EVOLUTION OF THE MICROSTRUCTURE DURING HIGH-TEMPERATURE CREEP AND OXYGENATION IN DIRECTIONALLY SOLIDIFIED YBA2CU3O7-X

The microstructure of YBa2Cu3O7-x-Y2BaCuO5 melt-textured composities d eformed in the secondary and tertiary creep regimes has been investiga ted by transmission electron microscopy. The high density of Y2BaCuO5 precipitates plays an important role in the microstructural developmen t as pinning sites for gliding dislocations. In the secondary regime, trapped dislocations are dissociated, leaving a stacking fault with di splacement vector [1/2 - delta 0 1/3]. second stacking fault, 1/6 [301 ] is typically associated with the former stacking fault. At this stag e, the deformation microstructure is dominated by diffusive processes between precipitates interconnected by the trapped dislocations. In th e tertiary stage, dislocation multiplication is the main factor contro lling the microstructure. which is characterized by a dramatic increas e in the density of perfect dislocations with Burgers vectors [100] an d [110]. Since deformation is performed above the orthohombic-to-tetra gonal transition temperature, the samples need to be oxygenated in ord er to achieve the superconducting phase. We have found that this oxyge nation step, performed at 450 degrees C, induces severe modifications of the as-deformed microstructure.