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 lengths, predicts that the Omega phase is ordered, i.e., that some crystallographic sites are preferentially occupied by Zr atoms. Such feature, which should in principle be observed in other, related Omega phases, has not yet been tested against direct measurements. This problem has now been studied in the Ti-V system, which is the analogue of Zr-Nb in the Sd-transition series. Neutron diffraction measurements have been performed in quenched Ti-V alloys with V contents between 14 and 17 at.%. The diffraction spectra have been analysed using the Rietveld method, and a systematic analysis is reported here of the possibility of deviations from the random occupation of the two sublattices which are distinguished in the Omega structure. Ln addition, these new diffraction data are used in an evaluation of the shortest interatomic 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 reserved.