On the hypothesis of bonding related atomic ordering in binary omega phases

A new model describing the structural and bonding properties of the omega p hase in Zr-Nb alloys has recently been presented [G. B. Grad ed nl.: Z. Met allkd. 87, (1996) 726]. This model, which was aimed at explaining the compo sition dependence of the bonding distances, predicts that the omega phase i n Zr-Nb alloys is ordered, i.e., that some of the crystallographic sites ar e preferently occupied for Zr atoms. This feature of the omega phase is not contradicted by the most recent studies of the lattice parameters, but it has not yet been tested against direct measurements of the atomic ordering. In this paper we identify, theoretically, the Bragg reflections that appea r only if the alloy is ordered, and show that among the three prototypical isoelectronic systems Ti-V, Zr-Nb and Hf-Ta, the former is the most suitabl e one for an experimental test of the model. The use of neutron scattering and transmission experiments is critically discussed, and the optimum compo sition for experiments with Ti-V alloys is presented.