D. Baars

Microstructural Refinement of Niobium for Superconducting RF Cavities

The mechanical properties of commercial polycrystalline pure niobium sheet used for superconducting radiofrequency cavities are known to provide inconsistent yield, springback and surface smoothness characteristics when plastically formed into a radiofrequency cavity. These inconsistent properties lead to significant variations in cavity geometry and thus superconducting cavity performance. One approach to reduce these problems is to refine the microstructure so that its properties are more uniform. Microstructural refinement of Nb sheet for RF cavities using multi-axis severe plastic deformation via equal channel angular extrusion (ECAE) was examined. ECAE was performed on 25 mm square cross-section bars of Reactor Grade Nb in a right angle die at room temperature following different extrusion routes to true strains above nine. This heavily worked material was rolled to 4 mm thick sheet and recrystallized. Measurements of hardness, springback, texture, and microstructural uniformity are reported and compared to those of commercial RRR Grade Nb sheet. Preliminary results show noteworthy promise for bulk Nb processed via severe plastic deformation prior to sheet rolling.

Mechanical Properties of High RRR Niobium With Different Texture

High purity bulk niobium RRR ~ 300 is the standard material used for SRF cavities since this has been the most promising material for over 10 years. Mechanical properties of the high RRR niobium play a critical role in the physical integrity of these structures. It is well known that the mechanical properties of polycrystalline high RRR niobium are affected by a variety of possible contributors: crystallographic texture, grain size, and impurity concentration. Tensile tests were carried out using three groups of niobium samples from MSU SRF cavity and Fermi International Linear Collider projects with different texture and grain size, which came from two different manufacturers. For some of the groups the tensile tests were done with different orientations with respect to the sheet. Orientation Imaging Microscopy (OIM) data were collected in these samples. The relationships between mechanical properties, texture, and grain size in these samples are analyzed.

Fabrication of Tantalum Sheet for Superconductor Diffusion Barriers

Poor deformation behavior of commercial Ta sheet used for Sn diffusion barriers in multifilamentary Nb3Sn superconductors leads to excessive Ta-Cu interface roughening and the use of a thick Ta layer to overcome layer fracture. The problem stems from three factors: Ta strain hardens faster and to a much higher flow stress than Cu, pure Cu is adjacent to Ta, and the Ta sheet microstructure is non-uniform and non-optimum. The objective of our work is to fabricate Ta sheet with improved deformation behavior over present day commercial Ta sheet. The hypothesis is that a uniform, fine-grained, and well-textured sheet with a preferred orientation will have better codeformation characteristics. To test this thinking, 25 mm square cross-section bars of Ta were deformed by equal channel angular extrusion (ECAE) to strains of nine, sliced in half, annealed to various levels, and rolled to 0.5 mm thick sheet, and Cu-Ta monofilaments fabricated with the processed sheet. Experimental results and roughness measurements on the Ta layers indicate that severe plastic deformation (SPD) processed sheet shows better characteristics than commercial Ta layers. This results from the uniform microstructure and fine grains in precursor Ta sheet processed by SPD.

Single crystal and large grain niobium research at Michigan State University

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 (β=0.081) cavities made from both fine and large grain niobium were compared bot...

DEVELOPMENT OF A CRYOGENIC MECHANICAL PROPERTY TESTING STATION FOR SUPERCONDUCTING RF CAVITY MATERIAL

Recent concerns with pressure vessel codes as they relate to the construction of superconducting linacs have raised questions about mechanical proprieties of materials used in their fabrication at cryogenic temperatures. Pressure vessel engineering codes will require demonstration of a level of safety equivalent to that provided by the various ASME pressure and piping codes, so low temperature mechanical properties of niobium, titanium, and their alloys are needed. Michigan State University (MSU), in collaboration with Fermi National Accelerator Laboratory (FNAL) and Florida State University (FSU), is constructing a materials testing station for tensile tests of materials at room and cryogenic temperatures (300, 77, and 4 K). Once complete, the testing station will allow researchers to relate effects of different microstructures arising from manufacturing pathways, including annealing processes, crystal orientations and microstructure characteristics (e.g. welds) to the resulting mechanical properties at ...

Tensile tests of niobium material for SRF cavities

Mechanical tests of cavity-grade niobium samples were conducted to provide engineering information for the certification of 3rd-harmonic superconducting radio-frequency cavities and cryomodules. Large changes of mechanical properties occur throughout the cavity fabrication process due to the cold work introduced by forming, the heating introduced by electron beam welding, and the recovery of cold work during the anneal used to degas hydrogen after chemical processing. Data is provided here to show the different properties at various stages of fabrication, including both weld regions and samples from the bulk niobium far away from the weld. Measurements of RRR were used to assure that any contamination during annealing was negligible.

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.

Deformation mechanisms, defects, heat treatment, and thermal conductivity in large grain niobium

The physical and mechanical metallurgy underlying fabrication of large grain cavities for superconducting radio frequency accelerators is summarized, based on research of 1) grain orientations in ingots, 2) a metallurgical assessment of processing a large grain single cell cavity and a tube, 3) assessment of slip behavior of single crystal tensile samples extracted from a high purity ingot slice before and after annealing at 800 °C / 2 h, 4) development of crystal plasticity models based upon the single crystal experiments, and 5) assessment of how thermal conductivity is affected by strain, heat treatment, and exposure to hydrogen. Because of the large grains, the plastic anisotropy of deformation is exaggerated, and heterogeneous strains and localized defects are present to a much greater degree than expected in polycrystalline material, making it highly desirable to computationally anticipate potential forming problems before manufacturing cavities.