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

Abstract

Abstract 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.