Density functional theory study of ω phase in steel with varied alloying elements

The presence of a long-abandoned hexagonal omega (omega) phase in steel samples is recently gaining momentum owing to the advances in transmission electron microscopy (TEM) measurements, even though it is already reported in other transition-metal alloys. The stabilization of this metastable phase is mainly investigated in presence of C, even though the formation of the omega phase is attributed to the combined effect of many factors, one among which is the enrichment of solute elements such as Al, Mn, Si, C, and Cr in the nanometer-sized regimes. The present study investigates the effect of the above alloying elements in omega-Fe using density functional theory (DFT) calculations. It is seen that the magnetic states of the atoms play a major role in the stability of omega-Fe. Cohesive energy calculations show that the alloying elements affect the energetics and stabilization of omega-Fe. Further, density of states calculations reveal the variation in d-band occupancy in the presence of alloying elements, which in turn affects the cohesive energy. Phonon band structure calculations show that only omega-Fe with substitutional C shows positive frequencies and hence possess thermodynamic stability. Finally, we confirm the existence of omega-Fe using TEM measurements of a steel sample containing the same alloying elements. Our results can shed light on the stabilization of the omega in other transition-metal alloys as well, in the presence of minor alloying elements.