Carbon-Induced Ordering in Manganese-Rich Austenite - A Density-Functional Total-Energy and Chemical-Bonding Study

Density-functional theory-based total-energy calculations have been performed in order to discover possible local atomic ordering effects in manganese-rich austenite phases. For manganese contents of 25 and 50% we found a thermochemical driving force that should lead to a manganese enrichment of the immediate proximity of the carbon atom. The energy lowers almost linearly from a pure iron- to a pure manganese-coordinated carbon atom with an energy difference between the two M-6 octahedra (M = Fe, Mn) of ca. 0.34 eV (33 kJ/mol) for both antiferromagnetic and nonmagnetic structures. This very effect and the energy differences are almost independent of (a) the manganese concentration, (b) the carbon concentration, and (c) the magnetic state. A comprehensive bonding analysis yields that the effect is caused by the destabilization of the carbon atom's surrounding metal-metal bonds which come out larger for iron than for manganese. The size of the energy differences indicate a strong tendency for carbon-induced short-range ordering.