or intermetallic compounds
is a term that is used in a number of different ways. Most commonly it refers to solid state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term. There is also a completely different use in coordination chemistry, where it has been used to refer to complexes containing two or more different metals.
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.
Note that many intermetallic compounds are often simply called alloys, even though strictly speaking they are not. For example, complex metallic alloys are intermetallic compounds with large unit cells.
This was stated by Schulze in 1967, and defines 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
- Electron (or Hume-Rothery) compounds
- Size packing phases. e.g. Laves phases, Frank-Kaspar phases and Nowotny phases
- Zintl phases
The definition of a metal is taken to include:
- the so-called poor metals, i.e. Aluminium, gallium, indium, thallium, tin and lead
- some, if not all, of the metalloids, e.g. silicon, germanium, arsenic antimony and tellurium.
Alloys, which are a homogeneous mixture 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 poor metals
, is extended to include compounds such as cementite
C. 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 Cp6
Intermetallics involving two or more metallic elements
Intermetallic compounds are generally brittle and high melting. 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
) bonding, respectively. Intermetallics have given rise to various novel materials developments. Some examples include alnico
and the hydrogen
storage materials in nickel metal hydride
, 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
Properties and examples
- magnetic materials e.g. alnico; sendust; Permendur, FeCo
- superconductors e.g. A15 phases; Niobium-tin
- hydrogen storage e.g. AB5 compounds (Nickel metal hydride batteries)
- shape memory alloys e.g. Cu-Al-Ni (alloys of Cu3Al and nickel
- coating materials e.g. Nitinol, NiTi; NiAl
- high temperature structural materials e.g. nickel aluminide, Ni3Al
- dental amalgams which are alloys of intermetallics Ag3Sn and Cu3Sn
The formation of intermetallics can cause problems, Intermetallics of gold and aluminium are a significant cause of wire bond failures in semiconductor devices and other microelectronics devices. There are 5 of them. AuAl2 is known as "purple plague". Au5Al2 is known as "white plague".
Examples of intermetallics through history include:
- Roman yellow brass, CuZn
- Chinese high tin bronze, Cu31Sn8
- type metal SbSn
German type metal is described as breaking like glass, not bending, softer than copper but more fusible than lead. . The chemical formula does not agree with the one above however the properties match with an intermetallic compound or an alloy of one.
- Intermetallics, Wiley-VCH, Weinheim 1995, 165 pages
- Intermetallics, Gerhard Sauthoff, Ullmann's Encyclopedia of Industrial Chemistry, Wiley Interscience. (Subscription required)