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An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The result is a metallic substance with properties different from those of its components.

Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements, and brass is more durable than copper, but more attractive than zinc.

Unlike pure metals, many alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. Special alloys can be designed with a single melting point, however, and these are called eutectic mixtures.

Sometimes an alloy is just named for the base metal, as 14 karat gold is an alloy of gold with other elements. The same holds for silver used in jewellery, and aluminum used structurally.

Alloys include:

See also list of alloys.