Magnetic ordering phase transition and abnormal brittleness in dilute Fe-Mn solid solution

TL;DR

This study investigates the magnetic phase transition in dilute Fe-Mn alloys and links it to the abnormal brittleness observed at certain Mn concentrations, using advanced computational methods.

Contribution

It reveals the magnetic phase transition mechanism and its relation to brittleness in Fe-Mn alloys through ab initio simulations.

Findings

AFM phase dominates at low Mn content

FM phase becomes dominant at ~1.85 at.% Mn and higher temperatures

Brittleness correlates with local magnetic ordering changes

Abstract

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.