Prebending Effect on Three-Dimensional Strain in ${\rm CuNb}/({\rm Nb},\ {\rm Ti})_{3}{\rm Sn}$ Wires Under a Tensile Load

Internal strains under a tensile load for CuNb/Nb₃Sn wires with and without the prebending treatment were measured directly by neutron diffraction at room temperature and 7 K. The axial residual strains changed by 0.30% at room temperature and by 0.38% at 7 K to the tensile side due to the prebending treatment. The effective Poisson's ratio evaluated from the relationship between axial and lateral strains under an axial stress was about 0.36 at 7K with or without the prebending treatment. The applied tensile strain dependences of the deviatoric and hydrostatic strains were estimated, and the tensile strain dependence of the superconducting properties were discussed on the basis of the three-dimensional strain model. A control of the residual strains such as the prebending treatment plays an important role in the superconducting properties of wires under axial stress/strain.

I. Introduction

It is well known that the superconducting properties of wires are very sensitive to stress and strain [1]. The strong strain/stress sensitivity is a problem for the application of wires. In particular, the superconducting properties are deteriorated by the compressive residual strain due to the thermal contraction difference between and the other materials composing of the wire. Meanwhile, it has become known that the superconducting properties of wires can be improved by a mechanical treatment at room temperature [2]–[4]. Neutron diffraction measurement revealed that the residual strains of the axial and the lateral directions were reduced by the prebending treatment (the repeated bending cycle at room temperature) [5]. The reduction of the residual strain enhances the superconducting properties of wires in the prebending effect.