Phase Evolution During Heat Treatment of Nb3Sn Wires Under Development for the FCC Study

Simon C Hopkins,Jonas Lachmann,Andreas Leineweber,Amalia Ballarino,Algirdas Baskys

doi:10.1109/tasc.2021.3063675

Abstract

In recent years, the phase formation sequence during heat treatment of Nb 3 Sn wires, and the influence of the microstructure and compositional homogeneity of Nb 3 Sn on in-field critical current (I c ), have received increasing attention. For RRP wires, the importance of understanding and managing the formation of the ternary phase nausite has been demonstrated. However, a published Cu-Nb-Sn phase diagram including this phase is still not available; and conductor development for the Future Circular Collider (FCC) study has introduced a variety of less-studied internal tin wire layouts. In this article, a study of phase transformations in the ternary Cu-Nb-Sn system is summarized, and selected isothermal sections of the re-evaluated phase diagram are presented. The phase transformations during low-temperature heat treatment steps of wires developed for the FCC study are also presented and analyzed in comparison to established RRP conductors.

Highlights

T HE development of Nb3Sn accelerator magnets for the High Luminosity LHC (HL-LHC) Project at CERN, and proposals for high-field Nb3Sn magnets for an energy-frontier Future Circular Collider (FCC), have led to a resurgence of research activity in Nb3Sn superconducting wires in recent years
Heat treatment of diffusion couples prepared by physical vapor deposition (PVD) on a Nb substrate at 220 °C resulted only in the formation of Cu6Sn5 (η), but Nb participated in intermetallic phase formation at 300 °C and above
Having resolved them in both PVD and powder samples, the composition and lattice parameters of each phase were obtained from energy-dispersive Xray (EDX) and X-ray diffraction (XRD), and the mean atomic volume calculated

Summary

Introduction

T HE development of Nb3Sn accelerator magnets for the High Luminosity LHC (HL-LHC) Project at CERN, and proposals for high-field Nb3Sn magnets for an energy-frontier Future Circular Collider (FCC), have led to a resurgence of research activity in Nb3Sn superconducting wires in recent years. For FCC, challenging performance targets have been set, notably a non-copper critical current density (Jc) of 1500 A mm-2 at 16 T and 4.2 K, and an effective filament diameter (deff) of 20 μm [1]. An important recent contribution was the reassessment of heat treatment design for Restacked Rod Process (RRP) wires (Bruker OST) undertaken by Sanabria et al [2], [3]. Sanabria et al were able to demonstrate the importance of controlling nausite formation, and that a heat treatment step at Manuscript received December 1, 2020; revised January 31, 2021; accepted February 12, 2021.

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