Anisotropic biaxial creep of stress-relief annealed Zircaloy-4 tubing

Abstract

Biaxial creep behavior of cold-worked stress-relief annealed Zircaloy-4 tubing is investigated under varied ratios of hoop to axial stresses by internal pressurization superimposed with axial load. The samples were also characterized in terms of creep loci. Zircaloy tubing was tested following two different heat treatments; both are stress-relieved and are referred to as A and B respectively. The textures were characterized using x-ray diffraction techniques and also in terms of the crystallite orientation distribution functions (CODFs). Anisotropic biaxial creep behavior was modeled based on prism slip dominance along with contributions, albeit minor, from other slip systems. Distinct deviations were noted between the prism-slip model predictions and the experimental loci whereas the earlier results on recrystallized Zircaloy cladding gave good correlation.(1) These differences are believed to arise from grain shape anisotropy in the stress-relieved material. The relatively small (equiaxed) grain size along the hoop and radial directions results in grain boundary sliding leading to stress enhancements along these directions. Incorporation of the directional dependence of such stress enhancements is shown to bring the predicted locus closer to the experimental results. However, for low stress-ratios (sigma(theta)/sigma(z) less than or equal to 0.5) where contractile hoop strains are noted, the data seem over-corrected due probably to the fact that GBS may not strongly influence the contractile strains required for volume conservation.