Abstract
The omicro phase has been identified by electron diffraction in a Fe-Al laminate material, corresponding to Fe Al, which was prepared by physical vapour 3 deposition (PVD). The phase has a hexagonal structure with a = 21/2 a and A2 c = 31/2 a /2, which is the same as the omicro structure reported by Silcock et al . in A2 1955. The morphology of the omicro-phase in the PVD Fe-Al laminate is abnormal, considering that omicro 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-tinorbital method has predicted omicro 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 omicro structure with increasing electron concentration in the parent beta phase. Introduction of Al into LTMs will stabilize the omicro structure via a decrease in the electron concentration in the parent beta phase. The remarkable stabilizing effect of Al on omicro formation in LTMs is attributed to the d-p bonding, while omicro formation in LTMs is controlled by the d-d bonding. Addition of Al to Fe is necessary to allow omicro phase formation in Fe-Al alloys.
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