Abstract
Melt-textured YBa2Cu3O x (123) superconductors have been shown to possess large critical current densities J c when the current is flowing along the strongly superconducting a-b planes. In order to further enhance J c, the number of effective flux pinning centres has to be increased. Whether or not twin boundaries are effective pinning sites is of particular interest since twins are inherent defects found in 123. In this investigation, the twin structure of 123 have been modified by mechanical deformation. It is found that the twin density has been increased by deformation processing, and secondary as well as tertiary twins have been generated to relieve the higher level of stresses. In addition, complex twin features and extensive twin interactions are seen in the deformed superconductors. Transport J c measurements, however, revealed that J c does not increase monotonically with decreasing twin spacing, thereby indicating that twin boundaries are not dominant pinning centres at moderate magnetic field strengths.