The mosaic structure of single domain YBa2Cu3O6+x/Y2BaCuO5(123/211) directionally solidified composites is investigated by x‐ray diffraction and transmission electron microscopy. This mosaic structure results from a polygonization process before the oxygenation step. A topological model is given based on the observed distribution of preferred subgrain boundary planes. This polygonization, characteristic of melt textured 123, is proposed to be an intrinsic critical current density limitation for these materials. ©1996 American Institute of Physics.