Mechanisms of hydride reorientation in zircaloy-4 studied in situ

Zirconium hydride platelet reorientation in fuel cladding during dry storage and transportation of spent nuclear fuel is an important technological issue. Using an in situ x-ray synchrotron diffraction technique, the detailed kinetics of hydride precipitation and reorientation can be directly determined while the specimen is under stress and at temperature. Hydrided Zircaloy-4 dogbone sheet samples were submitted to various thermo-mechanical schedules, while x-ray diffraction data was continuously recorded. Post-test metallography showed that nearly full hydride reorientation was achieved when the applied stress was above 210 MPa. In general, repeated thermal cycling above the terminal solid solubility temperature increased both the reoriented hydride fraction and the connectivity of the reoriented hydrides. The dissolution and precipitation temperatures were determined directly from the hydride diffraction signal. The diffraction signature of reoriented hydrides is different than that of in-plane hydrides. During cooling under stress, the precipitation of reoriented hydrides occurs at lower temperatures than the precipitation of in-plane hydrides, suggesting that applied stress suppresses the precipitation of in-plane hydrides. The analysis of the elastic strains determined by the shift in position of hydride and zirconium diffraction peaks allowed following of the early stages of hydride precipitation. Hydride particles were observed to start to nucleate with highly compressive strain. These compressive strains quickly relax to smaller compressive strains within 30°C of the onset of precipitation. After about half of the overall hydride volume fraction is precipitated, hydride strains follow the thermal contraction of the zirconium matrix. In the case of hydrides precipitating under stress, the strains in the hydrides are different in direction and trend. Analyses performed on the broadening of hydride diffraction peaks yielded information on the distribution of strains in hydride population during precipitation and cooldown. These results are discussed in light of existing models and experiments on hydride reorientation.