Steel is a metal alloy whose major component is
iron, with
carbon content between 0.02% and 2.14% by mass.
An
alloy is a partial or complete
solid solution of one or more
elements in a
metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be
homogeneous in distribution depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component elements.
Theory
Alloying one metal with other metal(s) or non metal(s) often enhances its properties.
For example,
steel is stronger than
iron, its primary element. The physical properties, such as
density,
reactivity,
Young's modulus, and
electrical and
thermal conductivity, of an alloy may not differ greatly from those of its elements, but engineering properties, such as
tensile strength and
shear strength may be substantially different from those of the constituent materials. This is sometimes due to the sizes of the
atoms in the alloy, since larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element occur. For example, impurities in semi-conducting
ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.
Some alloys are made by melting and mixing two or more metals.
Brass is an alloy made from
copper and
zinc.
Bronze, used for
bearings,
statues,
ornaments and
church bells, is an alloy of copper and
tin.
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 the temperature when melting is complete is called the
liquidus. However, for most alloys there is a particular proportion of constituents (in rare cases two) which has a single melting point. This is called the alloy's
eutectic mixture.
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 sometimes 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.
History
right|thumb| Bronze axe 1100 BCEThe use of alloys by humans started with the use of
meteoric iron, a naturally occurring alloy of nickel and iron. As no metallurgic processes were used to separate iron from nickel the alloy was used as it was.
See also
