Intergranular δ-hydride nucleation and orientation in zirconium alloys

A theoretical understanding of the intergranular δ-hydrides is still lacking, although hydride-related degradation of the mechanical properties of zirconium alloys has been studied for many years. In this paper a thermodynamic model is developed to analyze the nucleation and orientation of intergranular δ-hydrides. The results show that the grain boundary structure and zirconium grain orientation simultaneously govern hydride precipitation. An electron backscatter diffraction study of hydrided Zircaloy-4 provides direct evidence supporting theoretical predictions. A criterion is proposed to reveal the inherent relation between hydride precipitation at the grain boundaries and in the zirconium grains. Stress-induced susceptibility to hydride precipitation at the radial grain boundaries of zirconium alloy tubes is theoretically analyzed. This work provides a general framework based on which the correlation between the grain boundary structure, the grain orientation, the stress and their effects on the nucleation and orientation of the intergranular δ-hydrides can be clarified.