Intermetallic

An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic 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. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.

Although the term "intermetallic compounds", as it applies to solid phases, has been in use for many years, Hume-Rothery has argued that it gives misleading intuition, suggesting a fixed stoichiometry and even a clear decomposition into species.

Definitions

Research definition

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:

1. Electron (or Hume-Rothery) compounds
2. Size packing phases. e.g. Laves phases, Frank–Kasper phases and Nowotny phases
3. Zintl phases

The definition of a metal is taken to include:

1. post-transition metals, i.e. aluminium, gallium, indium, thallium, tin, lead, and bismuth.
2. metalloids, e.g. silicon, germanium, arsenic, antimony and tellurium.

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.

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 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 aluminium and iron, arranged as two interpenetrating simple cubic lattices of the component metals.

Properties and applications

Intermetallic compounds are generally brittle at room temperature and have high melting points. Cleavage or intergranular fracture modes are typical of intermetallics due to limited independent slip systems required for plastic deformation. However, there are some examples of intermetallics with ductile fracture modes such as Nb–15Al–40Ti. Other intermetallics 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 in many intermetallics. 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 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 super alloys, 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, inter-metallic involving silicon, are utilized as barrier and contact layers in microelectronics.

Examples

1. Magnetic materials e.g. alnico, sendust, Permendur, FeCo, Terfenol-D
2. Superconductors e.g. A15 phases, niobium-tin
3. Hydrogen storage e.g. AB₅ compounds (nickel metal hydride batteries)
4. Shape memory alloys e.g. Cu-Al-Ni (alloys of Cu₃Al and nickel), Nitinol (NiTi)
5. Coating materials e.g. NiAl
6. High-temperature structural materials e.g. nickel aluminide, Ni₃Al
7. Dental amalgams, which are alloys of intermetallics Ag₃Sn and Cu₃Sn
8. Gate contact/ barrier layer for microelectronics e.g. TiSi₂
9. Laves phases (AB₂), e.g., MgCu₂, MgZn₂ and MgNi₂.

The formation of intermetallics can cause problems. For example, intermetallics of gold and aluminium can be a significant cause of wire bond 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:

1. Roman yellow brass, CuZn
2. Chinese high tin bronze, Cu₃₁Sn₈
3. Type metal, SbSn
4. Chinese white copper, CuNi

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.

See also

• Complex metallic alloys
• Kirkendall effect
• Maraging steel
• Metallurgy
• Solid solution

References

• Gerhard Sauthoff: Intermetallics, Wiley-VCH, Weinheim 1995, 165 pages
• Intermetallics, Gerhard Sauthoff, Ullmann's Encyclopedia of Industrial Chemistry, Wiley Interscience. (Subscription required)

External links

• Intermetallics, scientific journal
• Intermetallic Creation and Growth – an article on the Wire Bond Website of the NASA Goddard Space Flight Center.
• Intermetallics project (IMPRESS Intermetallics project at the European Space Agency)
• Video of an AB₅ intermetallic compound solidifying/freezing