Effect of High Temperature Deformation on Grain Boundary Defects in Yba 2 Cu 3 O 7-X

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EFFECT OF HIGH TEMPERATURE DEFORMATION ONGRAIN BOUNDARY DEFECTS IN YBa 2 Cu3O7_x R. GARCIA, W.Z. MISIOLEK, R.N. WRIGHT AND K. RAJAN Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 ABSTRACT The results of transmission electron microscopy studies on YBa 2 Cu3O7_x subjected to high temperature extrusion are presented. Particular emphasis is put on high temperature accommodation processes of lattice dislocatirns into sub-grain boundaries. It is suggested from the electron microscopy observations that stress induced climb mechanisms for dislocations are operative. Also presented is evidence of localized lattice distortions near twin boundaries due to isothermal high temperature deformation. Preliminary results on the structure of grain boundary facets and steps is described. The implications of these results for texture development in bulk ceramic superconductors is also discussed. INTRODUCTION The development of superconducting materials depends not only on achieving as high a transition temperature as possible, but also on the ability of the material to carry large critical currents as well as its ability to be fabricated into forms useful for application. Recent work has demonstrated that with highly textured thin films or bulk materials, much higher critical currents can be achieved. These results indicate that grain boundaries and other internal interfaces (e.g. twin boundaries) can play a crucial role in determining the properties of high Tc superconductors. Thus an understanding of the structure of these boundaries and their interaction with defects (e.g. dislocations) is of fundamental importance for the development of practical applications of high Tc oxides. In previous papers [1-4] we have noted the importance of grain boundaries and twin boundaries in aiding plastic deformation at high temperatures as well as superconducting properties. In this paper it is shown that YBa2Cu3O7_x may exhibit extensive microplasticity at high temperatures. EXPERIMENTAL The YBa2Cu3O7_x starting material was prepared by the solid state technique. Superconducting powder ball milled for eighteen hours was hot extruded at 850 0C. The procedure involved pressing powder into a 13 mm internal diameter copper (or silver plated copper) tube, with the tube being located in a carbon steel bar of 51 mm outside diameter and 15 mm inside diameter. The billets were extruded on an 11.1 MN(1250 ton) Loewy extrusion press. They were prepared for extrusion using an 0.35 MPa grit blast hand degreaser and preheated in argon to the nominal extrusion temperature of 0850'C. The 5.40 cm diameter extrusion chamber is, itself, preheated to 315 C. Glass lubrication was used for the extrusion, and the billets were cooled in sand to minimize thermal stress development and related cracking. For the purpose of this study, extrusions at two different strain rates (2.6 and 6.4) were examined.

Mat. Res. Soc. Symp. Proc. Vol. 169. c1990 Materials Research Society

828

Figure 1. Interaction of dislocations with subgrain b