The effect of strand architecture on the fracture propensity on niobium-tin composite wires

Nb3Sn is a brittle superconducting material used to carry electric current in large fusion and high energy physics magnets. Usually fabricated in the form of a composite multifilamentary wire, Nb 3Sn strands can be susceptible to filament-level fracture under Lorentz forces during magnet operation. This can lead to degradation of the critical electric current density and subsequent underperformance of the magnet. To understand the effect of strand architecture (filament size, spacing, and arrangement) on fracture propensity, we have developed a metallographic technique to image Nb3Sn strands in longitudinal cross-section after mechanical and/or electrical testing. Deformation conditions included pure bending at 77 K, uniaxial tension at 4.2 K, electromagnetic (TARSIS) testing at 4.2 K, and microindentation at room temperature. Under bend strain, ITER-style strands exhibit primarily individual filament cracking, with some collective cracking occurring in internal tin strands that have agglomerated filaments near the sub-bundle core. This cracking occurred primarily at local bend strain values above 0.7% for strands with reacted filaments of 3 -- 4 mum diameter, and at local bend strain values above 0.4% for strands with reacted filaments of 6 mum diameter. The high energy physics strands, by contrast, exhibited long, collective cracks under bending that propagated across entire sub-bundles and even from sub-bundle to sub-bundle, beginning at the tensile bend axis and ending near the geometric neutral bend axis. Additionally, the fracture morphology is shown to be constant for a wide variety of HEP strands, with critical current density values ranging from 1700 A/mm2 to 3000 A/mm2.;Under indentation testing, crack propagation was shown to be a strong function of the local strain state, with even small tensile stresses producing extended fracture fields, while indents in a region under compressive stress showed almost no fracture at all. The clear implication is that the local stress state is the most critical parameter to be controlled for preventing fracture in the conductor. These findings should provide significant guidance to the Nb3Sn wire manufacturing community as they develop more fracture-resistant strands for magnet applications.