  We present ab-initio simulation of manganese, aluminum and carbon impurities
in austenite and demonstrate their inhomogeneous distribution, which involves
the repulsion of interstitial carbon atoms, the formation of bonded Mn-C pairs
as well as a short range Al-ordering of D03-type. The mechanisms for the
formation of stacking faults in Fe-Mn-Al-C are considered, and we find that the
impurities have influence on the stacking fault energies only when located
within a few interatomic layers near stacking fault. As a result, the stacking
fault energy does not depend on the average concentration of impurities in
matrix, but is highly sensitive to the concentration of the impurities in the
vicinity of stacking fault defect. We predict that manganese shows a slight
tendency for segregation near SF, while carbon prefers to be located far from
the stacking fault region. Both aluminum and carbon impurities linearly
increase the SFE, while the formation of Mn-C pairs and short range Al-ordering
restrain the SFE growth. Short range order in Fe-Al-C alloys strongly affects
the energy barrier for nucleation of dislocations and may lead to softening
phenomenon.
