Michigan State University, East Lansing
In the past decade, use of high purity Nb has enabled high Q (efficiency) values to be achieved in superconducting radio frequency (SRF) cavities, which enables design of more powerful accelerators and new concepts for benchtop accelerators. Fundamental understanding of the physical metallurgy of Nb that enables these achievements is beginning to shed light on where the challenges lie for reproducible and cost-effective production of high performance SRF cavities. Recent studies of dislocation substructure development and effects of recrystallization arising from welding and heat treatments and their correlations with Q are reviewed. Clearly, microstructural control throughout the manufacturing process is needed to insure that choices made for formability of complex shapes will also lead to optimal function of the cavity. With better fundamental understanding of the effects of dislocation substructure evolution and recrystallization on electron and phonon conduction, as well as the interior surface state, it will be possible to design optimal processing paths for cost-effective performance using approaches such as hydroforming, which minimizes or eliminates welds in a cavity.
Michigan State University, East Lansing
NSCL, East Lansing, Michigan
MSU, East Lansing, Michigan
Superconducting radio frequency (SRF) cavities are often formed from fine-grain niobium sheet produced by forging and rolling of ingots that contain very large grains. Texture heterogeneity is consistently observed in such sheets, and is related to heterogeneous deformation and recrystallization of the large grains in the ingot. Relationships between the deformed, recovered, and recrystallized microstructures are required to eventually produce a homogeneous recrystallization texture. Effects of dislocations and grain boundaries on recrystallization and recovery are examined in several kinds of samples: Differently oriented single crystal and bicrystal samples deformed by uniaxial tension and heat treated, and a bicrystal rolled to various reductions and then heat treated. Active slip systems, dislocation substructures, recovery, and recrystallization will be examined by orientation imaging microscopy at incremental stages of deformation, and compared with crystal plasticity model predictions, to determine the influence of dislocation substructure on the orientations of recrystallized grains.