Effect of cryogenic deformation on microstructure and critical current density in Ag/Bi-2223 tapes

The effect of deformation rate and silver hardness on J(c) and microstructu re of Ag/Bi-2223 tapes was studied under cryogenic processing conditions. I t was found that cryogenic deformation improved the grain connectivity, ali gnment,;Ag/core interface and critical current density, These benefits may be attributable to the increased extent of mechanical deformation, The hard ness of silver sheath during cryogenic mechanical deformation was significa ntly increased, allowing for a much larger load to be applied during the de formation process comparing to the mechanical deformation at room temperatu re. The maximum J(c) appeared at a 20% reduction rate of the tape thickness for cryogenically processed Ag/Bi-2223 tape. Higher deformation rate decre ases Je for cryogenically processed Ag/Bi-2223 tape which can be explained by the results of TEM examination. TEM images show that the sample having l ow J(c) had many small grains with large angle grain boundaries after recry stallisation when high rate of thickness reduction (58%) was applied during the process, while the sample having high J(c) had large grain with many s mall angle c-axis twist grain boundaries and small angle tilt grain boundar ies when 19% of thickness reduction was applied during the process.