Residual microstresses, a-b microcracking and twin structure in oxygen controlled melt grown NdBaCuO superconductors

Microstructure analysis by polarized light microscopy has shown that Nd4Ba2Cu2O10 (422) particles in oxygen controlled melt grown NdBaCuO bulk superconductors have similar influence on the NdBa2Cu3O7-x (123) microstructure as 211 particles in the Y123-211 system. The twin spacing is narrowed by decreasing the mean distance between 422 particles. a-b microcracking in the 123 matrix was ascribed to the thermal stresses around 422 particles set up in the 123 matrix during cooling due to the differential thermal expansion of 123 and 422 phases. The relaxation of thermal stresses in the < 100 > directions by twin boundary motion was evidenced by the observed rosette of detwinned regions or regions with a predominance of one twin variant formed around 422 particles. The level of thermal stresses was estimated.