An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic bonding alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties.
The term "intermetallic compounds" applied to solid phases has long been in use. However, Hume-Rothery argued that it misleads, suggesting a fixed stoichiometry and a clear decomposition into species.
Definitions
Research definition
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.
[G. E. R. Schulze: Metallphysik, Akademie-Verlag, Berlin 1967] This definition includes:
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Electron (or Hume-Rothery) compounds
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Size packing phases. e.g., , Frank–Kasper phases and
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The definition of metal includes:
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Post-transition metals, i.e. aluminium, gallium, indium, thallium, tin, lead, and bismuth.
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, e.g., silicon, germanium, arsenic, antimony and tellurium.
Homogeneous and heterogeneous of metals, and interstitial compounds such as and are excluded under this definition. However, interstitial intermetallic compounds are included, as are alloys of intermetallic compounds with a metal.
Common use
In common use, the research definition, including post-transition metals and
, is extended to include compounds such as
cementite, Fe
3C. These compounds, sometimes termed interstitial compounds, can be
stoichiometric, and share properties with the above intermetallic compounds.
Complexes
The term intermetallic is used
to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp
6Ni
2Zn
4.
B2
A B2 (also known as cesium chloride structure type) intermetallic compound has equal numbers of atoms of two metals, such as aluminium-iron, and
Nickel aluminide, arranged as two interpenetrating simple cubic lattices of the component metals.
Properties
Intermetallic compounds are generally brittle at room temperature and have high
melting point, though many also exhibit metallic conductivity or semiconducting behavior depending on the degree of covalent bonding. Cleavage or intergranular fracture modes are typical of intermetallics due to limited independent slip systems required for plastic deformation. However, some intermetallics have ductile fracture modes such as Nb–15Al–40Ti. Others can exhibit improved
ductility by alloying with other elements to increase grain boundary cohesion. Alloying of other materials such as
boron to improve grain boundary cohesion can improve ductility.
They may 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 display desirable
magnetism and chemical properties, due to their strong internal order and mixed (
metallic bond and
covalent bond/
ionic bond) bonding, respectively. Intermetallics have given rise to various novel materials developments.
+Physical properties of intermetallics |
| FeAl | 1250–1400 | 5600 | 263 |
| Ti3Al | 1600 | 4200 | 210 |
| MoSi2 | 2020 | 6310 | 430 |
Applications
Examples include
alnico and the
hydrogen storage materials in nickel metal hydride batteries.
Nickel aluminide, which is the hardening phase in the familiar nickel-base
superalloy, and the various
titanium aluminides have attracted interest for
turbine blade applications, while the latter is also used in small quantities for
grain refinement of
.
, intermetallics involving silicon, serve as barrier and contact layers in
microelectronics.
Others include:
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Magnetic materials e.g., alnico, sendust, Permendur, FeCo, Terfenol-D
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Superconductors e.g., A15 phases, niobium-tin
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Hydrogen storage e.g., AB5 compounds (nickel metal hydride batteries)
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Shape memory alloys e.g., Cu-Al-Ni (alloys of Cu3Al and nickel), Nitinol (NiTi)
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Coating materials e.g., NiAl
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High-temperature structural materials e.g., nickel aluminide, Ni3Al
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, which are alloys of intermetallics Ag3Sn and Cu3Sn
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Gate contact/ barrier layer for microelectronics e.g., TiSi2
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(AB2), e.g., MgCu2, MgZn2 and MgNi2.
The unintended formation of intermetallics can cause problems. For example, intermetallics of gold and aluminium can be a significant cause of wire bonding failures in semiconductor devices and other microelectronics devices. The management of intermetallics is a major issue in the reliability of solder joints between electronic components.
Intermetallic particles
Intermetallic particles often form during solidification of metallic alloys, and can be used as a dispersion strengthening mechanism.
History
Examples of intermetallics through history include:
German type metal is described as breaking like glass, without bending, softer than copper, but more fusible than lead.
See also
Sources
External links
