  Experiments showed that solute Mn in bcc iron is in antiferromagnetic (AFM)
coupling with iron neighbours below 2 at.% Mn, but is in ferromagnetic (FM)
coupling at higher concentrations. Surprisingly, although Mn is an important
alloying element in high-strength steels, it induces brittleness just at around
2 at.% Mn and higher concentrations. However, the mechanisms for the magnetic
ordering phase transition and the abnormal brittleness remain unclear. Based on
magnetism-constrained/unconstrained calculations and ab initio molecular
dynamics simulations within density functional theory, we show that while the
AFM phase prevails at low Mn contents, the FM phase becomes dominant at 1.85
at.% Mn and elevated temperatures. Our results suggest that the AFM-FM phase
transition with increasing Mn concentration can be ascribed to the thermal
effect. Furthermore, we find that the brittleness of the Fe-Mn alloys at
intermediate Mn content might be related to the stress variations within the
grains accompanying the local magnetic ordering changes.
