Intermetallic compounds
An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a solid-state compound exhibiting metallic bonding, defined stoichiometry and ordered crystal structure. Many intermetallic compounds are often simply called alloys.
Although the term "intermetallic compounds", as it applies to solid phases, has been in use for many years, its introduction was regretted, for example by Hume-Rothery in 1955.
Schulze in 1967, defined intermetallic compounds as solid phases containing two or more metallic elements, with optionally one or more non-metallic elements, whose crystal structure differs from that of the other constituents. Under this definition the following are included
The definition of a metal is taken to include:
Homogeneous and heterogeneous solid solutions of metals, and interstitial compounds such as the carbides and nitrides are excluded under this definition. However, interstitial intermetallic compounds are included as are alloys of intermetallic compounds with a metal.
In common use, the research definition, including post-transition metals and metalloids, is extended to include compounds such as cementite, Fe3C. These compounds, sometimes termed interstitial compounds can be stoichiometric, and share similar properties to the intermetallic compounds defined above.
The term intermetallic is used to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp6Ni2Zn4.
A B2 intermetallic compound has equal numbers of atoms of two metals such as aluminum and iron.
Intermetallic compounds are generally brittle and have a high melting point. They often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing. They can also display desirable magnetic, superconducting and chemical properties, due to their strong internal order and mixed (metallic and covalent/ionic) bonding, respectively. Intermetallics have given rise to various novel materials developments. Some examples include alnico and the hydrogen storage materials in nickel metal hydride batteries. Ni3Al, which is the hardening phase in the familiar nickel-base superalloys, and the various titanium aluminides have also attracted interest for turbine blade applications, while the latter is also used in very small quantities for grain refinement of titanium alloys. Silicides, intermetallics involving silicon, are utilized as barrier and contact layers in microelectronics.
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