Microstructural criteria for abrupt ductile-to-brittle transition induced by δ-hydrides in zirconium alloys

Hydride-induced ductile-to-brittle transition is an important feature for the failure of hydride-forming materials. Although this problem has been studied for many years, its relation to microstructure remains unclear. The degree of hydride-induced embrittlement depends on the hydride network. In this study, the formation mechanism of the hydride network with a high degree of continuity and its influence on the ductile-to-brittle transition of zirconium alloys are investigated. A criterion is proposed to determine the critical hydrogen content for the formation of an interlinked hydride configuration by considering the local stress-induced changes in hydrogen concentration and terminal solid solubility in the vicinity of the hydride tip. The theoretical results are in good agreement with the experimental observations. The propagation path of the hydride network and its dependence on microstructure are analyzed using electron backscatter diffraction and thermodynamic modeling. The results show that the grain-boundary structure, the grain-boundary misorientation and the angle of hydride inclination toward the grain-boundary plane control the formation and the growth direction of the hydride network. Although this work focuses on zirconium alloys, the obtained results are also of significance for other hydride-forming materials.