# Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C kappa carbides

## TL;DR

This study combines atom probe tomography, density functional theory, and thermodynamics to explain the atomic configuration, disorder, and off-stoichiometry in Fe-Mn-Al-C kappa carbides, revealing the origins of C content deviations.

## Contribution

It provides a detailed ab initio analysis of disorder and off-stoichiometry in Fe-Mn-Al-C carbides, linking experimental observations with theoretical modeling.

## Key findings

- Critical temperatures for chemical and magnetic disorder identified.
- Reduction in C content explained by energy and elastic strain considerations.
- Atomic configuration insights aid in understanding carbide stability and properties.

## Abstract

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