Oxygenation and defect generation processes in YBa2Cu3O7-delta bulk superconductors prepared by a melt process

Defect formation processes during oxygenation were investigated in a melt-processed YBa2Cu3O7-delta bulk superconductor. Oxygenation at 550 degrees C resulted in a formation of tweed-like twin with some microcracks. Non-uniform phase transition occurred for the specimen annealed at 450 degrees C as well as the microcracking by an oxygen-induced stress. It is suggested that there was a critical stress, sigma(t-c), where the stress relief mode was changed from twin formation into crack formation. Below sigma(t-c), the stress was relieved by a formation of twins. Above sigma(t-c), the stress was relieved by a generation of microcracks.