Characterization of Hydrides and the a-Zr Matrix in Zirconium Alloys: Effects of Stresses, Microstructure, and Neutron Irradiation on Hydride Texture, Terminal Solid Solubility, and Dislocation Structure

P. VIZCAÍNO; A. V. FLORES; M. A. VICENTE ALVAREZ; J. R. SANTISTEBAN; G. DOMIZZI; A. TOLLEY; A. CONDO; J. D: ALMER

Manchester

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

Synchrotron X-ray diffraction, transmission electron microscopy, and highresolution 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. ForZr-2.5Nb, studies included in situ constant stress tests performed on dog-bone specimens cycled between room temperature and 400C, where hydrides were completely dissolved and reprecipiated. For Zircaloy-4, analysis included exservice 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 d-hydride precipitates. The main findings are: (1) In Zr-2.5Nb pressure tubes, the terminal solid solubilityfor hydride dissolution and the terminal solid solubility for precipitation (TSSP) temperatures depend on the orientation of the parent a-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 TSSPtemperatures 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 d-hydride are heavily dislocated, yet display large variations (1.6 to 10 x 10^14 m^-2) depending on the microstructure and hydrogen content of the parent material.