Behavior of Nb₃Sn composite wires: Multiple room temperature bending cycles

Two practical Nb₃Sn composite wires (short samples) were investigated from the points of view of critical current, I_c, and cracks evolution versus number of bending load cycles. Bending was applied manually at room temperature using a blockform with curved surface corresponding to applied bending strains of 0.5% and 0.8%. It was applied in plane, alternatively in one direction and in the opposite one up to 130 times. We shall use for this mechanical treatment term pre-bending. The wires had approximately the same diameter but a different architecture: one was of conventional type (standard US-Japan wire, Hitachi Cable) and the second one was CuNb reinforced wire (Furukawa Electric Co. Ltd.) with reinforcement located near-the-edge. It was found that critical current maximizes gradually with the number of bendings and the maximum value of I_c is attained for a number of bending loadings, N_pb, of about 15. Beyond this optimum N_pb critical current is constant. Optimum N_pb is likely not dependent on the wire, applied field or pre-bending strains. Results suggest that Cu is important in the work hardening process. Work hardening during pre-bending is imposing limitations in application of this technically convenient mechanical treatment for release of the residual strain and further enhancement of I _c. Semi-quantitative analysis of the electron microscopy observations shows that for our experimental conditions the density of cracks is approximately constant being in agreement with described Ic behavior. Other possible consequences of our results are discussed. Among the most interesting issues is possibility of the 3D independent control of yielding and hence of residual strain release. Several new practical ideas that will need future confirmation are proposed: e.g. application of loading-unloading cycles of torsion treatment in combination with tensile or pre-bending treatments might be useful.