MODELING OF IN-SITU HYDRIDE GROWTH IN ZR-2.5NB

摘要

The presence of Zr hydrides can greatly reduce the ductility and fracture toughness in a pressure vessel made of Zr alloy. Understanding how the hydrides form and grow is critical to flaw assessment for the pressure tubes. The process of hydride growth in Zr-2.5%Nb has been monitored in-situ in high-energy synchrotron X-ray radiation. The C-shaped specimen with a V-notch was held under constant load at a temperature where hydride formation was ensured. The development of hydride size and elastic strains in both the hydrides and the Zr matrix was recorded. Afterwards, the hydride morphology was characterized by Scanning Electronic Microscopy. A finite element program in combination with a process zone model and a diffusion model has been used to interpret the experimental data for better understanding the hydride growth process. The hydride length and morphology are well predicted, given that the information of hydride size and hydrostatic stress is properly updated and exchanged between the process zone and diffusion models. The effect of creep has been included in the modeling but found relatively small compared to that of hydride volumetric expansion. The elastic strains in Zr are well reproduced except that disagreement with the experiment is found in the hydrided region. This analysis provides further evidence that the process zone and diffusion models can be used to predict the hydride size and morphology development. Further modeling at a micro-structural level is needed for improving predictions of the stress/strain state in the hydrides, which is essential to the development of a sound hydride crack initiation model.