Steel is an alloy of iron and carbon. © Wolfgang Kaeler/Corbis. Reproduced by permission.

An alloy is a mixture of two or more elements, at least one of which is metallic, that itself has metallic properties (ductility, conductivity, etc.). Compounds that involve metals but do not have metallic properties are not alloys. Alloying occurs naturally; most raw gold, for example, is alloyed with silver, and natural nickel-iron alloys occur both in terrestrial rocks and as a common ingredient of meteorites. However, all alloys used for modern technological purposes are created industrially. This is necessary both because most raw metals exist as chemical compounds in rocks and because the balance of ingredients in a useful alloy must be precise.

In a given alloy, one metal is usually present in higher concentration than any other element; this is termed the parent metal or solvent of the alloy. Most alloys are...