Direct microstructural observations and atomistic simulations on Hematite to Magnetite reduction

V Chavan and NN Pai and A Prakash and P Raut and B Hazra and HK Mehtani and H Sharma and S Girish and S Nag and S Kundu and S Basu and A Alam and AS Panwar and I Samajdar, ACTA MATERIALIA, 296, 121269 (2025).

DOI: 10.1016/j.actamat.2025.121269

This study used a combination of direct microstructural observations and atomistic simulations to examine Hematite to Magnetite reduction under molecular hydrogen. Reduction at 1123 K revealed noticeable microscopic strains, and associated grain fragmentation with increased defect densities plus residual stresses. Larger Magnetite grains exhibited deformation twinning, which reduced the transformation induced elastic- plastic deformation. Experimental orientation relationship (similar to 50(degrees)<110>) and lattice correspondence-based transformation strains were also established. Atomistic simulations involved both reactive force field molecular dynamics (ReaxFF MD) and density functional theory (DFT). None of the ReaxFF parametrizations were able to fully capture all the thermodynamic and physical properties of the oxide phases. Choice of a specific parametrization was based on its ability to model the transition from alpha-Hematite to gamma-Hematite or Maghemite. This was also shown with subsequent DFT calculations and direct microstructural observations. Experimentally, two distinct transformation paths emerged: (I) direct Hematite-to-Magnetite reduction and (II) Hematite-to-Magnetite transformation through Maghemite. Though (I) and (II) were crystallographically identical, (II) was observed >=similar to 973K and had significantly more microscopic strains. Maghemite transition, and corresponding atomic shear, emerged as a key factor for transformation-induced microscopic strains, which is critical to the overall reduction kinetics. ReaxFF MD simulated this atomistic shear of experimental Maghemite formation.

Return to Publications page