Alloys

by Chris Woodford. Last updated: February 4, 2022.

Almost every material we could ever want is lurking somewhere in the planet beneath our feet. From the gold we wear as jewelry to the oil that powers our cars, Earth's storehouse of amazing materials can supply virtually every need. Chemical elements are the basic building blocks from which all the materials inside Earth are made. There are 90 or so naturally occurring elements and the majority of them are metals. But, useful though metals are, they're sometimes less than perfect for the jobs we need them to do. Take iron, for example. It's amazingly strong, but it can be quite brittle and it also rusts easily in damp air. Or what about aluminum. It's very light but, in its pure form, it's too soft and weak to be of much use. That's why most of the "metals" we use are not actually metals at all but alloys: metals combined with other substances to make them stronger, harder, lighter, or better in some other way. Alloys are everywhere around us—from the fillings in our teeth and the alloy wheels on our cars to the space satellites whizzing over our heads. Let's take a closer look at what they are and why they're so useful!

Photo: High-strength superalloys like this one, made from "intermetallic" titanium aluminide (TiAl), are used in extreme applications (like aerospace engines) where "ordinary" aluminum and steel alloys are unsuitable. Photo by courtesy of NASA Glenn Research Center.

What is an alloy?

Photo: This sample of a titanium-zirconium-nickel alloy is being made to levitate (float in mid air) using electricity. It's one of many remarkable new materials being developed for possible use in space. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

You might see the word alloy described as a "mixture of metals", but that's a little bit misleading because some alloys contain only one metal and it's mixed in with other substances that are nonmetals (cast iron, for example, is an alloy made of just one metal, iron, mixed with one nonmetal, carbon).

The best way to think of an alloy is as a material that's made up of at least two different chemical elements, one of which is a metal. The most important metallic component of an alloy (often representing 90 percent or more of the material) is called the main metal, the parent metal, or the base metal. The other components of an alloy (which are called alloying agents) can be either metals or nonmetals and they're present in much smaller quantities (sometimes less than 1 percent of the total). Although an alloy can sometimes be a compound (the elements it's made from are chemically bonded together), it's usually a solid solution (atoms of the elements are simply intermixed, like salt mixed with water).

The structure of alloys

If you look at a metal through a powerful electron microscope, you can see the atoms inside arranged in a regular structure called a crystalline lattice. Imagine a small cardboard box full of marbles and that's pretty much what you'd see. In an alloy, apart from the atoms of the main metal, there are also atoms of the alloying agents dotted throughout the structure. (Imagine dropping a few plastic balls into the cardboard box so they arrange themselves randomly among the marbles.)

Artwork: Substitution alloys and interstitial alloys: In these diagrams, the black circles represent the main metal and the red circles are the alloying agents.

Substitution alloys

If the atoms of the alloying agent replace atoms of the main metal, we get what's called a substitution alloy. An alloy like this will form only if the atoms of the base metal and those of the alloying agent are of roughly similar size. In most substitution alloys, the constituent elements are quite near one another in the periodic table. Brass, for example, is a substitution alloy based on copper in which atoms of zinc replace 10–35 percent of the atoms that would normally be in copper. Brass works as an alloy because copper and zinc are close to one another in the periodic table and have atoms of roughly similar size.

Interstitial alloys

Alloys can also form if the alloying agent or agents have atoms that are very much smaller than those of the main metal. In that case, the agent atoms slip in between the main metal atoms (in the gaps or "interstices"), giving what's called an interstitial alloy. Steel is an example of an interstitial alloy in which a relatively small number of carbon atoms slip in the gaps between the huge atoms in a crystalline lattice of iron.

Photo: It's not just the basic ingredients (the metals and other constituents) that affect the properties of an alloy; how those ingredients combine is very important too. Pouring or stirring speeds, pouring temperatures, and cooling rates are some of the factors that can affect the physical properties of alloys. Photo of brass alloy casting by Jet Lowe, courtesy of US Library of Congress, Prints & Photographs Division, Historic American Engineering Record.

How do alloys behave?

People make and use alloys because metals don't have exactly the right properties for a particular job. Iron is a great building material but steel (an alloy made by adding small amounts of nonmetallic carbon to iron) is stronger, harder, and rustproof. Aluminum is a very light metal but it's also very soft in its pure form. Add small amounts of the metals magnesium, manganese, and copper and you make a superb aluminum alloy called duralumin, which is strong enough to make airplanes. Alloys always show improvements over the main metal in one or more of their important physical properties (things like strength, durability, ability to conduct electricity, ability to withstand heat, and so on). Generally, alloys are stronger and harder than their main metals, less malleable (harder to work) and less ductile (harder to pull into wires).

Chart: The same main metal can produce very different alloys when it's mixed with other elements. Here are four examples of copper alloys. Although copper is the main metal in all of them, each one has quite different properties.

How are alloys made?

Photo: Scientists at NASA Ames have developed a technique called high-pressure gas atomization for simplifying the production of magnesium alloys. Photo by courtesy of US Department of Energy.

You might find the idea of an alloy as a "mixture of metals" quite confusing. How can you mix together two lumps of solid metal? The traditional way of making alloys was to heat and melt the components to make liquids, mix them together, and then allow them to cool into what's called a solid solution (the solid equivalent of a solution like salt in water). An alternative way of making an alloy is to turn the components into powders, mix them together, and then fuse them with a combination of high pressure and high temperature. This technique is called powder metallurgy. A third method of making alloys is to fire beams of ions (atoms with too few or too many electrons) into the surface layer of a piece of metal. Ion implantation, as this is known, is a very precise way of making an alloy. It's probably best known as a way of making the semiconductors used in electronic circuits and computer chips. (Read more about this in our article on molecular beam epitaxy.)

Find out more

Photo: This fuel tank from the Space Shuttle was made from a super-light aluminum-lithium alloy, so it's a whopping 3400 kg (7500 lb) lighter than the tank it replaced. Cutting weight from the basic structure of the Shuttle meant it could carry heavier payloads (cargo). Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).

On this web site

• Iron and steel
• Materials science
• Metals (a simple introduction)
• Shape-memory alloys

Articles

• Unusual Alloy Brings Magnesium-ion Batteries Closer by Dexter Johnson. IEEE Spectrum, May 20, 2016. An alloy of tin and antimony holds promise for making viable magnesium-ion batteries that are safer than their lithium-ion rivals.
• The metal that brought you cheap flights by Laurence Knight. BBC News, May 16, 2015. A glimpse into the world of nickel aerospace alloys.
• Superalloys to the rescue: the marvellous metals that take us to the skies by Mark Miodownik. The Guardian, November 10, 2014. A materials science professor enthuses about nickel aersopace alloys.
• Bend Me, Shape Me: The Heavy-Metal Version by Anne Eisenberg. The New York Times. June 4, 2011. Caltech professor William L. Johnson develops a new method for shaping metal alloys like plastics.

Books

General introductions to materials science and engineering

These books explain the basic concept of matching materials to the jobs they need to do. That's the essential idea behind most alloys—essentially metals "enhanced" to do specific jobs better than they would in their pure, natural state.

• The New Science of Strong Materials (or Why You Don't Fall Through the Floor) by John E. Gordon. Penguin, 1991; Princeton University Press, 2006. Not about alloys at such, but a classic introduction to why different materials are good and bad at different jobs.
• Stuff Matters: Exploring the Marvellous Materials that Shape Our Man-Made World by Mark Miodownik. Houghton Mifflin Harcourt, 2014. An easy-to-follow, popular introduction to the scientific secrets of everyday materials.
• Engineering: A Beginner's Guide by Natasha McCarthy. OneWorld, 2009/2012. A short guide to what engineers do and how their work helps society.
• Invention by Design: How Engineers Get from Thought to Thing by Henry Petroski. Harvard University Press, 1998. Explains engineering as a cunning scientific way of solving everyday problems.

More detailed books

It's quite hard to find simple, general books about alloys; search instead for books on "engineering materials" and you should find something appropriate.

• The Superalloys: Fundamentals and Applications by Roger C. Reed. Cambridge University Press, 2008. An undergraduate student's introductory text.
• Materials Selection in Mechanical Design by Michael Ashby. Butterworth-Heinemann, 2016. Prof Ashby has written various college-level textbooks on materials science, including this one.
• Engineering Materials: Properties and Selection by Kenneth G. Budinski and Michael K. Budinski. Prentice Hall, 2010. A comprehensive introduction.
• Light Alloys: From Traditional Alloys to Nanocrystals by Ian Polmear. Butterworth-Heinemann, 2005. A good book about airspace-type alloys (aluminum, magnesium, titanium alloys).
• The Theory of Transformations in Metals and Alloys by John Christian. Pergamon, 1975/Elsevier, 2002. One of the all-time classics of modern materials science.

Organizations

• ASM: Materials Information Society: A society for materials engineers worldwide.
• The Minerals, Metals & Materials Society: US-based organization for minerals and materials professionals.