HYDRIDE EMBRITTLEMENT IN ZIRCALOY-4 PLATE .2. INTERACTION BETWEEN THETENSILE-STRESS AND THE HYDRIDE MORPHOLOGY

The effect of an applied tensile stress on the hydrides morphology in ZIRCALOY-4 was studied. To this end, the residual stresses around the hydride caused by the hydride precipitation was first evaluated. Consi dering the disability to predict hydride transformation stresses by or dinary macroscopical mechanical calculation in previous studies, X-ray diffraction (XRD) profile analysis and transmission electron microsco py (TEM) observations were carried out to quantify the microstructural evolution in hydrided ZIRCALOY-4. The residual microstrains and micro stresses in the matrix and around the hydride were thus estimated. The big discrepancy between our results and the existing studies were exp lained by the major self-accomodation of phase transformation deformat ion remaining inside the hydrides and the local plastic accommodation of ZIRCALOY-4. In order to study the stress effect on hydride orientat ion and to estimate the hydride orientation threshold stresses, hydrog en was introduced into the specimens under tensile stress. A quantitat ive technique was used to evaluate the susceptibility to perpendicular hydride formation under the influence of texture, residual stresses, and externally applied tensile stresses, following an improved approac h that had been first developed by Sauthoff and then applied to Zr-H s ystem by Puls. Both analytical and experimental results indicate that the threshold stress for producing perpendicular hydrides varies with the microstructural features, the yield strength, and the residual str esses.