Intermetallics

Intermetallics are crystalline compounds formed between two or more metallic elements that have a fixed stoichiometry and an ordered crystal structure. Unlike simple solid solutions, where elements mix in variable proportions, intermetallics possess distinct compositions and atom arrangements that lead to unique physical properties.

Most intermetallics exhibit high melting points and hardness, with properties that often vary with temperature. They tend to be brittle at room temperature because of their ordered bonding, though some show improved ductility at elevated temperatures. Common crystal structures include L12 (face-centered cubic-based, as in Ni3Al), L10 (tetragonal, as in TiAl), and B2 (CsCl-type, as in FeAl and NiAl).

Well-studied examples include TiAl and Ti3Al (gamma and alpha2 phases), Ni3Al and Ni3V, FeAl (B2), Fe3Al (D03), and Al3Ti. Silicides such as MoSi2 and Nb5Si3 are intermetallics formed with silicon and are valued for high-temperature stability and oxidation resistance.

Intermetallics are produced by diffusion-controlled reactions during alloy solidification or by processing techniques such as powder metallurgy, diffusion bonding, and aluminizing or siliciding coatings. They can offer high-temperature strength, creep resistance, and reduced density, but their brittleness and processing challenges limit widespread use.

Applications include high-temperature components in aerospace and automotive engines (for example, gamma-TiAl turbine blades and aluminide coatings) and various structural and functional materials where long-term stability is important. Ongoing research focuses on improving ductility, damage tolerance, and fabricability through alloying, microstructure control, and novel processing routes.