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/sub c/ and microstructure of Ag/Bi-2223 tapes was studied under cryogenic processing conditions. It was found that cryogenic deformation improved the grain connectivity, alignment, Ag/core interface and critical current density. These benefits may be attributable to the increased extent of mechanical deformation. The hardness of the silver sheath during cryogenic mechanical deformation was significantly increased, allowing for a much larger load to be applied during the deformation process comparing to the mechanical deformation at room temperature. The maximum J/sub c/ appeared at a 20% reduction rate of the tape thickness for cryogenically processed Ag/Bi-2223 tape. Higher deformation rate decreases J/sub c/ for cryogenically processed Ag/Bi-2223 tape which can be explained by the results of TEM examination. TEM images show that the sample having low J/sub c/ had many small grains with large angle grain boundaries after recrystallisation when a high rate of thickness reduction (58%) was applied during the process, while the sample having high J/sub c/ had large grains with many small angle c-axis twist grain boundaries and small angle tilt grain boundaries when 19% of thickness reduction was applied during the process.