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

Published

August 19, 2014

Author(s)

Najib Cheggour, Loren F. Goodrich, Z-H Sung, Theodore C. Stauffer, Jolene D. Splett, P. J. Lee, Matthew C. Jewell

Abstract

Systematic studies of the intrinsic irreversible strain limit ε_irr0, 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 ε_irr0 and the de-pinning field B_c2^* 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 ε_irr0 and B_c2^* with the heat-treatment duration in the Luvata wire and the lack of increase in the OST wires may suggest a possible correlation between ε_irr0 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 I_c when subjected to longitudinal tensile strain ε byond ε_irr0. The scarcity of cracks ind

Citation

Superconductor Science and Technology

Pub Type

Journals

Keywords

A15 Phase, Cracks, Critical Current, De-pinning Field, Diffusion, Effective Upper Critical Field, Heat Treatment, Irreversible Strain Limit, Kirkendall Voids, Microchemistry, Microstructure, Porosity, Strain, Stoichiometry, Ti Doping

Citation

Cheggour, N. , Goodrich, L. , Sung, Z. , Stauffer, T. , Splett, J. , Lee, P. and Jewell, M. (2014), 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, Superconductor Science and Technology (Accessed December 13, 2024)