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

Abstract

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 Lüders 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 Lüders bands. The similarities in the initiation site and the angle to the loading axis between the Lüders 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.

Preliminary investigation of Ic homogeneity along the longitudinal direction of YBCO coated conductor tape under tensile loading

“…For YBCO CC tapes without stabilizers, it was reported that annealed Hastelloy showed discontinuous yielding [13]. However, the I c degradation shown for the stabilized YBCO CC tape tested in this study, represented that the surround copper stabilizer may have contributed in the homogeneous I c degradation even with such behaviour of the Hastelloy substrate.…”

“…If the critical tolerance stress/strains of I c in CC tapes depend on the mechanical properties such as the yield stress/strain or proportional stress/strain limit, then characterization would be easy. But there are cases when the irreversible strain limit could be located prior to yielding of the substrate material, especially in the case of Ag-coated CC tapes [12][13]18]. What makes the characterization complicated is due to the multi-layered structure of the CC tapes.…”

“…Several groups have already investigated the strain dependence of I c in YBCO CC tapes under tensile, compressive, bending and torsion strain modes respectively [9][10][11][12][13][14][15][16][17][18]. It was observed that the I c behaves reversibly up to an irreversible strain limit, ε irr.…”

Review of progress in electromechanical properties of REBCO coated conductors for electric device applications

“…It is widely accepted that this significant irreversible degradation of I c in CC tapes is attributed to the cracking of the superconducting film due to its innate brittleness. Hence, for engineering purposes we need to measure the maximum strain that can be applied before the superconductor breaks [11][12][13].…”

Thermal behavior of 2G HTS tape for use in resistive fault current limiters

“…According to the engineering requirements to such a system, the surge and steady values of a fault current should not exceed a maximum permissible value. The high temperature superconducting tapes of the second generation (2G HTS) which have been shown to withstand high stress and strain without degradation [1,2] are seem to be the most suitable material for this application. Therefore, detailed studies of the electric and thermal characteristics of these tapes with multiple current overloads are needed.…”