Zr-2.5Nb is used as pressure-tube material in CANDU (CANada Deuterium Uranium) reactors. Under reactor operating conditions, pressure tubes undergo anisotropic dimensional changes, and thermal creep contributes to this deformation. In a previous study, the limited textures available to Zr-2.5Nb significantly restricted the understanding of the relationship between texture and creep anisotropy. Moreover, there has been no research performed to investigate textures and stress states simultaneously for this material, which would provide a valuable resource for developing creep anisotropy models and optimizing textures to improve creep resistance. Cold-worked Zr-2.5Nb fuel sheathing (FS) and micro pressure tubes (MPT) with various textures and microstructures were used as experimental materials. The tubes were machined as thin-wall standard (ratio of axial to transverse stress 0.5) and end-loaded (ratio of axial to transverse stress = 0.25~0.75) capsules and were internally pressurized and sealed. Stress and temperature dependence tests were performed on standard capsules under transverse stresses of 100~325MPa at 300~400°C to establish a regime in which dislocation glide is the likely strain producing mechanism. An average stress exponent vaule of 6.4 was obtained, indicating that dislcation creep is the likely dominant mechanism. Texture and stress state dependence tests were performed on standard and end-loaded capsules under a nominal transverse stress of 300MPa at 350°C. It was evident that creep anisotropy strongly correlates with textures under different stress states. A self-consistent polycrystalline model SELFPOLY7 based only upon crystallographic texture was employed to simulate the creep anisotropy of the tubes. However, the model cannot fit all the experimental data well by using a uniform critical resolved shear stress (CRSS) ratio of the operating slip systems. A modification was made, by taking into account the pre-existing dislocation distributions generated during cold work, and an improvement was achieved. This work provides a valuable resource for understanding the effect of texture, stress states and microstructure on anisotropic creep of cold-worked Zr-2.5Nb tubes. The current research also provides a strategic direction to improve creep anisotropy predictions. The large sets of experimental data supply a database to evaluate and develop improved models. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-08-14 11:16:08.67

There is no unified phenomenological model available for Zr alloys that allows for the inclusion of a complete set of texture input parameters in order to describe the anisotropic behaviour during plastic deformation at different strain rates and thermal creep. This research shows how AVBO, an anisotropic version of VBO, can be enhanced by introducing single crystal based tensors to describe the material response to different Kearns factors, which uniquely describe the texture of a sample with two numbers only. It is demonstrated with the aid of published thermal creep test data that small tensile deformation behaviour of Zr-2.5Nb is consistent with predominant slip of Zr alloy crystals parallel to their single crystal α basal planes, supporting the strategy to model the behaviour of these HCP materials with a phenomenological constitutive model. It is demonstrated that the new version of AVBO, ATXVBO, predicts trends consistent with a slip mechanism parallel to the basal planes. Multi-objective optimization was employed successfully to determine the set of 67 constants. It is proven that the theory relies on a limited number of tests to perform optimization of all the unknown constants. Numerous validation and sensitivity evaluations were performed to test the optimized solutions despite the limited availability of plastic deformation test data with documented texture information. It is demonstrated that, in order to capture dynamic strain aging effects, different strategies will be required at different evaluation temperatures. It is also shown that although texture variation can be accommodated that different manufacturing routes each requires a unique optimized set of constants. / Thesis / Doctor of Philosophy (PhD) / A unified phenomenological theory ATXVBO has been established that can predict the deformation of HCP materials, including Zr alloys, that can account for texture variation. It is postulated that the inelastic polycrystal properties can be derived from hypothetical single crystal properties and the Kearns factors, used to describe the texture.

Pressure tubes pick up hydrogen while they are in service within CANDU reactors. Sufficiently high hydrogen concentration can lead to hydride precipitation during reactor shutdown/repair at flaws, resulting in the potential for eventual rupture of the pressure tubes by a process called Delayed Hydride Cracking (DHC). The threshold stress intensity factor (KIH) below which the cracks will not grow by delayed hydride cracking of Zr-2.5Nb micro pressure tubes (MPTs) has been determined using a load increasing mode (LIM) method at different temperatures. MPTs have been used to allow easy study of the impact of properties like texture and grain size on DHC. Previous studies on MPTs have focused on creep and effects of stress on hydride orientation; here the use of MPTs for DHC studies is confirmed for the first time. Micro pressure tube samples were hydrided to a target hydrogen content of 100 ppm using an electrolytic method. For DHC testing, 3 mm thick half ring samples were cut out from the tubes using Electrical Discharge Machining (EDM) with a notch at the center. A sharp notch with a root radius of 15 µm was introduced by broaching to facilitate crack initiation. The direct current potential drop method was used to monitor crack growth during the DHC tests. For the temperature range tested the threshold stress intensity factors for the micro pressure tube used were found to be 6.5-10.5 MPa.m1/2 with the value increasing with increasing temperature. The average DHC velocities obtained for the three different test temperatures 180, 230 and 250oC were 2.64, 10.87 and 8.45 x 10-8 m/s, respectively. The DHC data obtained from the MPTs are comparable to the data published in the literature for full sized CANDU pressure tubes. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2009-04-24 12:55:36.917