Relationship between architecture, filament breakage and critical current decay in Nb₃Sn composite wires repeatedly in-plane bent at room temperature

Six Nb₃Sn composite wires with different architectures ('central and near-the-edge reinforcement') were repeatedly in-plane bent at room temperature (in-plane 'pre-bending'). Breakage behaviour was revealed from scanning electron microscopy observations by semi-quantitative analysis of the filament crack formation and evolution. Cracks are formed in the transversal and longitudinal directions. Transversal cracks show some tolerance to the applied bending strain due to the fact that filaments are composite materials; residual Nbcore can arrest development of a partial transversal crack initiated in the Nb₃Sn outer part of the filament. Together with the density of cracks C and the evolution of this parameter with pre-bending strain, ε_pb, in different regions of the wire, R-ε_pb curves are important to understand breakage behaviour of the wires. R is the ratio (number of full transversal cracks)/(number of full transversal cracks+number of partial transversal cracks). ParametersC and R allow us to reveal and satisfactorily understand the wire architecture - breakage - critical current decay relationship when pre-bending treatment is applied. As a consequence, breakage criteria necessary to minimize I_cdecay were defined and the positive influence of the reinforcement in preventing breakage was observed. It was also found that, in this regard, more Nb in the CuNbreinforcement, for the investigated wires, is better, if the heat treatment for the wire synthesis is performed at 670 °C for 96h. A different heat treatment, 650 °C for 240h, is less efficient in preventing filament breakage. Our results suggest the possibility of control and improvement of the breakage susceptibility of the filaments in the wires and, hence, of the bending I_cdecay, through the wise design of the wire architecture (i.e.by correlating design with the choice of composing materials and heat treatments).