Intermetallic

Research definition

In 1967 Gustav Ernst Robert Schulze 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. This definition includes:

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

The definition of metal includes:

• Post-transition metals, i.e. aluminium, gallium, indium, thallium, tin, lead, and bismuth.
• Metalloids, e.g., silicon, germanium, arsenic, antimony and tellurium.

Homogeneous and heterogeneous solid solutions of metals, and interstitial compounds such as 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 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 Cp₆Ni₂Zn₄.

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 aluminium-nickel, arranged as two interpenetrating simple cubic lattices of the component metals.

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 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.

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 attracted interest for turbine blade applications, while the latter is also used in small quantities for grain refinement of titanium alloys. Silicides, intermetallics involving silicon, serve as barrier and contact layers in microelectronics. Others include:

• Magnetic materials e.g., alnico, sendust, Permendur, FeCo, Terfenol-D
• 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), Nitinol (NiTi)
• Coating materials e.g., NiAl
• High-temperature structural materials e.g., nickel aluminide, Ni₃Al
• Dental amalgams, which are alloys of intermetallics Ag₃Sn and Cu₃Sn
• Gate contact/ barrier layer for microelectronics e.g., TiSi₂
• Laves phases (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 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 often form during solidification of metallic alloys, and can be used as a dispersion strengthening mechanism.

The intermetallic compounds formed by tin are hard and brittle. Some compounds of importance in electronics are Ag₃Sn, Cu₃Sn, and Cu₆Sn₅. As these particles grow they tend to compromise the integrity of a solder joint made by lead-free solder. Some additives can reduce the grain size of IMC and disperse them, turning them into strengthing elements.

Examples of intermetallics through history include:

• Roman yellow brass, CuZn
• Chinese high tin bronze, Cu₃₁Sn₈
• Type metal, SbSn
• Chinese white copper, CuNi

German type metal is described as breaking like glass, without 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.