Uncovering the hydride orientation-mediated hoop fatigue mechanism in a zirconium alloy cladding tube

Abstract

Hydride reorientation degrades the mechanical performance of Zr alloys. However, the mechanism by which hydride reorientation affects fatigue behavior is unclear. In this study, the effect of hydride orientation on crack initiation, propagation, and failure during hoop fatigue loading of a Zr–Sn–Nb alloy cladding tube was systematically investigated. The results show that the presence of ∼20% radial hydrides, in addition to circumferential hydrides, reduces fatigue life by 33%. Hydride-related cracking initially appears in intergranular or transgranular hydrides regardless of the orientation of the hydride. In circumferential hydrides, cracking initially appeared through the rupture of hydrides along their thickness, followed by crack growth either along the hydride–matrix interface or inside the hydrides. By contrast, for radial hydrides, cracks initially appeared at the hydride–matrix interface and always propagated along the interface. Radial hydrides provide favorable paths for fatigue crack growth, which contributes to the lower fatigue life of the hydride-reoriented samples. Subsequently, the rupture criterion of hydrides is discussed. A method for the quantitative calculation of grain-boundary stress was proposed, which allowed for the establishment of an accurate relationship between the critical size of the hydride and the critical strain of the matrix. Finally, a possible rationale for the crack deflection of circumferential hydrides is provided.