Manufacture of Fine-Grained Niobium Bar

For advanced magnets used in high energy physics research, commercially available NB3Sn superconductor wire needs to have smaller superconducting filament regions, while retaining a high critical current density (Jc). However, the currently available Nb bars used in such conductors have a large grain structure, which cause poor deformation behavior as the Nb is worked into fine filaments in Nb-Sn-Cu composite wire. When isolated Nb filaments deform irregularly, they can break or contact other nearby filaments, which decreases conductor performance. In this project, the problem of large grains in the starting Nb will be addressed by breaking up the original coarse and nonuniform microstructure with carefully applied plastic deformation and annealing. These procedures can be applied via Equal channel angular extrusion (ECAE) at high efficiency and reasonable cost. The approach demonstrated in Phase I included (1) processing bulk Nb to a fine-grained microstructure, (2) observing improved deformation behavior of grain-refined Nb in Cu-Nb composite wire, and (3) demonstrating a high yield of fully-worked material for multipass ECAE processing of single bars. In Phase II, the equipment for ECAE processing will be scaled up, tested, and evaluated for economic viability. The larger equipment will be used to produce large fine-grained Nb bars for prototype Nb3Sn conductor experiments. Commercial Applications and other Benefits as described by the awardee: The technology should be suitable for the commercial production of advanced high Jc, fine-filament Nb3Sn wire for high energy physics and fusion energy applications. In addition, the ECAE approach should be applicable to the fabrication of Ta, Ti, interstitial free steel, and superalloys, with improved mechanical properties for a host of applications.