A first-principles study of the impact of paramagnetism on grain boundary segregation in FeMn alloys

To understand the impact of paramagnetism on defect energetics and kinetics is, though conceptually and computationally challenging, important for designing Fe-based alloys. Since magnetic degrees of freedom change faster than atomic degrees of freedom in the high-temperature paramagnetic state, the atoms move according to an averaged force instead of instantaneous forces attained from each spin configuration.

Therefore, a new computationally efficient method based on spin-space averaging [1] has been developed to handle magnetic disorder next to defects, which uses the spin constraint tool developed in the DFT code S/PHI/nX. First, we focus on vacancies in the FeMn system, for which we demonstrate that paramagnetism significantly affects atomic relaxations as well as vacancy diffusion barriers and thereby explain why Mn diffusion shows a different temperature dependence than Fe-self diffusion in α-Fe. Next, we expand our method to extended defects and show that paramagnetism has a significant effect on the grain boundary segregation of Mn. Finally, we combine our results to reveal the chemo-magneto-structural coupling underlying Mn segregation to grain boundaries in Fe alloys.

[1] Körmann et al., Phys. Rev. B 85(12):125104, (2012)