Review on ω Phase in Body-Centered Cubic Metals and Alloys

An omega phase with a primitive hexagonal crystal structure has been found to be a common metastable phase in body-centered cubic (bcc) metals and alloys. In general, omega phase precipitates out as a high density of nanoscale particles and can obviously strengthen the alloys; however, coarsening of the omega particles significantly reduces the alloy ductility. The omega phase has coherent interfacial structure with its bcc matrix phase, and its lattice parameters are a omega = root 2 x a(bcc) and c omega, root 3/2 x a(bcc). The common {112}< 111 >-type twinning in bcc metals and alloys can be treated as the product of the : bcc phase transition, also known as the omega-lattice mechanism. The omega phase's behavior in metastable beta-type Ti alloys will be briefly reviewed first since the omega phase was first found in the alloy system, and then the existence of the omega phase in carbon steels will be discussed. Carbon plays a crucial role in promoting the omega formation in steel, and the omega phase can form a solid solution with various carbon contents. Hence, the martensitic substructure can be treated as an alpha-Fe matrix embedded with a high density of nanoscale omega-Fe particles enriched with carbon. The recognition of the omega phase in steel is expected to advance the understanding of the relationship between the microstructure and mechanical properties in bcc steels, as well as the behavior of martensitic transformations, twinning formation, and martensitic substructure.