Evolution of the microstructure during high-temperature creep and oxygenation in directionally solidified YBa₂Cu₃O_7-x

Abstract

The microstructure of YBa₂Cu₃O_7−^x −Y₂BaCuO₅ melt-textured composities deformed in the secondary and tertiary creep regimes has been investigated by transmission electron microscopy. The high density of Y₂BaCuO₅ precipitates plays an important role in the microstructural development as pinning sites for gliding dislocations. In the secondary regime, trapped dislocations are dissociated, leaving a stacking fault with displacement vector [½—δ 0 ⅓]. A second stacking fault, ⅙〈301〉, is typically associated with the former stacking fault. At this stage, the deformation microstructure is dominated by diffusive processes between precipitates interconnected by the trapped dislocations. In the tertiary stage, dislocation multiplication is the main factor controlling the microstructure, which is characterized by a dramatic increase in the density of perfect dislocations with Burgers vectors 〈100〉 and 〈110〉. Since deformation is performed above the orthohombic-to-tetragonal transition temperature, the samples need to be oxygenated in order to achieve the superconducting phase. We have found that this oxygenation step, performed at 450°C, induces severe modifications of the as-deformed microstructure.