A study on the mechanism of iodine-induced stress-corrosion cracking of Zircaloy-4

The iodine-induced stress-corrosion cracking (SCC) of Zircaloy-4 plate specimens has been studied by the constant elongation rate test (CERT) and U-bend test methods. In order to systematically evaluate the effects of stress states on the SCC behavior, four kinds of specimens were prepared from as-annealed Zircaloy-4 plate specimens. The uniaxial tensile and single edge-notched (SEN) specimens fractured in the ductile manner, not by SCC. However, SCC resulted in the plane strain tensile specimens and deep-notched U-bend specimens. As the strain rate was lowered, the susceptibility to SCC increased. The stress-corrosion (SC) cracks propagated by the trans-granular brittle fracture mode for as-annealed specimens, whereas by the inter- and trans-granular mixed fracture mode for reannealed specimens. The SCC processes of Zircaloy-4 in iodine-gas are discussed in terms of effects of stress states, elongation rates and yield stresses. The present experimental results suggest that the iodine-induced SCC processes are based upon the iodine-diffusion model. (orig.)