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

Nanosized (∼2 nm) ω-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 ω-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 (ω′-Fe3C) during coarsening of the ultrafine ω-Fe3C particles is inevitable. Coarsening or aggregation of fine ω-Fe3C particles may result in a phase transition due to the arrangement of interstitial carbon atoms. A ω-Fe3C → ω′-Fe3C transition was analyzed at the atomic scale. The ω′-Fe3C phase can exhibit an orthorhombic structure with lattice parameters aω′ = 4.033 Å, bω′ = 2.470 Å, and cω′ = 6.986 Å based on aω′ = aω, bω′ = cω, and cω′=3aω for abcc or aα-Fe = 2.852 Å (aω=2abcc, cω=3/2abcc). The simulated ω′-Fe3C electron diffraction patterns were experimentally confirmed. The ω-Fe3C → ω′-Fe3C transition can explain why the ω-Fe3C phase never becomes larger than several nanometers in carbon steel.