We present an experimental study and a systematic phenomenological analysis of the structural properties and the high temperature reactions in Zr-Nb alloys quenched from high temperatures, where bcc (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 objective of the present work is to study the nucleation and evolution of the W phase upon aging (“a”). Neutron thermodiffraction studies are discussed of the structural properties of Wq+bq two-phase alloys and bq single-phase alloys at 300 K £ T £ 650 K. In the first place, the structural properties of the Wa phase formed isothermally upon aging are used to obtain information about the evolution of the system towards thermodynamic equilibrium. A method of analysis is applied which involves the concept of a reference behaviour describing the probable structural properties of each individual phase if it were unaffected by the other structures in the heterogeneus alloys. We show how the present method of analysis may be used to obtain indirect information of both theoretical and practical interest, in view of the composition changes and phase reactions occuring at high temperatures. Finally, the nucleation of the Wa phase at high temperature is monitored combining neutron thermodiffraction experiments with transmission electron microscopy observations, and a discussion in presented of the possible mechanisms for the ba® Wa phase transition at high temperatures.
e-mail: gaurelio@cab.cnea.gov.ar