Coarse Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

• Published in: IEEE transactions on applied superconductivity 2011-06, Vol.21 (3), p.2554-2558
• Main Authors: Scheuerlein, C, Di Michiel, M, Arnau, G, Flukiger, R, Buta, F, Pong, I, Oberli, L, Bottura, L
• Format: Article
• Language: English
• Subjects: Diffraction; Heat treatment; Metallurgy; Microstructure; Niobium; Scanning electron microscopy; superconducting wires and filaments; Temperature measurement; Tin; X-ray diffraction; {\rm Nb}_{3}{\rm Sn}
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2010.2082476

The formation of coarse Nb 3 Sn grains in Internal Tin (IT) strands has been studied at the example of a prototype strand with high Sn content. Metallographic examination revealed that the comparatively low critical current density (J c ) of this strand is partly due to the formation of a significant fraction of coarse grained Nb 3 Sn at the periphery of the individual filaments within the subelements. The phase evolution during the reaction heat treatment has been determined in situ by high energy synchrotron X-ray diffraction as well as ex situ by Energy Dispersive X-ray Spectroscopy in a Scanning Electron Microscope (SEM) in order to identify the conditions under which the coarse grains form. Similar to what is observed in the tubular type strands, Nb 3 Sn coarse grain formation occurs in the filament areas that had first been transformed into NbSn 2 and Nb 6 Sn 5 , prior to Nb 3 Sn formation, and it accounts for an estimated Jc reduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn 2 that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb 6 Sn 5 formed appears to be hardly influenced by the different heat treatments that have been tested.