Analysis of steady-state thermal creep of Zr-2.5Nb pressure tube material

The steady-state thermal creep rate in the axial and transverse directions of Zr-2.5Nb of pressure tubes, used in CANDU nuclear reactors, was determined. The data were obtained both from tensile samples having their tensile axes cut along the axial and transverse directions of the pressure tubes and from small-sized, thin-walled tubes, i.e., "mini" tubes stressed either in torsion or by internally pressurizing capsules manufactured from the mini tubes, or by additionally applying an external, axial load on these internally pressurized capsules. The temperature range of the data was from 373 to 596 K (100 degreesC to 323 degreesC) and the duration of the tests was from about 1500 hours to over 12,000 hours. The tests were carried out over a sufficiently long time for the creep rate to be measurable in the steady-state creep regime. It was found that the steady-state creep rate depends on stress in a nonlinear fashion and the stress exponent over the entire temperature range was about four. This value is consistent with the values measured earlier on other zirconium alloys. The activation energy Q was found to be about 21 and 10 kcal/mol for temperatures above and below 475 K (similar to200degreesC), respectively. These values are lower than those measured by other investigators on the same material at higher temperatures but similar to values found on other Zr alloys at low temperatures. It appears that Q is dependent on temperature and its value is consistent with the presence of dynamic strain aging (DSA). The results of this study were analyzed with a polycrystalline, nonlinear self-consistent model that take into account the crystallographic texture of the material. This model was used to derive the values of critical resolved shear stress (CRSS), which are consistent with prismatic, basal, and pyramidal glide. By using these values and the apparent temperature dependence of Q, it was shown that this model predicts well the steady-state creep rate over the entire temperature range and under very different stress states.