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Reversibility of micro-yielding and critical current in a YBCO-coated conductor caused by a uniaxial tensile load

The deformation behaviour of a YBa2Cu3Ox (YBCO)-coated conductor based on Ni–W substrate and its influence on the critical current were precisely investigated. From a room temperature tensile test, the change of permanent strain as a whole of the coated conductor was divided into two regions of micro- and macroscopic yielding. The precise lattice constant measurement by synchrotron radiation at Spring8 revealed that the permanent strain in the YBCO layer was characterized by two regimes related just to the two regions of yielding mentioned above. In the micro-yielding region, the permanent strain in the YBCO layer was identical with that in the metallic components, suggesting that the YBCO layer behaves elastically and any damage-like cracks were not introduced. In the macroscopic yielding region, the smaller permanent strain in the YBCO layer compared with the metallic components suggested that damage-like cracks were introduced in the YBCO layer during loading and remains under zero external load. There the invariant of total length is accommodated by the existence of damage-like cracks. The reversibility of the critical current was evaluated by loading/unloading experiments. The permanent strain ensuring the reversible limit and the corresponding applied strain were found to be 0.17% and 0.52%, respectively. This reversible limit coincided with the limit of micro-yielding. Furthermore the irreversible degradation of the critical current as a function of strain is suggested to relate to the macroscopic yielding.