Fatigue behaviour at room temperature and its influence on superconducting properties at 4.2 K of multifilamentary Nb₃Al composite wire

Fatigue behaviour at room temperature and its influence on superconducting current and upper critical magnetic field at 4.2 k were studied using Nb₃Al multifilamentary composite wire with a copper ratio of 1.55 and an overall diameter of 0.812 mm in which 241 Nb₃Al filaments are embedded. The Nb₃Al filament itself was not fractured by the fatigue when tested separately. In the composite, the fatigue cracks nucleated in the copper, which grew and caused the fracture of the Nb₃Al filaments. When the maximum stress in the fatigue test was low, one crack among many grew preferentially; when the maximum stress was intermediate, many cracks could grow at different cross-sections before the overall fracture of the composite; when the maximum stress was high, the composite was fractured before large fatigue crack growth. Because of such a maximum stress dependence of the growth of the fatigue damage, the critical current measured using the segments of the fatigue-fractured specimens for the intermediate maximum stress range was lower than that for the low and high maximum stress ranges. The mode I fracture toughness of the present composite wire, estimated from the size of the strength-determining fatigue crack in combination with the maximum stress, was around 7 MPa m^1/2.