Combined atom probe tomography and density functional theory investigation of the Al off stoichiometry of k carbides in an austenitic Fe Mn Al C low density steel
We report on the investigation of the off stoichiometry and site occupancy of k carbide precipitates within an austenitic (g) 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 k 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 k/g mismatch, which facilitates occupation of Al sites in k carbide by Mn atoms (MngAl 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.