A popular metastable omega phase in body-centered cubic steels

Steel remains to be one of the most common structural materials in the world as human civilization advances from the Iron Age to the ongoing Silicon Age. Our knowledge of its microstructure evolution and structure–performance relationship is nevertheless still incomplete. We report the observation and characterization of a long ignored metastable phase formed in steels with body-centered cubic (bcc) structure using both transmission electron microscopy and density functional theory calculations. This ω phase has a hexagonal structure and coherent interface with the matrix: aω = √2 × abcc and cω = √3/2 × abcc. It is 3.6% smaller in volume and 0.18 eV higher in energy than bcc-Fe, with atoms in alternating close- and loose-packed layers couple anti-ferromagnetically. Carbon plays a crucial role in promoting bcc to ω transformation. At a concentration higher than 4 at.% they tend to segregate from the bcc matrix to the ω-phase; at about 14 at.%, they can induce bcc to ω transformation; and finally at 25 at.%, they stabilize the ω phase as ω-Fe3C. The ω phase in bcc Fe can serve as sinks for vacancies, H, and He atoms, leading to improved resistance of martensitic steels to irradiation damage.

► A long-ignored metastable ω phase in body-centered cubic (bcc) steel. ► The ω phase has hexagonal structure with lattice parameters aω = √2 × abcc and cω = √3/2 × abcc. ► Carbon enrichment is found to play a crucial role on the bcc-to-ω phase transformation. ► The ω phase is strongly related to the martensitic transformation and twinning structure. ► The ω phase in bcc Fe can serve as sinks for vacancies, H, and He atoms.