The Structure of Sublattices of Carbon and Iron Atoms in Cementite

Abstract

Published data on positions of carbon and iron atoms in cementite are reviewed. Experimental results of the last two decades are presented together with the data obtained by ab initio modelling. The positions of carbon atoms in the iron sublattice are considered in tree types of interstice, i.e., normal, distorted prismatic, and normal octahedral ones. According to the ab initio computations, the energy differences in displacement of a carbon atom from the most energy-favorable normal prismatic positions into positions of the two remaining types do not exceed 0.3 eV and the energy of formation of a carbon vacancy is also low (0.5 eV). It is shown thermodynamically that even at a temperature of 1000 K, the proportion of the not occupied normal prismatic pores may reach 0.2. This makes it possible to treat the iron sublattice of cementite, in addition to the bcc and fcc modifications, as a manifestation of the concentration polymorphism of iron, and to treat the cementite itself as and interstitial solid solution based on this third modification.