Fracture mechanics behavior of a NiFe superalloy sheath for superconducting fusion magnets. Part 1. property measurements

A seamless extruded conduit for superconductor cabling was fabricated and subjected to mechanical tests. The conduit is made of a nickel-iron alloy having aging and thermal contraction characteristics comparable with Nb3Sn conductors. The conduit in liquid helium at 4 K retains its ductility and offers high strength, toughness, and fatigue resistance. Specimens with surface cracks in tension offer substantial fracture resistance for the practical case of crack propagation in the through-wall direction. Fatigue tests indicate that surface cracks adopt a nearly semicircular shape as they grow through the conduit wall (L-S orientation) at rates in the power-law region that are no faster than rates in the transverse direction (L-T orientation). The serviceability of this material is discussed.