Dislocation substructures and phase stability of melt-textured YBa2Cu3O7-Y2BaCuO5 composites at high oxygen pressures

The evolution of the microstructure of melt-textured YBa2Cu3O7-Y2BaCuO5 (123-211) composites in O-2 at 100 bar and 600degreesC has been characterized by transmission electron microscopy (TEM). Under these conditions the 123 phase is brought out of its stability field, within a region of the phase diagram where the weakly superconducting YBa2Cu4O8 (124) polytype is stable against a CuO + 123 mixture. The nucleation of 1/6 [301] stacking faults is not restricted to the 211-123 interfaces, as found in samples processed at 350 and 450degreesC but is strongly enhanced in the bulk matrix. Simultaneously, matrix decomposition into a BaCu oxide, 211 and presumably CuO occurs at 211 interfaces, which is accompanied by an exaggerated growth of the stacking faults. After 12 h at PO2 = 100 bar and 600degreesC, the whole volume imaged by TEM appears to be covered by large stacking faults overlapping along the c axis. It is found that, when the 211 interfaces become saturated by stacking faults, new stacking faults have to nucleate in the bulk matrix. Strikingly, such isolated loops appear to be arranged along lines parallel to [100], close to neighbouring straight dislocations and associated with twin boundaries. The observed processes give some insights into novel routes towards the engineering of microstructures for high critical current bulk-scale 123-type superconductors.