Improvement of mechanical and superconducting properties in CuNb/(Nb,Ti)₃Sn wires by applying bending strain at room temperature

The improvement of stress/strain properties has been observed for the CuNb/(Nb,Ti)₃Sn coil made by a react and wind (R&W) method. In the winding and insulation processes, a certain bending strain is applied to the wire and then released at room temperature, which is defined as 'pre-bending' treatment. In order to understand this behaviour, we examine the influences of bending strain at room temperature on the mechanical and superconducting properties in (Nb,Ti)₃Sn composite wire. Experimental results for the pre-bent CuNb/(Nb,Ti)₃Sn wires show the improvement of the stress dependence of I_c/I_cm and the increase of the critical current I_c at zero stress. The calculation on the basis of the distribution of the uniaxial local stress/strain describes the experimental results, qualitatively. We have found that the improvement of the critical current and the mechanical properties is due to the local work hardening effects by applying the bending strain at room temperature.