No abstract

No abstract

No abstract

Delayed Hydride Cracking (DHC) is a fracture mechanism responsible for several failures of zirconium alloy components. The Zr-2.5Nb alloy is used to manufacture CANDU pressure tubes, which are the primary pressure boundary in CANDU reactors.The rate of crack propagation in these tubes depends on direction. CANDU pressure tubes have anisotropic grain structure and texture due to extrusion and heat treatment.Disk-shaped hydrides preferentially form on certain crystallographic planes that are hypothesized to be responsible for the difference in DHC growth rates in certain directions. If bulk hydrides slow crack growth, then no directional difference in crack velocity will be observed when no bulk hydrides are present; i.e, where the DHC test temperature is reached by cooling to temperatures above the terminal solid solubility of precipitation (TSSP). Furthermore, below TSSP, where bulk hydrides are present, the crack velocities should begin to diverge as temperature is decreased, as oriented hydrides impede the crack growth to different extents in different directions. In this study, DHC crack velocities are determined using cantilever beam specimens with cracks propagating in both the longitudinal and through-thickness directions with and without bulk hydrides present, i.e., below and above TSSP. The DHC crack velocities in the longitudinal direction were observed to be higher than the corresponding through-thickness crack velocities with or without bulk hydrides present. Thus, the hypothesis is disproved; the orientation of bulk hydrides relative to the direction of DHC cracking does not explain the direction difference in crack velocities. Alternative hypotheses are presented.ii

No abstract