Article

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

Michigan State Univ., East Lansing
IEEE Transactions on Applied Superconductivity (Impact Factor: 1.32). 07/2007; DOI: 10.1109/TASC.2007.898014
Source: IEEE Xplore

ABSTRACT 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.

0 Bookmarks
 · 
110 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: As Superconducting Radio Frequency (SRF) technology is used in more accelerator designs, research has focused on increasing the efficiency of these accelerators by pushing gradients and investigating cost reduction options. Today, most SRF structures are fabricated from high purity niobium. Over years of research, a material specification has been derived that defines a uniaxial, fine grain structure for SRF cavity fabrication. Most recently a push has been made to investigate the merits of using single or large grain niobium as a possible alternative to fine grain niobium. Michigan State University (MSU), in collaboration with Fermi National Accelerator Laboratory (FNAL) and Thomas Jefferson National Accelerator Facility (JLAB), is researching large grain niobium via cavity fabrication processes and testing, as well as exploring materials science issues associated with recrystallization and heat transfer. Single-cell 1.3 GHz (beta=0.081) cavities made from both fine and large grain niobium were compared both in terms of fabrication procedures and performance. Two 7-cell cavities are currently being fabricated.
    08/2007; DOI:10.1063/1.2770682
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Alternative fabrication techniques for superconducting radio frequency (SRF) cavities are being investigated. The main goals are to reduce cavity fabrication costs and expand possibilities for advanced cavity designs. At present, SRF cavities are fabricated via deep drawing of parts from sheet material and electron beam welding (EBW) to join the parts together. EBW produces welds of high quality, but the procedures are costly and time-consuming. Alternative technologies being explored include tungsten inert gas (TIG) welding of Nb, hydroforming of Nb, and electron-beam free form fabrication (EBFFF) of Nb. If techniques can be developed which do not degrade the Nb purity, TIG welding could reduce or eliminate the need for EBW. Hydroforming could also be an alternative to deep drawing and EBW. As has been demonstrated by several other groups, complete cavities can be hydroformed from Nb tubes in one step using internal pressure and outer dies. Hydroforming of cavities in an industrial setting is presently being explored. EBFFF is a new technique for forming parts from wire stock with an electron beam. Though it may not be suitable for fabrication of a complete cavity, EBFFF could be used to produce tubes for hydroforming or parts for drift tube cavities. Additionally, the possibility of producing single crystal tubes using EBFFF is being explored.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Large grain∕single crystal Nb is currently being examined for fabricating superconducting radiofrequency (SRF) cavities as an alternative to using rolled sheet. Three ingot slices from different suppliers have been characterized and are compared. It is desirable to know the grain orientations of an ingot slice before fabrication. Characterization of an ingot slice has been done using electron backscattered pattern (EBSP) orientation imaging microscopy (OIM), which requires cutting out pieces from the slice, a destructive technique. Use of a Laue camera allows nondestructive characterization of grain orientations. The OIM method was used to examine ingot slices from CBMM and Ningxia, while the Laue method was used to examine a Heraeus ingot slice. The three ingot slices are compared in terms of their crystal orientations and grain boundary misorientations, indicating no obvious commonalities. The Laue method has practical advantages over OIM for evaluating ingot slices during the manufacturing process.
    03/2011; 1352(1):90-99. DOI:10.1063/1.3579228

Full-text (2 Sources)

Download
88 Downloads
Available from
May 28, 2014