An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. 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 resulting metallic substance generally has properties significantly 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. It 'inherits' some of the characteristics of the elements it was made from, usually physical properties like density, reactivity and electrical and thermal conductivity. However, its engineering properties (Tensile strength, Young's modulus, shear strength) can be vastly different from its constituent materials. Among other factors, this is due to the differing sizes of the atoms in the alloy - larger atoms exert a compressive force on neighbouring atoms, and smaller atoms will exert a tensile force on their neighbours. Unlike a pure metal, where the atoms are more free to move, this helps the alloy resist deformation.
Unlike pure metals, most 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. However, for most pairs of elements, there is a particular ratio which has a single melting point, and this is called a eutectic mixture.
In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 carat (58%) gold is an alloy of gold with other elements. Similarly, the silver used in jewellery and the aluminiumused as a structural building material are also alloys.

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