Phase Formation, Composition and ${\rm T}_{\rm c}$ Distribution of Binary and Ta-Alloyed ${\rm Nb}_{3}{\rm Sn}$ Wires Produced by Various Techniques

Abstract

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters and in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625 /320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower , and a broad peak due to the fine grains, with a higher . The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Nb3Sn Wires for the Future Circular Collider at CERN: Microstructural Investigation of Different Wire Layouts

“…4. Considering that grain morphology is closely related to Sn content and the latter is higher in equiaxed grains [10], [11], [12], approximately round-shaped grains were selected. For such measurements, very thin lamella regions were chosen (approximately 50 nm) to avoid grains overlapping.…”

The EuCARD High Field Magnet Project

“…This strand has been widely studied and the work has been published (see [6], [7], [8], [9], [10]). In the US, for the past 10 years the DOE has financed the development of Nb 3 Sn strand for High Energy Physics applications.…”