Elucidation of the critical current enhancement mechanism due to the repeated bending treatment for high-strength Nb_3Sn wires

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Research subjective was to understand the basic mechanism for the critical current enhancement effect due to the applying repeated bending strain at room temperature fur pre-reacted NbaSn strands and to find the important guiding principle for the manufacturing process of NbaSn strand cables, employing pre-reacted NbaSn strands.To apply the critical current enhancement effect for NbaSn strands with the repeated bending strain treatment, a test oil was prepared by NbaSn strands reinforced with CuNb composite, and the critical current enhancement was confirmed in the test coil applied by a large hoop stress. The SEM observation was carried out for NbaSn strand-directional filaments etched out from the repeated-bending-treatment sample. It is found that the repeated bending treatment introduces micro-crack defects into NbaSn filament cores, and that such micro-crack defects do not increase until a tolerated strain limit of 0.8 %, in the case of NbaSn strands with. CuNb reinforcement in the outer portion. The NbaSn strands with and without the prebending treatment were measured by the neutron diffraction, which is a high resolution and high intensity powder diffractmeter at KEK in Tsukuba. The residual strains for the prepared NbaSn strands with and without the prebending treatment reveal the clear difference in the lateral direction between the two. We succeeded in indicating the residual strain reduction for the repeated bend NbaSn directly by the neutron diffraction. This means that the important understandings for the critical current enhancement elect of Nb_3Sn strands were obtained.The three + four (3+4) strand cable Consisting of three Nb_3Sn strands and four stainless steel strands were fabricated to develop new NbaSn stand cables applying the critical current enhancement effect It is found that the prepared 3+4 strand cable surely maintains such enhancement in the practical strand cabling process.