| | Single and large-grain Nb SRF cavities are of interest
due to possible reduction of cost and problems associated
with inconsistent texture and surface finish among batches
of rolled polycrystalline Nb sheet. The effect of crystal
orientation on dislocation density, surface quality, and
recrystallization after plastic deformation and e-beam
welding was investigated, as understanding of their
interrelations is needed. These were evaluated for three
samples of different orientations at steps similar to those
in typical cavity forming, with deformation modeled
using a crystal plasticity approach. Initial dislocation
density was higher than expected, increased with
deformation, after welding was reduced in recovered
areas, and was similar to initial density in recrystallized
grains; there was also evidence that Nb has a higher
tolerance for dislocations than other metals. Surface
quality depends on a complex relation of crystal
orientation, slip system activity, and prior surface
treatment. Recrystallization nucleated outside the melt
pool, and the new orientations grew both epitaxially into
the weld as it solidified, and away until heat and time
were insufficient to continue growth. | |