Hydride platelets in light water reactor fuel claddings are typically circumferentially oriented along the texture. Reoriented hydrides raise the risk of embrittlement and reduce the fracture toughness. While much research has been performed on the impact of hydrides on tensile and fracture properties, work on the influence of hydrides reorientation on fatigue properties is scarce.
In this work, un-irradiated and recrystallized Zircaloy-4 sheet material was tested; the flat samples allowed easily performing hydrides reorientation and the determination of hydride platelets, texture and fatigue cracks directions. The samples were hydrogen-loaded to a concentration of 250 wppm; some samples underwent a reorientation treatment. All specimens were fatigue-tested at ambient temperature under load control. The samples were analyzed with respect to the hydrides orientation by light optical microscopy and fractography by scanning electron microscopy.
Hydrides which are in-plane with the loading direction do not reduce the fatigue lifetime compared to hydrogen-free samples. Reoriented hydrides, however, cause a lifetime reduction. While in-plane hydrides are perpendicular to the crack front, reoriented hydride platelets are parallel to the crack propagation direction; fractography reveals brittle features indicating broken out hydride platelets.
The hydrogen concentration only is not decisive alone for the fatigue lifetime. Important is the share between reoriented and in-plane hydrides, which depends on the hydrogen concentration and the conditions of the thermomechanical reorientation cycle, specifically the temperature. Complete hydrides dissolution at peak temperature favors re-oriented hydride growth provoking reduced fatigue lifetime, while residual circumferential hydrides can inhibit its growth.