Patrick Girault

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

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.

Hydride Precipitates in Zirconium Alloys: Evolution of Dissolution and Precipitation Temperatures During Thermal Cycling Correlated to Microstructure Features

The fast and spontaneous hydrogen diffusion in zirconium alloys used in the nuclear industry leads to the hydride precipitation which is often pointed as causing embrittlement and rupture. Our studies using X-ray Diffraction (XDR), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy coupled to Back-Scatter Diffraction (SEM-EBSD) have been demonstrating that the nature of the hydride phase precipitate depends on the hydrogen content, and can show crystallographic orientation relationships (ORs) with the substrate. Differential Scanning Calorimetry (DSC) has been used to identify the dissolution and precipitation energies at global scale. The difference of both can be associated to the misfit dislocations contribution to the precipitation. Local In-situ TEM dissolution observations confirm the dissolution temperature identified at a global scale and show the depinning of some misfit dislocations during dissolution process. The consequence of this mechanism is that dissolution and precipitation temperatures shift during thermal cyclic loading. This situation will be correlated to the nature of crystallographic hydride phases and their ORs.