Local interactions of carbon in FeAl alloys

A model study of the local interactions of carbon in a FeAl host is presented, employing the first principles density functional theory within the generalized gradient approximation. The geometric, electronic and magnetic structure of the perovskite K-carbide or Fe3AlC is investigated for both the stoichiometric and non-stoichiometric phases. For the perovskite K-carbide, a 15-atom unit cell is used for the calculations. The bulk environment for this unit cell is simulated by including all 14 nearest neighbors for each of the six Fe-atoms on the face centers of the central unit cell. The investigations on the K-carbide reveal interesting patterns of atomic relaxations. While the stoichiometric phase constrains the Fe-atoms on the faces of the cube, a partial removal of carbon drives the Fe-clustering accompanied by the generation of large magnetic moments. Aluminum atoms, occupying the corners of the perovskite structure, are not perturbed significantly in either of the above situations. Based on the B2(CsCl)-lattice and the possible vacant site occupations of carbon atoms, a mechanism for the formation of Fe3AlC is proposed. Further, the calculations on solute carbon in FeAl reveal a preferential occupation of octahedral sites to the tetrahedral sites. The optimal AlC bond requirements for these site occupations result in large tetragonal distortions of the host lattice.