Formation and Dissolution of Hydride Precipitates in Zirconium Alloys: Crystallographic Orientation Relationships and Stability after Temperature Cycling

Abstract

Hydride precipitation due to the spontaneous and fast hydrogen diffusion is often pointed as causing embrittlement and rupture in zirconium alloys used in the nuclear industry. Transmission Electron Microscopy (TEM) and X-Rays Diffraction (XRD) have been used to study the precipitation of hydride phases in zirconium alloys as a function of the hydrogen content. The orientation relationships observed between the hydride phase and the substrate were similar to those previously observed in Titanium hydrides grown on Titanium. Dislocation emission from the hydride precipitates has been directly related to the relaxation of the misfit stresses appearing during the transformation. The stability of the hydride phases after several dissolution-reprecipitation cycles have been studied by DSC, TEM and XRD for different total hydrogen content in several alloys. The energy of precipitation observed is lower than that of the dissolution in each case studied. The temperature associated with these two processes slightly increase as a function of the cycle number, as a result of the homogenizing hydrogen distribution in the alloy bulk. The same hydrides phases present before cycling were also observed after 20 cycles. However, transition phases poorer in hydrogen than the dominant one may precipitate at the interface with the substrate. The evolution of these transitions phases with the temperature increase will be investigated by TEM in-situ heating in the next future.