Microstructural evolution in zirconium based alloys

Zirconium alloys which exhibit practically all possible solid state phase transformations offer some unique opportunities for studying the fundamental aspects of phase transformations. The crystallographic features associated with the phase changes are much simpler in zirconium based systems, making them more suitable for providing a basic understanding of the relevant phenomena. Some of the important issues related to phase transformations research are identified and the manner, in which studies in zirconium alloys have contributed to the understanding of these issues, is discussed in this paper. Both the types of displacive transformations – shear-dominated and shuffle dominated – are encountered in zirconium alloys. The crystallography and the mechanism of such transformations are discussed, giving special reference to the morphological and sub-structural changes in zirconium martensites and to the fine particle and plate-shaped morphologies of the ω-phase. Morphological and crystallographical features of displacive and diffusional transformations have been compared. Several examples of mixed mode transformations, viz., the formation of γ-hydride phase, the active eutectoid decomposition and the formation of ordered ω-phases are cited and their respective mechanisms described.