Omega phase formation in a physically vapour-deposited Fe-Al alloy

The omega phase has been identified by electron diffraction in a Fe-Al laminate material, corresponding to Fe-3---Al, which was prepared by physical vapour deposition (PVD). The phase has a hexagonal structure with a = 2(1/2)a(A2) and c = 3(1/2)a(A2)/2, which is the same as the omega structure reported by Silcock et at. in 1955. The morphology of the omega-phase in the PVD Fe-Al laminate is abnormal, considering that omega structures have generally been observed as very fine precipitates evenly distributed in a beta (A2 or B2 structure) matrix. Calculation of binding energies of competing phases using the full-potential linear-muffin-tin-orbital method has predicted omega stability in transition metals (TMs) and TM-Al alloys which is in agreement with experimental observations. The calculation shows that addition of later-transition-metal (LTM) elements to group-IV TMs will destabilize the omega structure with increasing electron concentration in the parent beta phase. Introduction of Al into LTMs will stabilize the omega structure via a decrease in the electron concentration in the parent beta phase. The remarkable stabilizing effect of Al on omega formation in LTMs is attributed to the d-p bonding, while omega formation in LTMs is controlled by the d-d bonding. Addition of Al to Fe is necessary to allow omega phase formation in Fe-Al alloys.