Structural properties of metastable phases in Zr-Nb alloys III.: The athermal Ω phase

By means of high-resolution neutron diffraction experiments, a new study of the lattice parameters and other structural properties as well as the mass fraction of the Omega phase formed by quenching Zr-Nb alloys, has been made. With the present results and those from the recent neutron diffraction work by Benites and co-workers (J. Alloys Comp. 299 (2000) 183; 302 (2000) 192) reported on papers I and 11 of this series, an analysis is performed of the composition dependence of various key aspects of the diffusionless ('athermal') Omega phase transition in the Zr-Nb system. In particular, we discuss the misfit between the Omega particles and the parent bcc phase, the hexagonal to trigonal symmetry change occurring in Omega and the effect of the latter upon the shortest interatomic distance in this phase. The picture emerging from the present study is also used to investigate the interatomic distance correlation previously reported by Grad et al. (Z. Metallkd. 87 (1996) 726). (C) 2002 Elsevier Science B.V. All rights reserved.