The structural properties and relative stability of metastable phases have been studied in the Zr-Nb and Ti-V systems.
An extensive neutron diffraction study will be presented on a series of Zr-Nb and Ti-V alloys quenched from high temperatures, where b is the stable phase. Upon quenching, three metastable structures are formed, viz., the martensitically formed hcp (aq) phase, the Wq phase, and the untransformed bq phase. The structural properties of these metastable phases are determined as a function of the Nb and V contents to generate a reliable experimental database. With such data, a series of issues are discussed related to the structure, relative stability, and phase relations in the alloys and its constitutive elements. The observed effect of composition upon the lattice parameters of the metastable bq and Wq phases is combined in a consistent way with a critical analysis of structural and thermophysical data on the metastable phases of Ti and Zr.
Finally, the relative stability of the metastable aq, Wq and bq phases in Zr-Nb alloys, and its evolution towards thermodynamic equilibrium will be discussed. The study combines neutron thermodiffraction and analytical electron microscopy techniques. During isothermal heat treatments performed at high temperature, the structural properties of the alloys are determined as a function of temperature, time and composition. Our analysis leads to a consistent picture of the complex reactions occurring at high temperature, i.e., of composition changes in the W and b phases and phase transitions. The results in the present work open up the possibility of using neutron diffraction experiments to gain insight into the stable and metastable phase diagram and phase reactions in the Zr-Nb and Ti-V systems.