Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel

We report on the investigation of the off-stoichiometry and site-occupancy of kappa-carbide precipitates within an austenitic (gamma), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the kappa-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L'12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the kappa/gamma mismatch, which facilitates occupation of Al sites in kappa-carbide by Mn atoms (Mn-Al(gamma) anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.