Structural properties and high-temperature reactions of the metastable Ω 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 Ω phase formed by quenching ('q') bcc (β) Zr-Nb alloys. In the first part of the work an extensive database with lattice parameters (LPs) for the Ωq and the untransformed βq 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 Ωq/βq 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 Ωq + βq two-phase alloys at 300 K ⩽ T ⩽ 650 K. Extensive LP vs. time results are discussed. On this basis, the structural properties of the Ωa phase formed isothermally upon aging ('a') are established, and compared with those of the Ωq phase. In addition, new information is discussed on the evolution of the Ωa + βa 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 Ωa phase decreases upon aging whereas that of βa 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 Ωq and Ωa metastable phases.