Phase Transformations During the Reaction Heat Treatment of Internal Tin Nb<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex> </formula>Sn Strands With High Sn Content

Abstract

Abstract-The phase transformations that occur during the reaction heat treatment (HT) of Nb 3 Sn superconductors depend on the overall elemental composition of the strand subelements. In the case of modern high strands with a relatively low Cu content, liquid phases are present during large temperature intervals and phases that can be detrimental for the microstructural and microchemical homogeneity of the fully reacted strand are formed. We report synchrotron X-ray diffraction measurements during in-situ reaction HT of a state-of-the-art high Nb 3 Sn internal tin strand. In this strand, Cu 3 Sn is formed upon Cu 6 Sn 5 decomposition at 415 C, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345 C-575 C, and NbSn 2 is present in the temperature interval 545 C-630 C. The formation of voids in the strand subelements has been monitored by synchrotron microtomography during in-situ reaction HT.

Effect of the fabrication route on the phase and volume changes during the reaction heat treatment of Nb3Sn superconducting wires

“…The Cu to non-Cu volume ratio is 0.92. The phase and porosity volume changes during RHT of this wire are summarized in references [10] and [21].…”

“…The relative conductor volume changes before and after RHT have been reported for different conductor types, and in particular, the relative diameter changes of unconfined wires are readily available. The length changes of unconfined Nb 3 Sn wires [9,10] and cables [11] during the entire RHT procedure have also been reported. Typically, the length of unconfined multifilament wires is slightly reduced during RHT, and their diameter substantially increased.…”

“…However, knowledge of the unconfined wire or cable volume changes during RHT is insufficient for the prediction of volume changes of magnet coils during the same RHT, because in a coil the conductor volume changes are influenced by the thermomechanical properties of the other coil constituents, such as wedges and spacers, and of the reaction mould. As an example, the 11 Tesla dipole [12] coil length increases during RHT, despite the fact that the unconfined 11 Tesla dipole RRP Rutherford cable length decreases by about 0.3% [11], and the unconfined RRP wire length is barely affected by RHT [10,11]. In contrast, the length of the new LHC inner triplet quadrupole (MQXFB) magnet [13] coils made of Rutherford cable with a similar RRP wire is decreased after RHT. This illustrates that in order to predict the coil volume changes, a deeper understanding is required of the mechanisms of conductor volume changes, and of the effect of the thermomechanical behavior of the mould and other coil constituents, and of the friction coefficients between the various pairs of materials during RHT.…”

The crystal structure of (Nb0.75Cu0.25)Sn2 in the Cu-Nb-Sn system

“…Jellyroll (MJR) [5] Rod-In-Tube (RIT) [8], PIT [9] and RRP [10] and in in-situ diffraction experiments [4,10,11]. In PIT wires at~575 C the ternary compound disappears followed by a reappearance of the NbSn 2 phase and a fcc Cu-based solid solution containing up to 9.5 at% of Sn (a bronze) [10].…”

“…In PIT wires at~575 C the ternary compound disappears followed by a reappearance of the NbSn 2 phase and a fcc Cu-based solid solution containing up to 9.5 at% of Sn (a bronze) [10]. Through the dissolution of the cooper sheath, the NbSn 2 phase reacts with the outer Nb tube forming Nb 6 Sn 5 .…”