Ab initioexplanation of disorder and off-stoichiometry in Fe-Mn-Al-Cκcarbides

Carbides play a central role for the strength and ductility in many\nmaterials. Simulating the impact of these precipitates on the mechanical\nperformance requires the knowledge about their atomic configuration. In\nparticular, the C content is often observed to substantially deviate from the\nideal stoichiometric composition. In the present work, we focus on Fe-Mn-Al-C\nsteels, for which we determined the composition of the nano-sized kappa\ncarbides (Fe,Mn)3AlC by atom probe tomography (APT) in comparison to larger\nprecipitates located in grain boundaries. Combining density functional theory\nwith thermodynamic concepts, we first determine the critical temperatures for\nthe presence of chemical and magentic disorder in these carbides. Secondly, the\nexperimentally observed reduction of the C content is explained as a compromise\nbetween the gain in chemical energy during partitioning and the elastic strains\nemerging in coherent microstructures.\n