Tensile fracture behaviour of RE-123 coated conductors induced by discontinuous yielding in Hastelloy C-276 substrate

We investigated the intrinsic strain effect on critical current, Ic, and tensile fracture behaviour in the RE-123 coated conductors (RE =  Y, Dy or Sm) with Hastelloy C-276 substrates. It is found that the SmBCO coated conductors exhibit an intrinsic strain effect on Ic which is similar to that in the DyBCO ones. Ic decreases monotonically with increasing applied tensile strain but it can recover reversibly up to the strain of 0.32% when the strain is relieved. Quenching occurred for the coated conductors with Hastelloy C-276 substrate in the narrow strain region of 0.27–0.35% regardless of the buffer layers and superconducting materials. In order to reveal the reason for such quenching, we measured mechanical properties of the Hastelloy C-276 tapes alone and coated conductors at 77 K. The as-received and as-polished Hastelloy tapes show continuous yielding, which results from homogeneous plastic deformation. On the other hand, the Hastelloy tape annealed at 963 K corresponding to the deposition condition for the superconducting layer exhibits discontinuous yielding. In the annealed Hastelloy tapes, occurrence of Luders bands is observed, indicating that plastic deformation progresses inhomogeneously. Discontinuous yielding is also confirmed in the coated conductor. The yield strains of the coated conductors are almost identical to the strains at quenching. Characteristic fracture behaviour is observed, that transverse crack arrays are initiated from the tape edge and they propagate into the central part. The crack arrays orient to a certain angle to the loading axis, which is similar to that of the Luders bands. The similarities in the initiation site and the angle to the loading axis between the Luders bands and the crack arrays and close values of quenching strains to the yielding one strongly suggest that quenching is attributable to the discontinuous yielding in the Hastelloy substrate.