We have investigated the microstructures of large-grain RE123 (RE = Y, Sm) bulk superconductors with an emphasis placed on the effects of subgrains on superconducting properties. For the samples grown by slow-cooling, the misorientation angles of the subgrains were larger at the later stage of the growth. For isothermally grown samples, subgrains are likely to form under large undercooling. Based on the results of micro-area x-ray pole figure analyses, the possible mechanism of subgrain formation was concluded to be the accumulation of dislocations formed as a result of entrapment of RE211 inclusions during crystal growth. Both magnetization and transport measurements showed that the subgrains act as weak-links. Hence the amount of subgrains should be minimized for the fabrication of high performance large-grain RE 123 materials.
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