Effect of solute atoms on the chemical bonding of Fe₃C (cementite)

Abstract

First-principles molecular orbital calculations for Fe₃C (cementite) containing solute atoms have been made by the use of the discrete-variational(DV)-Xα method. Metal-metal (M-M) bonds around solute atoms show strong dependence on the atomic number of solute atoms, but metal-C (M-C) bonds are nearly constant irrespective of the solute atoms. Overlap population diagrams show that the Fermi level in pure Fe₃C lies in the antibonding band of the M-M bonding. The solute atoms induce the antibonding band to shift; thus the M-M bonds around the solute atoms are changed. The change in the magnitude of the M-M bonds with solute atoms is in good agreement with the variation in Vickers hardness of the Fe₃C solid solutions. Since the M–M bond is dominant between (001) planes in Fe₃C, it is natural that the M–M bond strength shows strong effects on the hardness which is mainly determined by the (001) slip. The present result is contrary to the previous views; it had been believed that the M–C bond strength rather than the M–M bond determines the hardness of the Fe₃C solid solutions.