A recently developed Fe-C interatomic potential has been used to investigate carbon in highly supersaturated $\alpha$ iron with an emphasis on the possible ways in which carbon can arrange itself on the octahedral sites of the tetragonally distorted $\alpha$-iron lattice. Focusing particularly on the composition ${\text{Fe}}_{16}{\text{C}}_2$, the embedded atom method potential used gives the same ground-state structure of ${\text{Fe}}_{16}{\text{C}}_2$ as density-functional-theory calculations. Moreover, when computing C-C interactions between two carbon atoms at 0 K with energy minimization, the preferred separation distance corresponds exactly to the octahedral site positions expected in ${\text{Fe}}_{16}{\text{C}}_2$. It has been shown with molecular dynamics that the carbon atoms in ${\text{Fe}}_{16}{\text{C}}_2$ change how they order on octahedral sites during heating and during cooling. Finally, when investigating the interaction energy of ${\text{Fe}}_{1-x}{\text{C}}_x$ for different carbon compositions, it is found that there is a clear minimum at $x=0.11$ for the fully ordered structure, corresponding to ${\text{Fe}}_{16}{\text{C}}_2$ composition. The results of these calculations are discussed with particular reference to carbon ordering in ferrous martensite.

• Received 14 December 2009

DOI:https://doi.org/10.1103/PhysRevB.81.224204