In Situ S/TEM Study of Hydride Dissolution/Precipitation Behavior in Zirconium Alloys

Abstract

The dissolution/precipitation behavior of hydrides in Zircaloy-2 was investigated by electron microscopy. The in-situ S/TEM heating/cooling experiments were carried out on hydrides observed in the TEM foil at different locations. For a hydride in a thin area, where dislocations can be imaged, a distinct memory effect was seen with reprecipitation occurring at the same location where the hydride existed before dissolution. Dissolved hydrides left behind dislocation nests, and precipitation-induced local plasticity was observed. For hydrides in a thicker part of the TEM foil clear dissolution/precipitation hysteresis was seen with a temperature difference of approximately 50°C. When reprecipitating, hydrides in thick parts of the foil also formed at the same position as before, displaying a memory effect. The existence of a memory effect for hydride reprecipitation was also observed during ex situ SEM imaging. After one cycle of heating/cooling, hydrides were observed to predominantly precipitate at their original positions. Electron backscatter diffraction analysis revealed that the precipitated hydride obeyed the conventional orientation relationship with the parental zirconium, with a habit plane close to the zirconium (0002) plane. The plasticity induced by hydride precipitation was evaluated by geometrically necessary dislocation analysis, demonstrating localized deformation in the zirconium matrix around hydrides. The precipitation-induced dislocations were found to be mainly <a>-type dislocations by TEM. The effect of high-temperature annealing on hydride precipitation was investigated. A 3-day annealing at 800°C caused significant grain growth of the zirconium that resulted in the subsequent hydride distribution being relatively uniform, and predominantly along grain boundaries. Last, a small number of hydrides were found to precipitate with the atypical (0001)Zr//(001)hydride orientation relationship. These observations suggest two contributory causes to the memory effect: at the grain level dislocation nests act as preferential nucleation sites for hydrides, and intergranular stress acts as a driver moving hydrogen to preferred grain locations/interfaces.