Browsing by Author "K. Linga Murty, Committee Chair"

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Creep-Rupture Study of Annealed Zircaloy 4: Stress and Temperature Effects
(2005-11-22) Marple, Brian Wesley; Gerd Duscher, Committee Member; Mohamed A. Bourham, Committee Member; K. Linga Murty, Committee Chair
Zircaloys are widely used as fuel rod cladding in light water reactors (LWRs) because of their low cross-section for absorption of thermal neutrons. Currently, the United States does not permit reprocessing of spent fuel so the primary barrier for the spent fuel in a repository will be the fuel rod cladding. Due to the decay heat of the spent fuel, creep rupture is considered to be the primary cause of failure in spent fuel cladding over the long period of time that it will be stored. A fundamental understanding of the creep mechanisms in Zircaloys is crucial to accurately predicting the integrity of the fuel cladding over long periods of time. Zirconium has a hexagonally close-packed crystal structure and because of this, exhibits creep anisotropy that is affected not only by the texture, but also by temperature, stress, and loading. Since the stress imposed on the spent fuel during long-term storage will be relatively low compared to service conditions, the low stress creep behavior must be characterized and mechanistically understood to avoid non-conservative estimates based on in-pile creep data. In addition, loading of spent fuel in a repository will be due to the internal pressure generated by fission product gasses and from the inert gas introduced at the time of fuel fabrication. This work focuses on the creep rupture behavior and microstructural characterization of annealed Zircaloy-4 at temperatures ranging from 250°C-600°C and stresses from 27 MPa-350 MPa. Typically, fuel assemblies that have been fabricated from Zircaloy-4 are not in the annealed condition. Instead, they are cold-worked and stress relieved (CWSR). Since low stress creep rupture testing would take years at low temperatures, high temperatures are used to observe the effects of low stress in a reasonable amount of time. At such high temperatures, the grain structure of the CWSR material would change drastically. Therefore the material was annealed prior to testing to avoid this complication. Testing on unirradiated material will yield higher strain rates because of irradiation hardening. Therefore, estimates based on unirradiated creep rupture data would be conservative. Prior to testing, optical metallographs were taken to characterize the grain structure. A limited texture study was performed to evaluate the texture coefficients for each direction in the rod. Transmission electron microscopy (TEM) was also performed to characterize the initial dislocation microstructure. After testing, diametric measurements were taken and the strain rate determined. From data at various stresses, the activation energy was derived along with equations predictive of rupture such as the Larson-Miller parameter and the Monkman-Grant relationship. Specimens of interest were selected for optical metallography and to obtain TEM micrographs of the dislocation microstructure. The activation energy deduced was in excellent agreement of that for self-diffusion. Optical metallography showed slight grain elongation in samples tested at high stresses while grains remained equiaxed at low stresses. TEM showed significant sub-grain formation at low stresses and random dislocation organization at higher stresses.
Crystallographic Texture and Creep Anisotropy in Cold Worked and Recrystallized Zirlo
(2005-08-09) Yan, Jinyuan; Ron O.Scattergood, Committee Member; Mohamed A. Bourham, Committee Member; Man-Sung Yim, Committee Member; K. Linga Murty, Committee Chair
Zirlo, a special zircaloy material alloyed with niobium, tin and iron is a successor of Zircaloy-4. Zirlo is materials used in fuel rod cladding, structural and flow mixing grids, instrumentation tubes, and guide thimbles. It increases margin to fuel rod corrosion limits and enhance fuel assembly structural stability in Pressurized Water Reactor. Zirconium and its alloys, being hexagonally close packed, have limited number of slip systems, and exhibit preferred orientations following thermo-mechanical treatments, which result in anisotropic mechanical properties. The objective of this project is to investigate the anisotropic mechanical properties, crystallographic texture, and microstructure of crept zirlo materials. The anisotropic mechanical properties were investigated using uniaxial and biaxial creep tests. The specimen was loaded axially by a dead weight pan, and the hoop stresses was achieved by internally pressurizing the specimen with inert argon. Different axial and hoop stress, which produced different stress ratios (0, 0.67,0.75, 1, and 2) are selected for creep tests at 450°C. The axial displacement was measured by a linear variable differential transducer and the diameter change by a laser extensometer. Creep data are used to determine strain rate ratios vs stress ratios, the anisotropic parameters ( R and P), and creep loci for cold-worked and recrystallized zirlo. The crystallographic textures were characterized in terms of inverse and direct pole figures using X-ray diffraction techniques. Inverse pole figures were constructed for specimens in the rolling direction, transverse direction, and normal direction for both cold worked and recrystallized tubes. Direct pole figures were constructed for specific reflection planes, such as basal (0002), prismatic (10 0) and pyramidal (10 2). Crystallite orientation distribution function (CODF) was derived from the pole figure data. Euler plots were obtained from crystallite orientation distribution coefficients (wlmn ) and subsequently therefore, ideal orientations were calculated. These CODFs were combined with the Lower-Bound model to predict creep anisotropy assuming the dominance of prismatic, basal and pyramidal slip systems. Creep strain rate ratios vs stress ratios, creep loci and anisotropy parameters (R and P) were predicted. The predictions based on the prismatic dominance matche with the experimental data very well. Microstructure of the crept specimens was characterized by Transmission Electron Microscopy for different stress ratios ( 0, 0.75 and 1). The results show mainly dislocations in the matrix with no subgrain formation. The samples tested under equibiaxial loading revealed deformation twins. More detailed work is called for in characterizing the influence of stress-states and stress levels as well as cold work on deformation microstructures.