Cementite

Cementite occurs in steels, in meteorites, possibly at the core of the Earth and has uses in its pure form. It's composition can deviate from , but not by much because the Fe-C bond contributes to its cohesion. Its crystallographic unit cell is orthorhombic and primitive, with large lattice parameters, explaining its hardness. Many of its properties are anisotropic. Its single-crystal elastic properties have been investigated using first-principles calculations and by clever experiments. The iron atoms in the cell occupy two types of positions with different point symmetries; the four carbon atoms lodge within prismatic interstices. The structure can develop defects such as dislocations, faults and vacancies. Cementite is metallic and ferromagnetic with a Curie temperature of about 187C. When alloyed, metallic solutes substitute on to the iron sites; smaller atoms such as boron replace carbon at interstitial sites. This review focuses on cementite as a single phase.