Coarse ${\rm Nb}_{3}{\rm Sn}$ Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

Scheuerlein, C.; Michiel, Marco Di; Arnau, Gemma; Flükiger, R.; Buta, F.; Pong, Ian; Oberli, L.; Bottura, L.

doi:10.1109/tasc.2010.2082476

Cited by 13 publications

(11 citation statements)

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“…Nausite has been suspected to decompose into NbSn 2 [27], and our observations confirm this, given that these Nausite grains (as well as the Nausite ring itself) decompose into porous structures of NbSn 2 ( figure 9(b)). On further heating, the NbSn 2 transforms via Nb 6 Sn 5 into the disconnected pieces of Nb 3 Sn (associated with the so-called Nb dissolution of figure 4), as well as coarse A15 grains characteristic of low performance wires [27]. Finally, during the 665°C step (A15 reaction), Nb 3 Sn forms rapidly throughout the filament pack (as observed by the large extent of reaction in figure 9(b)).…”

Section: Standard Two-stage Cu/sn Mixing Heat Treatment Resultssupporting

confidence: 82%

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Sanabria¹,

Field²,

Lee³

et al. 2018

Supercond. Sci. Technol.

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Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb 3 Sn wires, in particular a critical current density (J c ) of more than 1500 A mm −2 at 16 T and 4.2 K with an effective filament diameter (D eff ) of less than 20 μm. The restacked-rod-process (RRP ® ) is the technology closest to meeting these demands, with a J c (16 T) of up to 1400 A mm −2 , residual resistivity ratio>100, for a subelement size D s of 58 μm (which in RRP ® wires is essentially the same as D eff ). An important present limitation of RRP ® is that reducing the sub-element size degrades J c to as low as 900 A mm −2 at 16 T for D s =35 μm. To gain an understanding of the sources of this J c degradation, we have made a detailed study of the phase evolution during the Cu-Sn 'mixing' stages of the wire heat treatment that occur prior to Nb 3 Sn formation. Using extensive microstructural quantification, we have identified the critical role that the Sn-Nb-Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300°C-400°C rangegreatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408°C) η phase (Cu 6 Sn 5 ) and the more desirable ε phase (Cu 3 Sn), which decomposes at 676°C. After the mixing stages, when heated above 408°C towards the Nb 3 Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn 2 on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb 3 Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350°C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ε phase through better Cu-Sn mixing. At a D s of 41 μm, the Nausite control heat treatment increases the J c at 16 T by 36%, reaching 1300 A mm −2 (i.e. 2980 A mm −2 at 12 T), and moving RRP ® closer to the FCC targets.

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Section: Standard Two-stage Cu/sn Mixing Heat Treatment Resultssupporting

confidence: 82%

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Sanabria¹,

Field²,

Lee³

et al. 2018

Supercond. Sci. Technol.

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“…There is evidence that at least some types of these peri-filamentary coarse grains develop from a ternary Cu-Nb-Sn phase with composition in the vicinity of 15-18 at% Cu, 23-28 at% Nb, 56-60 at% Sn which exists at temperatures below 500 [2] and which is similar to that reported by Naus [3]. It is thought that and may be responsible for some of the other types of these peri-filamentary coarse grains [4].…”

Section: A Peri-filamentary Coarse Grainsmentioning

confidence: 57%

Longitudinal and Transverse Cross-Sectional Microstructure and Critical Current Density in ${\rm Nb}_{3}{\rm Sn}$ Superconductors

Pong

Oberli

Bottura

et al. 2011

IEEE Trans. Appl. Supercond.

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Abstract-The longitudinal and transverse cross-sectional microstructures of several internal tin strands have been systematically investigated. The critical current densities of these strands were then correlated with their design parameters. It is observed that the occurrence of certain coarse grain structures is related to the location of the filaments with respect to the subelements as well as to the strand. Experimental evidence suggests that the existence of these coarse grains is related to Sn distribution during the early stages of the heat treatment. It is also noticed that some coarse grains have high aspect ratio features, confirming the need to study the longitudinal fracture surface. We report in this paper the observation of some unusual grain sizes and morphologies. It appears that, of the high-Sn content conductors investigated in this paper, a strand's global composition has a weaker influence on the critical current density than local and structural factors, such as the local Cu:Nb area ratio (LAR) and filament layout.

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“…However, no traces of Ta were found in the TEM/EDX measurements. Moreover, the phase was found also in wires containing only Cu, Nb and Sn, without any Ta traces [11]. Hence, this phase is expected to exist as a purely ternary Cu-Nb-Sn phase.…”

Section: Discussionmentioning

confidence: 84%

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Coarse ${\rm Nb}_{3}{\rm Sn}$ Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

Cited by 13 publications

References 11 publications

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Longitudinal and Transverse Cross-Sectional Microstructure and Critical Current Density in ${\rm Nb}_{3}{\rm Sn}$ Superconductors

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

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Coarse ${\rm Nb}_{3}{\rm Sn}$ Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

Cited by 13 publications

References 11 publications

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP®Nb3Sn wires

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP®Nb3Sn wires

Longitudinal and Transverse Cross-Sectional Microstructure and Critical Current Density in ${\rm Nb}_{3}{\rm Sn}$ Superconductors

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

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Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires