Refractory Metals: What Are They?

Refractory metals are a class of metallic materials that offer superior heat resistance and very high melting points.

While the exact definition of which elements are included in this group varies, this article centers on the four most widely used refractory metallic elements: tantalum, niobium, molybdenum and tungsten. The class can be more broadly defined to include hafnium, chromium, rhenium and multiple other high-melting-point elements.¹

These elements, along with the multiple compounds and alloys produced from them, have many shared characteristics. In addition to high melting points (the elements included in this article all have a melting point greater than 2,200 °C), they have high corrosion resistance, high densities and all of them retain superior mechanical strength at high temperatures.^2,3

These features mean that refractory metals are used in many applications, such as glass-melting electrodes, furnace parts, sputtering targets, heat sinks and crucibles.³ This article closely examines the most commonly utilized refractory metals and their applications.

Tungsten

Renowned for its use in filament lightbulbs, tungsten is the most abundant refractory metal, along with being one of the most versatile elements.⁴

Elemental tungsten has the highest thermal conductivity of all refractory metals (173 W/(mK)), the highest melting point (3410 °C) of any metal and is one of the densest metals (19.3 g/cm³ near room temperature). Tungsten is often employed to create high-temperature components like crucibles, furnace boats and sintering trays due to these properties.

Tungsten Heavy Alloys (WHAs) can be produced by alloying tungsten with nickel, iron-nickel, or copper. The brittleness of tungsten is decreased by alloying it with these elements and materials are produced that can be economically fabricated by powder metallurgy and machined using traditional techniques.

Alloys like HPM and Kulite^® are included in this class of materials. The extreme density of these alloys makes them beneficial in applications like vibration dampeners, defense components and balance weights.

A high resistance to radiation is also provided by tungsten. This characteristic makes it highly desirable for applications like anti-scatter grids, radiation shielding and collimators.

Molybdenum

The most widely used refractory metal is molybdenum, providing excellent mechanical properties at high temperatures, low thermal expansion and high thermal conductivity.

These characteristics mean molybdenum is used in the creation of durable components for high heat applications, such as bearing parts, elevator brake pads, furnace parts and forging dies. Molybdenum is additionally utilized in heat sinks because of its high thermal conductivity (138 W/(m·K)).⁵

Along with its mechanical and thermal properties, the highest electrical conductivity of all refractory metals belongs to molybdenum (2×10⁷ S/m), which leads it to be used in the creation of glass melting electrodes.

Molybdenum is usually alloyed with different metals for applications where hot strength is needed, even though molybdenum has superior strength at high temperatures.⁶

TZM is a notable molybdenum-base alloy that contains 0.08% zirconium and 0.5% titanium. This alloy displays around twice the strength of unalloyed molybdenum at 1100 °C, along with having low thermal expansion and high thermal conductivity.

Tantalum

Tantalum is valued for its exceptional resistance to corrosion. It is resistant to hydrochloric, nitric and sulfuric acids, in addition to many liquid metals and organic chemicals. Due to this, tantalum is widely applied in chemical equipment like heat exchangers, reaction vessels and tantalum tubes.

The resistance of tantalum to chemical reactions also makes it advantageous for thin-film deposition, normally attained by sputtering. Sputtering is where atoms are ejected from the surface of a sputtering target to create highly uniform, functional coatings.

Commonly used in the production of semiconductors, electron beam melted (EBM) tantalum sputtering targets deliver inherently high purity with proven performance in a range of high vacuum environments.

Niobium

For a refractory metal, niobium delivers a high level of ductility. Niobium is highly workable even at low temperatures and is available in a variety of forms such as foil, plate and sheet.⁷

Niobium has a low density for a refractory metal, meaning that niobium alloys can be employed to create high-performance refractory components with a comparatively low weight. Due to this, niobium alloys like C-103 are often utilized in rocket engines for aerospace applications.⁸

C-103 provides superior high-temperature strength, allowing it to tolerate temperatures up to 1482 °C. It is also highly formable where TIG (Tungsten Inert Gas) processes can be used to weld it without significantly impacting machinability or ductility.⁹

It additionally has a low thermal neutron cross-section in comparison with different refractory metals, showing potential to be used in next-generation nuclear applications. ³

Refractory Metal Components H.C. Starck Solutions

H.C. Starck Solutions supplies high-performance refractory metal parts for a wide range of sectors. Alloyed and pure refractory metals are available as sheet, plate, wires, foil, tubes, coils, disks, rods, machined parts and sputtering targets.

Complex fabrications from refractory metals are also supplied by H.C. Starck Solutions through the use of advanced manufacturing technologies like 3D printing. Get in touch with H.C. Starck Solutions here to gain additional information on the refractory metal products and services available.

References and Further Reading

1. International Journal of Refractory Metals and Hard Materials.
2. Bauccio, M. ASM Metals Reference Book, 3rd Edition. (ASM International, 1993).
3. Snead, L. L., Hoelzer, D. T., Rieth, M. & Nemith, A. A. N. Refractory Alloys: Vanadium, Niobium, Molybdenum, Tungsten. in Structural Alloys for Nuclear Energy Applications 585–640 (Elsevier, 2019). doi:10.1016/B978-0-12-397046-6.00013-7.
4. Harvell, M. B. What are Refractory Metals. https://www.pickpm.com/wp-content/uploads/2016/08/What-Are-Refractory-Metals.pdf.
5. Shields, J. A. & Lipetzky, P. Molybdenum applications in the electronics market. JOM 52, 37–39 (2000).
6. Molybdenum - Element information, properties and uses | Periodic Table. https://www.rsc.org/periodic-table/element/42/molybdenum.
7. Nobium | Alloys | Properties | Applications. H.C. Starck Solutions https://www.hcstarcksolutions.com/niobium/.
8. Tanaka, R. et al. Research and Development of Niobium-Based Superalloys for Hot Components of Gas turbines. 5.
9. C-103 Nb Alloy: Properties & Applications. H.C. Starck Solutions (2020). https://www.hcstarck.com/
10. Technology Metals Product Information - H.C. Starck. https://www.hcstarck.com/

This information has been sourced, reviewed and adapted from materials provided by H.C. Starck Solutions.

For more information on this source, please visit H.C. Starck Solutions.