Intermetallic

Intermetallic or intermetallic compounds are solid-state phases involving metals with a more ordered structure than an alloy, and often a well-defined, fixed stoichiometry.

Many intermetallic compounds are often simply called 'alloys', although this is somewhat of a misnomer. Both are metallic phases containing more than one element, but in alloys the various elements substitute randomly for one another in the crystal structure, forming a solid solution with a range of possible compositions; in intermetallic compounds, different elements are ordered into different sites in the structure, with distinct local environments and often a well-defined, fixed stoichiometry. Complex structures with very large unit cells can be formed.

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.[1]

Definitions

Research definition

Schulze in 1967,[2] 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:

Alloys, which are homogeneous 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.

Common use

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.

Complexes

The term intermetallic is used[3] to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp6Ni2Zn4.

B2

A B2 intermetallic compound has equal numbers of atoms of two metals such as aluminum and iron.[4]

Intermetallics involving two or more metallic elements

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.[5]

Properties and examples

The formation of intermetallics can cause problems. Intermetallics of gold and aluminium can be a significant cause of wire bond failures in semiconductor devices and other microelectronics devices. There are five intermetallic compounds in the binary phase diagram of Al–Au. AuAl2 is known as "purple plague". Au5Al2 is known as "white plague".

Intermetallic particles

Intermetallic particles form during solidification of metallic alloys.

History

Examples of intermetallics through history include:

German type metal is described as breaking like glass, not bending, softer than copper but more fusible than lead.[7] The chemical formula does not agree with the one above; however, the properties match with an intermetallic compound or an alloy of one.

References

  1. ↑ Electrons, atoms, metals and alloys W. Hume-Rothery Publisher: The Louis Cassier Co. Ltd 1955
  2. ↑ G. E. R. Schulze: Metallphysik, Akademie-Verlag, Berlin 1967
  3. ↑ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
  4. ↑ "Wings of steel: An alloy of iron and aluminium is as good as titanium, at a tenth of the cost". The Economist. February 7, 2015. Retrieved February 5, 2015. E02715
  5. ↑ S.P. Murarka, Metallization Theory and Practice for VLSI and ULSI. Butterworth-Heinemann, Boston, 1993.
  6. ↑ Milton Ohring, Materials Science of Thin Films, 2nd Edition, Academic Press, San Diego, CA, 2002, p. 692.
  7. ↑ Type-pounding The Penny Cyclopædia of the Society for the Diffusion of Useful Knowledge By Society for the Diffusion of Useful Knowledge (Great Britain), George Long Published 1843

See also

External links