Atomic ordering and systematics of bonding lengths in the Ti-V omega phase: a neutron diffraction study

A new model describing the structural and bonding properties of the omega ( Omega) phase in Zr-Nb alloys has recently been presented [12]. This model, which was aimed at explaining the composition dependence of the bonding len gths, predicts that the Omega phase is ordered, i.e., that some crystallogr aphic sites are preferentially occupied by Zr atoms. Such feature, which sh ould in principle be observed in other, related Omega phases, has not yet b een tested against direct measurements. This problem has now been studied i n the Ti-V system, which is the analogue of Zr-Nb in the Sd-transition seri es. Neutron diffraction measurements have been performed in quenched Ti-V a lloys with V contents between 14 and 17 at.%. The diffraction spectra have been analysed using the Rietveld method, and a systematic analysis is repor ted here of the possibility of deviations from the random occupation of the two sublattices which are distinguished in the Omega structure. Ln additio n, these new diffraction data are used in an evaluation of the shortest int eratomic distances which are relevant for a comparison with the predictions of the model of Grad et al. (C) 1999 Elsevier Science S.A. All rights rese rved.