Alloys
Introduction
An alloy is a combination of metals or of a metal and another element. Alloys are defined by a metallic bonding character. An alloy may be a solid solution of metal elements (a single phase) or a mixture of metallic phases (two or more solutions). Intermetallic compounds are alloys with a defined stoichiometry and crystal structure. Zintl phases are also sometimes considered alloys depending on bond types.
History
The use of alloys by humans started with the use of meteoric iron, a naturally occurring alloy of nickel and iron. It is the main constituent of iron meteorites which occasionally fall down on Earth from outer space. As no metallurgic processes were used to separate iron from nickel in the meteorite, it is considered the first use of an alloy. Around 3000 BC, people in the ancient Near East began adding tin to copper to make bronze, giving rise to the Bronze Age.
Types of Alloys
Alloys can be classified in several ways. One way is based on the extent to which the elements forming the alloy mix. Alloys can be homogeneous (consisting of a single phase), or heterogeneous (consisting of two or more phases). They can also be classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy.
Substitutional Alloys
In a substitutional alloy, the atoms of the components are approximately the same size, and the atoms of one element are substituted for those of another in the crystal lattice. An example of a substitutional alloy is brass, made from copper and zinc.
Interstitial Alloys
In an interstitial alloy, the atoms of one element fill in the interstices (holes) in the crystal lattice of another element. The atoms of the two elements are different sizes, with the atoms of the smaller element fitting into the spaces between the atoms of the larger element. An example of an interstitial alloy is steel, made from iron and carbon.
Properties of Alloys
Alloys are designed to have properties that are superior to those of their component elements. For example, steel is stronger than iron, its primary element. Alloys may have enhanced corrosion resistance, higher electrical or thermal conductivity, better malleability, or other desirable characteristics.
Applications of Alloys
Alloys are used in a wide variety of applications. For example, in the aircraft industry, alloys of aluminium, titanium, and nickel are widely used. In the construction industry, steel is used for its strength and corrosion resistance. In the electronics industry, alloys of gold and aluminium are commonly used in connector applications.
