On the role of pre-existing, unhealed cracks on the bending strain response of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract

Studies of the transport critical current (Ic), magnetization, magnetic flux penetration, and microstructure of pressed and rolled Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox tapes (2223) have been made as a function of bending strain. Pressed tapes exhibited markedly less degradation of Ic from strain than did rolled tapes, while the magnetization of pressed tapes declined much more rapidly with bending strain than did either of the transport currents. Magneto-optical imaging of nonbent pressed samples revealed a network of flux-penetrated defect channels that were primarily oriented parallel to the tape axis. Bending such samples to a small strain increased the visibility of these defects, believed to be cracks. This network correlates well to the cracks produced in intermediate thermomechanical processing deformation steps. The greater sensitivity of the transport current of rolled samples to bending is further direct proof of the fact that the tape “remembers” the cracks induced in the core during intermediate deformation and that heat treatment after the deformation does not heal all damage.