The solidification process introduces defects into melt-grown REBa 2 Cu 3 O y (123) bulks, which then influence the overall superconducting and mechanical properties of the fabricated tiles. The most important microstructural features related to the growth process are the subgrain boundaries and the macroscopic inhomogeneity in the concentration of the RE 2 BaCuO 5 (211) particles. It is shown, in this paper, that the growth of the 123 bulks in the growth sectors is responsible for the macroscopic arrangement of these defects in a sample. A mechanism of subgrain formation, which is based on the formation of dislocations during the engulfing of the 211 particles by the growth of the 123, is proposed. Macroscopic residual stresses caused by 211 macroscopic inhomogeneity are estimated. The change of the macroscopic sample shape is associated with the liquid transport from the slower a-axis growth front to the faster c-axis growth front at the edge between the aand c-growth sectors.

In this review paper the results on the cracking phenomena in melt-grown 123 bulk superconductors are summarized. The reasons for cracking in this material are mechanical stresses, which arise in the sample during its fabrication. Two main sources of stresses appearing during fabrication were identified: the different thermal expansion coefficient of 123 and 211 phases and the dependence of 123 phase lattice parameters on the oxygen stoichiometry. The formation mechanisms of three basic types of cracks are characterized. The most typical are a/b-microcracks, which are observed as dense lines parallel to the a/b plane, and their length does not exceed some 211 interparticle distances. The a/b-microcracks are formed at 211 particles due to tangential tensile stress developed around each 211 particle during cooling from the crystallization temperature and during oxygen uptake. The second and the third type of observed cracks are so-called a/b-and c-macrocracks. They extend along larger sample areas and are formed under the combined influence of tensile stresses developed during sample oxygenation and stresses induced by 211 concentration macroinhomogeneity. Some possibilities of suppression of cracking are considered. According to our analysis it is possible to find oxygenation conditions under which the formation of the c-macrocracks or both c-and a/b-macrocracks is suppressed. Other possibilities of suppression of cracking such as oxygenation under uniaxial pressure, sample bandage and sample impregnation are discussed.

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