base alloys and pure niobium metal including superconducting niobium-titanium alloy, which accounts for over one-half of all niobium alloys produced. Originally, niobium metal was produced by powder metallurgy methods which involved high temperature vacuum sintering and carbon reduction.

Powder Production. Powders are produced from ingot by hydriding, crushing, and dechydriding; in addition, some recent efforts have been directed toward producing complex metals-table alloy powders, such as niobium-aluminum and niobium-silicon alloys, by liquid metal atomization and rapid quenching. The particle structure of degassed hydride niobium powder is completely analogous to that of a tantalum powder produced in a similar process for capacitors.

Molybdenum

More molybdenum is consumed annually than any other refractory metal. Molybdenum ingots, produced by melting of P/M electrodes, are extruded, rolled into sheet and rod, and subsequently drawn to other mill product shapes, such as wire and tubing. These materials can then be stamped into simple shapes. Molybdenum is also machined with ordinary tools and can be gas tungsten arc and electron beam welded, or brazed.

Molybdenum has outstanding electrical and heat-conducting capabilities and relatively high tensile strength. Thermal conductivity is approximately 50% higher than that of steel, iron or nickel alloys. It consequently finds wide usage as heatsinks. Its electrical conductivity is the highest of all refractory metals, about one third that of copper, but higher than nickel, platinum, or mercury. The coefficient of thermal expansion of molybdenum plots almost linearly with temperature over a wide range. This characteristic, in combination will raise heat-conducting capabilities, accounts for its use in bimetal thermocouples. Methods of doping molybdenum powder with potassium aluminosilicate to obtain a non-sag microstructure comparable to that of tungsten also have been developed.

The major use for molybdenum is as an alloying agent for alloy and tool steels, stainless steels, and nickel-base or cobalt-base super-alloys to increase hot strength, toughness and corrosion resistance.

In the electrical and electronic industries, molybdenum is used in cathodes, cathode supports for radar devices, current leads for thorium cathodes, magnetron end hats, and mandrels for winding tungsten filaments.

Molybdenum is important in the missile industry, where it is used for high-temperature structural parts, such as nozzles, leading edges of control surfaces, support vanes, struts, reentry cones, heal-radiation shields, heat sinks, turbine wheels, and pumps.

Molybdenum has also been useful in the nuclear, chemical, glass, and metallizing industries. Service temperatures, for molybdenum alloys in structural applications arc, is limited to a maximum of about 1650°C (3000°F). Pure molybdenum has good resistance to hydrochloric acid and is used for acid service in chemical process industries.

Molybdenum Alloy TZM. The molybdenum alloy of greatest technological importance is the high-strength, high-temperature alloy TZM. The material is manufactured either by P/M or arc-cast processes.

TZM has a higher recrystallization temperature and higher strength and hardness at room and at elevated temperatures than unalloyed molybdenum. It also exhibits adequate ductility. Its superior mechanical properties arc due to the dispersion of complex carbides in the molybdenum matrix. TZM is well suited to hot work applications because of its combination of high hot hardness, high thermal conductivity, and low thermal expansion to hot work steels.

Major uses include:

In an attempt to improve the high-temperature strength of P/M TZM alloys, alloys have been developed in which titanium and zirconium carbide is replaced by hafnium carbide. Alloys of molybdenum and rhenium are more ductile than pure molybdenum. An alloy with 35% Re can be rolled at room temperature to more than 95% reduction in thickness before cracking. For economic reasons, molybdenum-rhenium alloys are not widely used commercially. Alloys of molybdenum with 5 and 41% Re are used for thermocouple wires.

Rhenium

Among the elements, rhenium has the highest melting point, except for tungsten and carbon. Its density is exceeded only by osmium, iridium, and platinum. A ductile-to-brittle transition temperature does not exist in pure rhenium. Rhenium is the only refractory metal that does not form carbides.

Rhenium has a high electrical resistivity over a wide temperature range. This characteristic, combined with a low vapor pressure, makes it ideally suited for filament applications; additionally, it maintains ductility and is not affected by the oxidation/reduction cycle experienced in these applications, as is tungsten. One of the largest applications for rhenium is for mass spectrometer filaments, where it is available in commercial (99.99%) and zone-refined (99.995%) purities.

Rhenium is not attacked by molten copper, silver, tin, or zinc. It dissolves readily in molten iron and nickel, but is stable in the presence of aluminum. Rhenium has a significant hardening effect on platinum. At elevated temperature, rhenium resists attack in hydrogen and inert atmospheres. It is resistant to hydrochloric acid and seawater corrosion and to the mechanical effects of electrical erosion.

Rhenium Alloys. Rhenium is a beneficial alloying addition with other refractory metals. Rhenium greatly enhances the ductility and tensile strength of refractory metals and their alloys Rhenium alloys arc used in nuclear reactors, semiconductors, electronic tube components, thermocouples, gyroscopes, miniature rockets, electrical contacts, thermionic converters, and other commercial and aerospace applications. Tungsten-rhenium alloys, applied by vapor deposition, are used to coat the surface of molybdenum targets in x-ray tube manufacture. Other rhenium alloys (with tungsten or molybdenum) are used for filaments, grid heaters, cathode cups, and igniter wires in photoflash bulbs.