Published by Australian Academy of Science
Making light of metals Box 2 |  Changing metals: Alloys

Many of the metals we use in everyday life are not pure. They are a solid mixture of two or more metals (or metals with non-metals), what is generally termed an alloy. Steel is an alloy of iron, carbon and often various other elements. The brass used in trumpets and other wind instruments is an alloy that combines copper with zinc. Anything less than 24 carat gold has copper or silver mixed with the gold.

We make alloys because the pure metal does not have the properties we need; the mixture does. Iron is too brittle for many purposes; adding precisely controlled amounts of carbon turns it into steel which has a better combination of strength and toughness. Alloying copper with zinc (to make brass) makes it harder and more durable. Pure gold is very soft; adding silver or copper to gold changes its colour and hardness.

Alloys are a major line of research in the light metals business. Knowing which metals to mix and in what proportions is a complex process. For highly demanding situations, such as the blades of a jet engine, the mix might contain ten different elements, carefully chosen to deal with challenges like heat, wear and mechanical stress. Alloying can change the properties of a metal in more than one way. Mixing together atoms of different sizes can make it harder for the metal atoms to slide past each other, so the metal becomes stronger and less malleable. Also by adding atoms with extra valence electrons, the bonds between the metal atoms and electrons become stronger (Box 1: The magic of metals).

There are other ways to change the properties of metals. Heat treatments like quenching and tempering have been used by blacksmiths for millennia to harden and toughen iron to make swords or ploughs. Nowadays we have other ways to alter surface properties, say by adding very thin layers, implanting ions or heating with lasers. Getting the technology right depends very much on understanding the science.

Related sites

• Metals and alloys (British Broadcasting Corporation)
• Metals and alloys (San Jose State University, USA)
• Heat treatment (eFunda, USA)
• Heat treating: Process description (Metals Advisor, USA)

Other box

Box 1. The magic of metals