High critical current density (J c ) Nb 3 Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a J c (12 T, 4.2 K) up to ~2500 A/mm 2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible J c must be addressed by forming the maximum fraction of high J c small-grain (SG) A15 and minimizing low J c large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial J c improvement.
An international RRT has been carried out in order to establish the test method for mechanical properties of
commercial Nb3Sn
superconductive wires under the cooperation of eleven worldwide research groups. From the
stress–strain curve, the following quantities were evaluated; modulus of elasticity, transition
of elastic to plastic deformation, proof strength, tensile strength and elongation to fracture.
The scatter of measured values was analyzed to evaluate the COV, which is the standard
deviation divided by the average. The results made clear how the experimental conditions
influence the determination of physical quantities. The most important point is
that large COVs for modulus of elasticity and proof strength from the initial
slope are caused by the narrow elastic limit. Methods have been discussed to
improve the statistics of experimental results obtained from the international RRT.
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