engineering

Refractory metals

Refractory metals is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Refractory metals rather than just read about it. In short: Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering.

Key takeaways

Refractory metals belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Refractory metals to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Refractory metals from memory before moving on to harder problems.

Reference excerpt

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definitions of which elements belong to this group differ. The most common definition includes five elements: two of the fifth period (niobium and molybdenum) and three of the sixth period (tantalum, tungsten, and rhenium). They all share some properties, including a melting point above 2000 °C and high hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make powder metallurgy the method of choice for 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 their high melting points, refractory metals are stable against creep deformation to very high temperatures.

Definition 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 includes iridium, osmium, niobium, molybdenum, tantalum, tungsten, rhenium, rhodium, ruthenium and hafnium. The five elements niobium, molybdenum, tantalum, tungsten and rhenium are included in all definitions, while the widest definition includes all elements with a melting point above 2,123 K (1,850 °C), such as titanium, vanadium, zirconium, and chromium. Technetium is not included because of its radioactivity, though it would otherwise have qualified under the widest definition.

Properties

Physical

Refractory metals have high melting points, with tungsten and rhenium the highest of all elements, and the others' melting points exceeded only by osmium and iridium, and the sublimation of carbon. These high melting points define most of their applications. All the metals are body-centered cubic except rhenium which is hexagonal close-packed. The physical properties of the refractory elements vary significantly because they are members of different groups of the periodic table. The hardness, high melting and boiling points, and high enthalpies of atomization of these metals arise from the partial occupation of the outer d subshell, allowing the d electrons to participate in metallic bonding. This gives stiff, highly stable bonds to neighboring atoms and a 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. 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 in jet engines, or tools used during forging.

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.

Applications Refractory metals, and alloys made from them, are used in lighting, tools, lubricants, nuclear reaction control rods, as catalysts, and for their chemical or electrical properties. Because of their high melting points, refractory metal components are never 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 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. Molybdenum is used in mercury-wetted reed relays, because molybdenum does not form amalgams and is therefore resistant to corrosion by liquid mercury. Molybdenum is the most commonly used of the refractory metals. Its most important use is as a strengthening alloy of steel. Structural tubing and piping often contains molybdenum, as do many stainless steels. 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 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.

Tungsten and its alloys

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

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Refractory metals

Start with the simplest possible case. Write down what Refractory metals claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Refractory metals before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Refractory metals ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Refractory metals

In research: Refractory metals appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Refractory metals in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Refractory metals is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metallurgy, Metals, Refractory materials, so understanding it makes those chapters shorter.
In everyday life: Look for Refractory metals outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Refractory metals in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Refractory metals means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Refractory metals out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Refractory metals in simple terms?

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering.

Why does Refractory metals matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Refractory metals?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Refractory metals.