Ab initio thermodynamics of carbides in high Mn steels

The addition of Cr to high-Mn steels improves their corrosion resistance. At the same time, it has been experimentally reported that the alloying with Cr yields a substantial increase in the number of carbides, like cementite and Fe₂₃C₆. They can contribute to a precipitation hardening of the material, but also provide interfaces that might be critical for hydrogen embrittlement. In the present work, we therefore use density functional theory to determine the thermodynamic driving force for the formation of carbides as a function of the chemical composition of the alloy. These investigations are performed at finite temperatures considering the vibrational, electronic and magnetic contributions to the free energy of formation. We analyze the critical role of Cr by determining the partitioning of Cr and Mn into the carbides. At the same time, the role of these alloying elements for the solution enthalpy of H in the carbides is determined. We report that the chemical trends for the carbides show surprising differences from the behavior in the Fe-Mn matrix.