Metastable ω′-Fe3C carbide formed during.-Fe3C particle coarsening in binary Fe-C alloys

Nanosized (similar to 2 nm) omega-Fe3C particles with hexagonal structures have been observed only at body-centered cubic (BCC) {112}< 111 >-type twinning boundaries in twinned Fe-C martensite of the Fe-C alloy system. However, these ultrafine.-Fe3C particles never grow large enough to be observed easily. The present structural modeling and electron diffraction calculations reveal that the formation of the new carbide (omega'-Fe3C) during coarsening of the ultrafine omega-Fe3C particles is inevitable. Coarsening or aggregation of fine omega-Fe3C particles may result in a phase transition due to the arrangement of interstitial carbon atoms. A omega-Fe3C -> omega'-Fe3C transition was analyzed at the atomic scale. The omega'-Fe3C phase can exhibit an orthorhombic structure with lattice parameters a(omega') = 4.033 angstrom, b(omega') = 2.470 angstrom, and c(omega') = 6.986 angstrom based on a(omega') = a(omega), b(omega') = c(omega'), and c(omega') = root 3 a(omega) for a(bcc) or a(a-Fe) = 2.852 angstrom (a(omega) = root 2 a(bcc), c(omega) = root 3/2 a(bcc)). The simulated omega'-Fe3C electron diffraction patterns were experimentally confirmed. The omega-Fe3C -> omega'-Fe3C transition can explain why the omega-Fe3C phase never becomes larger than several nanometers in carbon steel. Published under license by AIP Publishing.