Alloy

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An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance usually has different properties (sometimes substantially different) from those of its components.

[edit] Properties

Alloys are usually prepared to improve on the properties of their components. For instance, steel is stronger than iron, its primary component. The physical properties of an alloy, such as density, reactivity and electrical and thermal conductivity may not differ greatly from the alloy's elements, but engineering properties, such as tensile strength, shear strength and Young's modulus, can be substantially different from those of the constituent materials. This is sometimes due to the differing sizes of the atoms in the alloy—larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the alloy resist deformation, unlike a pure metal where the atoms move more freely.

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; this is called the eutectic mixture.

[edit] Classification

Alloys can be classified by the number of their constituents. An alloy with two components is called a binary alloy; one with three is a ternary alloy, and so forth. Alloys can be further classified as either substitution alloys or interstitial alloys, depending on their method of formation. In substitution alloys, the atoms of the components are approximately the same size and the various atoms are simply substituted for one another in the crystal structure. An example of a (binary) substitution alloy is brass, made up of copper and zinc. Interstitial alloys occur when the atoms of one component are substantially smaller than the other and the smaller atoms fit into the spaces (interstices) between the larger atoms.

[edit] Terminology

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 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminium used as a structural building material are also alloys.

The term "alloy" is sometime used in everyday speech as a synonym for a particular alloy. For example, automobile wheels made of "aluminium alloy" are commonly referred to as simply "alloy wheels". The usage is obviously indefinite, since steels and most other metals in practical use are also alloys.