Influence of the heat-treatment conditions, microchemistry, and microstructure on the irreversible strain limit of a selection of Ti-doped internal-tin Nb sub(3)Sn ITER wires

• Published in: Superconductor science & technology Vol. 27; no. 10; pp. 1 - 21
• Main Authors: Cheggour, N, Lee, P J, Goodrich, L F, Sung, Z-H, Stauffer, T C, Splett, J D, Jewell, M C
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Heat treatment; Microchemistry; Microstructure; Strain; Strands; Superconductivity; Superconductors; Wire
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/0953-2048/27/10/105004

Systematic studies of the intrinsic irreversible strain limit epsilon sub(irr,0), microstructure, and microchemistry were made on several internal-tin Nb sub(3)Sn pre-production wires, fabricated for the domestic agencies of the USA and China participating in the International Thermonuclear Experimental Reactor. These wires were produced by Luvata, Oxford Superconducting Technology (OST), and Western Superconducting Technologies (WST), and were intended for the tokamak's toroidal-field coils. The results of this study show that, for a final heat-treatment at 650 [degrees]C to form the A15 phase, both epsilon sub(irr,0) and the de-pinning field B sub(c2)* improved by increasing heat-treatment duration beyond 100 h for the Luvata wires. Investigation of the samples' microstructure revealed only a small number of cracked Nb sub(3)Sn filaments despite the significant and permanent degradation of their critical current I sub(c) when subjected to longitudinal tensile strain epsilon beyond epsilon sub(irr,0). All the strands contained substantial Kirkendall porosity, but we found that the quantity and distribution of the Kirkendall voids vary significantly with strand design.