Structural properties and high-temperature reactions of the metastable Omega phase in Zr-Nb alloys.

This article presents an experimental study and a systematic phenomenological analysis of the structural properties and the high-temperature reactions of the Omega phase formed by quenching (‘q’) bee (beta) Zr-Nb alloys. In the first part of the work an extensive database with lattice parameters (LPs) for the Omega(q) and theuntransformed beta(q) phase is developed on the basis of neutron diffraction measurements. Various striking features of the LP vs. composition relations are detected, and a new method of analysis is applied which involves the concept of a `reference behavior’ (RB) describing the probable properties of each individual phase if it were unaffected by the other structures in the heterogeneous alloys. In this way, a detailed evaluation is performed of the effects of Omega(q)/beta(q) coherence strains upon the LPs of these two phases. In the second part of the work, neutron thermodiffraction studies are reported of the structural properties of the Omega(q) + beta(q) two-phase alloys at 300 K <= T <= 650 K. Extensive LP vs. time results are discussed. On this basis, the structural properties of the Omega(a) phase formed isothermally upon aging ('a') are established, and compared with those of the Omega(q) phase. In addition, new information is discussed on the evolution of the Omega(a) + beta(a) system toward thermodynamic equilibrium. Moreover, it is shown how the present method of analysis may also be used to obtain indirect information of both theoretical and practical interest. In particular, considerable evidence is presented indicating that the Nb content of the Omega(a) phase decreases upon aging whereas that of beta(a) increases. Finally, it is shown how these composition changes might be used to reconcile much of the long-standing conflicting reports concerning the structural properties of the Omega(q) and Omega(a) metastable phases. (c) 2005 Elsevier B.V. All rights reserved.