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 °C to 323 °C) 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 (∼ 200 °C), 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.

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

Characterization of Al 2 O 3 /ZrO 2 composite coatings deposited on Zr-2.5Nb alloy by plasma electrolytic oxidation

Both in situ tension and compression tests have been carried out on textured Zr–2.5Nb plate material at room temperature. Deformation along all the three principle plate directions has been studied and the evolution of interphase and intergranular strains along the loading and the principle Poisson's directions has been investigated by neutron diffraction. The evolution of interphase and intergranular strain was determined by the relative phase properties, crystal properties and texture distribution. The average phase behaviors are similar during tension and compression, where the β-phase in this material is stronger than the α-phase. The asymmetric yielding of the α-{0 0 0 2} grain family results in a relatively large intergranular strain in the loading direction during compression and different dependence of strength during tension and compression on texture. The combination of the thermal residual stress and the asymmetric CRSS in the 〈 c 〉 axis gives the {0 0 0 2} grain family a higher strength in compression than in tension.

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

Effect of neutron irradiation on deformation mechanisms operating during tensile testing of Zr-2.5Nb

Formation of duplex microstructure in Zr–2.3Nb alloy and its plastic behaviour at various strain rates

The anisotropy of yielding, as measured by the ratio of yield stress in the axial and transverse directions of Zr-2.5Nb pressure tubes used in Canada Deuterium Uranium (CANDU) nuclear reactors, was determined experimentally by testing samples in uniaxial tension. The yield anisotropy was measured in uniaxial tension in samples obtained from the three directions of a Zr-2.5Nb plate and in shear, by testing in torsion “mini” pressure tubes from the same material. From these experiments, the temperature and strain-rate dependence of the yield stress and the dependence of the anisotropy of yielding on temperature were also determined. It is shown that the yield anisotropy of pressure tube material is constant for temperatures up to about 800 K and that the strain-rate sensitivity is also constant up to about 700 K and is equal to ∼0.02. In addition, the activation energy (Q) of this material was estimated by using the temperature dependence and rate sensitivity of the yield stress. It was found to be of the same order of magnitude as that determined earlier by other investigators. A polycrystalline, nonlinear self-consistent model that takes into account the crystallographic texture of the material was used to derive the values of the critical resolved shear stress (CRSS) which are consistent with prismatic, basal, and pyramidal glide and the values of the Hill’s plastic anisotropy coefficients which are consistent with the observed anisotropy of yielding. The model provided an estimate of the complete stress tensor, describing yielding of a Zr-2.5Nb pressure tube material.

N. Christodoulou

Papers

Anisotropy of yielding in a Zr-2.5Nb pressure tube material