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

Abstract

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 ЊC to 323 ЊC) 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 (ϳ200 ЊC), 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.

Effect of hydrogen on the creep behavior of Zr–2.5%Nb alloy at 723K

“…The n value, in the case of transverse specimen, was noted to be 1.86 in the unhydrided condition which reduced with the addition of hydrogen and reached a value closer to 1.59 for a hydrogen level of 160 wt ppm. The n value obtained in this work is about 1/3 of that reported by Shewfelt [25] and closer to that reported by Chirstodolu et al [26] for a similar alloy in the longitudinal orientation. The value of 2.81 obtained in this study, in the case of longitudinal specimen, indicates that dislocation glide is the possible deformation mechanism and the mechanism tends towards grain boundary sliding (2.41) with the addition of hydrogen.…”

Mechanical properties and dynamic strain aging behavior of Zr–1.5Nb–0.4Sn–0.2Fe alloy

“…It was between the activation energy (30 kcal/mol) of oxygen in a zirconium matrix [13] and that (62 kcal/mol) of a self-diffusion in the zirconium alloy [14].…”

Evolution of interphase and intergranular stresses in Zr–2.5Nb during room temperature deformation

“…Its in-reactor properties such as hydride cracking, irradiation growth and creep have been of interest for decades and models have been established to simulate the irradiation behavior [2][3][4][5][6]. It has been found that the irradiation behavior of Zr-2.5Nb pressure tubes is strongly dependent on the texture, microstructure and intergranular residual stresses [7][8][9][10], which are controlled by the manufacturing schedule.…”

“…To the authors' knowledge, a deformation model which can be used and applied directly to such a material taking into account the anisotropy and the dual phase nature has not yet been constructed. Indeed, most of the studies to date have ignored the ␤-phase because of its relatively small volume fraction and have treated the material as single phase polycrystalline [3,4,23,26] with the aim of accommodating any related error within the fit of the parameters of the single phase models to the experimental data. However, recent studies [27] have suggested that the overall properties of Zr-2.5Nb may be highly dependent on the properties and the distribution (both geometric and texture) of the ␤-phase.…”

“…Neglecting the ␤-phase is thus likely to introduce significant errors in modeling the deformation response of the overall material and in particular in extrapolating model predictions to new textures and microstructures. This is one likely reason why the models in [3] and [4] are accurate only for certain texture and microstructure combinations.…”