Improved Nb For Superconducting RF Cavities

79394B Superconducting radio frequency (SRF) cavities, such as those fabricated at Jefferson Laboratory and used in charged particle linear accelerators, are formed by deep drawing. In this process, a non-uniform or poorly textured microstructure in the starting sheet can cause poor deformation behavior and surface roughness in the formed part. Poor deep drawing behavior is manifested by excessive thinning, unpredictable cavity shape "spring-back", or the development of "orange-peel" surface roughness, all of which can degrade SRF cavity and linear accelerator performance. In this project, severe plastic deformation (SPD) via equal channel angular extrusion (ECAE) will be used to refine and homogenize the microstructure of bulk Nb, which will then be rolled to sheet for fabrication into SRF cavities. By carefully selecting the deformation and heat treatment conditions during the processing steps, we expect to be able to manufacture Nb sheet with a fine and uniform grain size, preferred texture and improved formability for subsequent fabrication into SRF cavities. Phase I will: (1) refine the microstructure of bulk Nb by several different SPD routes, (2) fabricate 2-3 mm thick Nb sheet from the ECAE processed bulk by rolling, and (3) conduct mechanical and microstructural characterization of the bulk and sheet Nb to determine which sequence of steps and fabrication conditions gives the best microstructure for deep drawing formability. Comparisons with commercial Nb sheets will be made to evaluate success. Commercial Applications and Other Benefits as described by the awardee: In addition to the fabrication of superconducting radio frequency (SRF) cavities, the technology should be directly applicable to the fabrication of other improved sheet materials, including Ta and steel.