Influence of the heat-treatment conditions, microchemistry, and microstructure on the irreversible strain limit of a selection of Ti-doped internal-tin Nb3Sn ITER wires
Najib Cheggour, Loren F. Goodrich, Z-H Sung, Theodore C. Stauffer, Jolene D. Splett, P. J. Lee, Matthew C. Jewell
Systematic studies of the intrinsic irreversible strain limit εirr0, microstructure, and microchemistry were made on several internal-tin Nb3Sn pre-production wires fabricated for the domestic agencies of the U.S.A. and China participating in the international thermonuclear experimental reactor (ITER). 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°C to form the A15 phase, both εirr0 and the de-pinning field Bc2* improved with increasing the heat-treatment duration beyond 100 hours in the Luvata wires. On the other hand, we saw no improvement in these two parameters as a function of the heat-treatment duration in the OST wires. Furthermore, micro-chemical analysis of OST wires revealed that some Nb3Sn filaments have a Sn- and Ti-rich phase at the interface between Cu(Sn) matrix and Nb3Sn in the form of a shell around individual filaments. This phase is far less prominent in the Luvata and WST conductors, and could inhibit diffusion of Sn and Ti into Nb3Sn filaments during the reaction and could potentially be the reason for the lack of noticeable change in B^c2^* with the heat-treatment duration in the OST wires. The increase of εirr0 and Bc2* with the heat-treatment duration in the Luvata wire and the lack of increase in the OST wires may suggest a possible correlation between εirr0 and the stoichiometry of the A15 composition. Investigation of the samples' microstructure revealed only a small number of cracked Nb3Sn filaments despite the significant and permanent degradation of their critical current Ic when subjected to longitudinal tensile strain ε byond εirr0. The scarcity of cracks ind
, Goodrich, L.
, Sung, Z.
, Stauffer, T.
, Splett, J.
, Lee, P.
and Jewell, M.
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</sub>Sn ITER wires, Superconductor Science and Technology
(Accessed July 31, 2021)