In order to understand the effects of strain on the superconducting properties in composite Nb3Sn wires and cables, the three-dimensional (3D) strain is very important. Quantum beams such as neutron and synchrotron radiation enable us to quantify the detailed internal strain in any direction nondestructively. Therefore, quantum beams are recognized as a powerful tool to evaluate the 3D strain inside composite materials. The internal strain states of Nb3Sn strands in thick conduits such as cable-in-conduit conductors can also be detected because of the large penetration depth of neutrons. Because of advances in neutron and synchrotron radiation facilities, recent studies have examined the internal strains in composite superconducting wire and cable conductors. This paper reviews recent studies on 3D strains and their effects on the superconducting properties of Nb3Sn wires and cable conductors, along with some experimental data. Other applications of quantum beams for superconducting wires are also introduced briefly.

https://iopscience.iop.org/article/10.1088/0953-2048/26/7/073001/pdf

Quantitative strain measurement in Nb3Sn wire and cable conductors using high-energy x-ray and neutron beams

We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb∕Nb3Sn wires at room temperature and at 7K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7K, respectively. From the obtained data, we estimated the deviatoric strain. At 7K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain epb=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative e...

Room and low temperature direct three-dimensional-strain measurements by neutron diffraction on as-reacted and prebent CuNb∕Nb3Sn wire

The strain induced degradation of Nb3Sn superconductors can hamper the performance of high field magnets. The authors report elastic strain measurements in the different phases of entire non-heat treated and fully reacted Nb3Sn composite strands as a function of uniaxial stress during in situ deformation under neutron beam. After the reaction heat treatment the Cu matrix loses entirely its load carrying capability and the applied stress is transferred to the remaining Nb–Ta alloy and to the brittle (Nb–Ta)3Sn phase, which exhibits a preferential ⟨110⟩ grain orientation parallel to the strand axis.

/pdf/in-situ-neutron-diffraction-under-tensile-loading-of-powder-2bdqjx25iv.pdf

In situ neutron diffraction under tensile loading of powder-in-tube Cu∕Nb3Sn composite wires: Effect of reaction heat treatment on texture, internal stress state, and load transfer

To prepare for projects such as the Large Hadron Collider upgrade, International Thermonuclear Experimental Reactor and Demonstration reactor, it is important to form a clear understanding of stress-strain properties of the materials that make up superconducting magnets. Thus, we have been studying the mechanical properties of superconducting wires using neutron diffraction measurements. To simulate operational conditions such as temperature, stress, and strain, we developed a cryogenic load frame for stress-strain measurements of materials using a neutron diffractometer at Japan Proton Accelerator Research Complex (J-PARC) Takumi beam line. The maximum load that can be applied to a sample using an external driving machine is 50 kN. Using a Gifford–MacMahon cryocooler, samples can be measured down to temperatures below 10 K when loaded. In the present paper, we describe the details of the cryogenic load frame with its test results by using type-304 stainless steel wire.

Development of a cryogenic load frame for the neutron diffractometer at Takumi in Japan Proton Accelerator Research Complex.

To demonstrate the applicability of the prebending strain effect to the R&W coil winding process, we have developed the prebent react-and-wind (R&W) process, which is the combination of the conventional R&W process and the prebending treatment. The superconducting and mechanical characteristics were investigated for CuNb/Nb3Sn superconducting wires which were picked up from the prebent R&W coil winding process. The results were compared with that of CuNb/Nb3Sn wires which were prebent manually using a prebending former. The prebending treatment using pulleys enhanced Ic more than the manual prebending treatment. This is explained qualitatively by a difference of the prebending strain distribution. The prebending strain distribution is plain symmetric and axis symmetric in cases of the manual prebending and the pulleyed prebending, respectively. The axis symmetrically distributed prebending strain reduces the residual strain more than the plain symmetrical one. The mechanical property was also enhanced. The slope of the stress-strain curve for the pulleyed prebent samples were steeper than that of as-heat-treated sample. The maximum Ic of the Ic -strain curve was enhanced and the strain value corresponding to the maximum Ic shifted to smaller strain by the pulleyed prebending treatment. Though in the large stress region over 200 MPa, the Ic-stress curves of pulleyed prebent samples and as-heat-treated sample were in good agreement, suggesting that the Ic enhancement effect by the prebending treatment was not beneficial, in the small stress region, it is quite beneficial

Prebending Strain Effect on $rm CuNb/Nb_3rm Sn$ Superconducting Wire During Practical React-and-Wind Process

The critical current, upper critical field and critical temperature of bronze route Nb3Sn commercial wires are enhanced by applying a repeated bending strain at room temperature, i.e., "prebending strain". In order to investigate the prebending effects from a viewpoint of a residual strain, axial and lateral residual strains were evaluated directly by neutron diffraction at room temperature. We found that the axial residual strain changes from -0.10% to 0.02 % but the lateral one is unchanged by applying a prebending strain of 0.5% for an ordinary bronze route (Nb,Ti)3Sn wires without reinforcement. Hence, in the case of the ordinary Nb3Sn wires without reinforcement, the prebending treatment modifies only the axial residual strain states independently to the lateral one, although it may depend on the wire structure. The critical current properties under the axial tensile strain suggest that the axial residual strain is reduced by about 0.11% but the radial residual strain unchanged by the prebending treatment of 0.5%. This is consistent with the results of the neutron diffraction

S. Harjo

Papers

Neutron Diffraction Study on Prebending Effects for Bronze Route $rm Nb_3rm Sn$ Wires Without Reinforcement