Effects of Recrystallization and Neutron Irradiation on Creep Anisotropy of Zircaloy Cladding

Zirconium alloys have hcp crystal structure with low c/a ratio at and below the reactor operating temperatures and exhibit preferred orientations or textures. These textures result in anisotropic mechanical properties, which in-turn affect their in-service behavior such as in-pile creep down of the cladding tubes. The purpose of the present study has been to investigate the effects of recrystallization and neutron irradiation on the anisotropic biaxial creep behavior of Zircaloy cladding tubes. The creep anisotropies of the cold-worked and recrystallized tubes were considered using results from the closed-end internal pressurization tests superimposed with axial loading. The in situ biaxial strain measurements were made using laser and linear variable differential transformer (LVDT) extensometers. Creep data were obtained at various stress ratios, and creep loci were constructed at constant energy dissipation for both cold-worked and recrystallized tubes. X-ray diffraction techniques were used to measure the textures which were then described quantitatively in terms of crystallite orientation distribution functions (CODFs). These CODFs were employed to predict the anisotropy parameters R and P, and the anisotropic creep behavior. The creep behavior of Zircaloy tubes changed with recrystallization. The effect of neutron irradiation on the recrystallized material is modeled by invoking secondary slip systems. The considerable amount of plastic anisotropy observed in the unirradiated recrystallized tubes shows a tendency to decrease and to become isotropic at high fluences. On the other hand, neutron irradiation does not produce any significant changes in the anisotropy of the cold-worked material when radiation growth is taken into consideration.