Stability of the Fe23Zr6 phase in Zr alloys

Abstract

Alloyed with iron (Fe), niobium (Nb) and tin (Sn), zirconium (Zr) is the main element in the Zirlo-type alloys, vastly used as structural elements and as containers of burnable elements in nuclear reactors. Although Zr is a major component in this type of alloys, it is most important to know the phase diagrams of their components as well as possible. The binary phase diagram of the Fe-Zr system has been studied for some time now by several authors. However, in the Fe-rich region the existence of the Fe23Zr6 compound, which was first described in 1962, remains controversial. To clarify the origin of this phase, the present work deals with the manufacture and prolonged heat treatments at different temperatures of alloys located in the Fe-rich region of the Fe-Nb-Zr, Fe-Sn-Zr and Fe-Zr phase diagram. The experiments have been performed with raw materials whose degree of purity was varied. The phases present were identified by using X-ray diffraction (XRD), semi quantitative microanalysis by using scanning electron microscopy analysis with energy dispersive spectrometry (SEM-EDS) and quantitative microanalysis by using electron microprobe with wavelength dispersive spectrometry (SEM-WDS). Finally, by using the results of characterization of heat-treated alloys for long annealing times at different temperatures, it is suggested that the presence of the Fe23Zr6 compound is an equilibrium phase of the Fe-Zr binary system.