Interfaces and microstructural defects in broad face and longitudinal cross-sectional specimens of Ag-sheathed tape were studied by means of transmission electron microscopy. It was found that most of the twist boundaries in the colonies had a small rotation angle, which was accommodated by a screw dislocation network. The twist boundaries were usually atomically flat; with no second phase. Most of the colony boundaries were of mixed type, with a misorientation angle of up to 30. A thin amorphous layer usually formed at colony boundaries, with adjacent basal planes on both sides of the boundary. Most of the c-direction colony boundaries were of mixed type, and second phases were sometimes observed at these boundaries. Tilt boundaries, which consisted of a wall of edge dislocations, were clean and the lattices on both sides of the boundary were well matched. Second phases were often found at the triple colony junction. Small colonies were found, randomly oriented and seriously distorted, around large isolated particles of second phase. Bending and interruption of the (001) planes, and [001] edge dislocations, were often observed. The results indicated that the predominant weak links in the tapes were colony boundaries, and that the critical current density was controlled by colony boundaries in low fields.

R.K.Wang, H.K.Liu, S.X.Dou: Superconductor Science and Technology, 1995, 8[3], 168-73