Scanning electron microscopy observations have been made on fracture surfaces induced by tensile tests of Ag-added SmBa2Cu3Oy/Sm2BaCuO5 composite bulk superconductors melt-grown in O2 and 1%O2/Ar atmospheres. It was deduced that the inferior tensile strength in the bulk grown in l%O2/Ar, as compared with that of the bulk grown in O2, was caused by the larger size and volume fraction of the included voids. The tensile strength of a specimen with a fracture surface consisting of multiple facets was lower than that with a single facet for the bulks grown in both conditions. It was found that a rectangular facet on the fracture surface is a possible origin for the fatal crack and that the secondary phase particles on that area act as anchoring points for initial crack propagation.
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