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*

Abstract

Systematic studies of the intrinsic irreversible strain limit εirr,0, microstructure, and microchemistry were made on several internal-tin Nb3Sn 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 °C to form the A15 phase, both εirr,0 and the de-pinning field Bc2* improved by increasing heat-treatment duration beyond 100 h for the Luvata wires. On the other hand, we saw no improvement in these two parameters as a function of 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 may potentially be the reason for the lack of noticeable change in Bc2* with heat-treatment duration in the OST wires. The increase of εirr,0 and Bc2* with heat-treatment duration in the Luvata wires and the lack of increase in the OST wires may suggest a possible correlation between εirr,0 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 beyond εirr,0. The scarcity of cracks indicate that Ic(ε) measurements are highly sensitive to crack formation in Nb3Sn filaments, especially at low electric-field criteria ≦̸0.1 μV cm−1, even when the sizes of the individual filaments are only few micrometers. All the strands contained substantial Kirkendall porosity, but we found that the quantity and distribution of the Kirkendall voids vary significantly with strand design. Luvata wires have the least porosity, followed by WST wires, and then by OST strands. However, even though the presence of cracks in the Nb3Sn filaments that are in close proximity to Kirkendall voids suggest a correlation between crack initiation and the proximity of the filaments to these voids, the porosity investigation established no definitive relationship between porosity and εirr,0 in the wires studied.