Crystal Orientations Near Welds in High RRR Niobium With Very Large Grains

Superconducting radio frequency (SRF) cavities made of single crystal niobium are under development for use in charged particle accelerators. Use of single crystals may simplify manufacturing and reduce cost, as well as improve properties over the currently used fine grain niobium material. However, the processes of forming by deep drawing, and subsequent welding of the formed parts to assemble the cavity, might lead to recrystallization in regions of high strain or curvature near the weld. Orientation imaging microscopy (OIM) was used to assess these possibilities in some preliminary experiments. A sample of single crystal niobium strip was arbitrarily bent and electron beam (EB) heated across one end to simulate welding. The bent sample had no more than 14% strain, and it did not exhibit definitive recrystallization near or away from the EB heated area. Another sample was prepared by halving a large grain niobium bicrystal across the boundary of two grains, flipping one half and EB welding the halves back together, such that the weld had three different grain misorientations along its length, including two triple points. There was no formation of new orientations along the weld where it joined two crystal orientations. However, some new orientations solidified where the weld encountered three different grain orientations. This preliminary data is encouraging, suggesting that minimal generation of new grain orientations during EB welding may be practical. However, more work with different orientations and strains is needed to determine how tolerant Nb is for maintaining a flat solidification interface and resisting recrystallization.