We have studied the microstructures of subgrains in large single-domain RE-Ba-Cu-O bulk superconductors and their effects on superconducting properties. Subgrains with high misorientation angles were seen at the positions of the later stage of melt growth, or the positions where the second-phase particles are large in size and volume. An electron backscattering diffraction technique revealed that the orientation of the second-phase particles affects the orientation of the matrix phase, some of which forms the subgrain boundaries. Electric and magnetic measurements have been performed for several samples having different subgrain geometries and the results of I-V and resistivity measurements indicated that subgrains act as weak links, especially in a high magnetic field region.
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