Large grain niobium (Nb) has been used to fabricate superconducting radiofrequency (SRF) cavities for particle accelerators over the past couple of decades, as a promising alternative to the well-established but expensive approach of using rolled polycrystalline sheet Nb. While the large grain approach to make cavities provides a reduced cost process, the performance is comparable to fine grain cavities. Cavities fabricated from both approaches exhibit variability in performance, a costly yet common issue. Understanding the origin of the variability will enable informed design decisions to be made, which necessitates studying the underlying physical metallurgy of Nb.One source of the variability is the starting material for fabricating cavities. Large and fine grain Nb was characterized to determine if ingots have commonalities and examine how different ingots result in heteogeneity in the microstructure of rolled sheets. Eight large grain ingots were analyzed using electron backscatter diffraction (EBSD) and Laue X-ray diffraction. The lack of similarity in crystal orientations and grain boundary misorientations of the ingots suggests random orientation nucleation/growth, which gives rise to variability in subsequent forming and processing. Fine grain rolled Nb sheets to be used for the Facility for Rare Isotope Beams (FRIB) were evaluated using tensile tests and EBSD to ensure the acceptability of the material. With these data, performance variability in future FRIB cavities can be traced to the initial microstructure. While the mechanical properties and texture vary significantly from one batch to another, correlations between texture, microstructure, and mechanical properties are generally weak.A multi-crystal rolling experiment was devised to investigate the connection between ingot and sheet microstructure. Wedged pieces from an ingot were rolled flat, from which samples with different amounts of cold work were extracted and analyzed with EBSD before and after annealing. Bands with orientations different from the parent grains developed due to rolling, and small grains nucleated from the bands upon annealing. The banding and recrystallization patterns approximate those observed in sheets subjected to more rolling passes, which implies that the microstructural heterogeneity in the rolled sheets originated from the randomly oriented large grains in the ingots.Hot spots are regions in a cavity with a localized temperature increase that may destroy the superconducting state. To identify sources of hot spots and performance variability in large grain cavities, EBSD was used to examine the cross-sections at the equator and iris of a cavity half-cell. The results suggest that cavity surface damage (locations with higher dislocation content reflected by greater orientation gradients) associated with the friction from deep drawing depends on crystal orientation, and the magnitude of such damage is different at the iris and equator. The orientation gradients at the equator were not uniformly removed after annealing at 1000 oC/2hr. This explains why the equator is more susceptible to hot spots in additional to it having a higher magnetic field and suggests that annealing at higher temperatures or longer times may be necessary.Modeling microstructural evolution during cavity processing can help predict performance variability and reduce the number of trial and error experiments. A fundamental understanding of deformation mechanisms of Nb is needed to establish such a model. For this purpose, two sets of single crystals with the same orientations were extracted from an ingot slice, and one set was heat treated to alter the initial condition. Both sets of samples were deformed to about 40% engineering strain in uniaxial tension. The differences in flow stress, crystal rotation, and active slip systems between the two sets are likely due to the removal of preexisting dislocations caused by the anneal.
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