Thermal expansion of cementite in hypereutectoid iron-carbon alloy

Iron forms a number of carbides, of which cementite Fe3C has been studied best in steels. However, the nature of cementite has not been determined completely. It is known that carbon atoms can be arranged in two possible ways in the cementite lattice, namely, at the centers of trigonal prisms with iron atoms at the vertexes and at the centers of slightly deformed octahedrons (octahedral surroundings of iron atoms). The first variant is considered to be the most probable and is characterized by a lamellar structure that causes strong anisotropy of the physical properties of cementite. In the case of octahedral surroundings the structure should be quasi-isotropic with weak anisotropy of the cementite properties. In the present work the method of x-ray dilatometry is used to study the thermal expansion of cementite extracted from a hypereutectoid steel containing 1.72% C.