Characterization of hydrides precipitated in the a-Zr matrix of zirconium alloys: effects of stress, microstructure and neutron irradiation on hydride dislocation structure, texture, and terminal solid solubility

P. VIZCAINO; A. V. FLORES; M. A. VICENTE ALVAREZ; J. R. SANTISTEBAN; G. DOMIZZI; A. TOLLEY; A. CONDÓ; J. ALMER

Simposio; 19th International Symposium on Zirconium in the Nuclear Industry; 2021

Synchrotron X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy experiments were performed on samples from Zr-2.5Nb alloy pressure tubes and Zircaloy-4 fuel channels materials in order to define hydride precipitation temperatures, hydride texture, the matrix hydride orientation relationship, and dislocation densities. For Zr-2.5Nb, studies included in situ constant stress tests performed on dog-bone specimens cycled between room temperature and 400°C, where hydrides were completely dissolved and reprecipiated. For Zircaloy-4, analysis included ex-service Zircaloy-4 irradiated to ~ 1022 neutrons/cm2. Careful interpretation and analysis of azimuthal variations around the Debye-Scherrer rings allowed a clear description of both texture and dislocation densities of δ-hydride precipitates. The main findings are: (1) In Zr-2.5Nb pressure tubes, the terminal solid solubility for hydride dissolution and the terminal solid solubility for precipitation (TSSP) temperatures depend on the orientation of the parent α-Zr grain. Hydrides both precipitate and dissolve at slightly lower temperatures (~ 5°C and ~ 15°C, respectively) in grains having their c-axis parallel to the tube hoop direction than those with their c-axis ~ 20° off from the hoop direction. (2) Application of a stress along the tube hoop direction during precipitation increases TSSP temperatures and favors hydride precipitation on grains with c-axes stretched by the load. For grains with the c-axis parallel to the applied load, TSSP temperatures increase at a rate of (0.08 ± 0.02)°C/MPa, nearly two orders of magnitude higher than previous estimates. (3) Precipitates of δ-hydride are heavily dislocated, yet display large variations (1.6 to 10 × 1014 m−2) depending on the microstructure and hydrogen content of the parent material.