Bronze tin content and grain size in Nb3Sn composite superconductors

The bronze matrix contributes in an interrelated way to both the electrical and mechanical properties of Nb₃Sn composite superconductors. The bronze provides precompression and contributes to the load bearing capability of the conductor, and these aspects of mechanical performance are related to residual tin content. The Nb3Sn conductors used in the NHMFL 900 MHz NMR magnet are examples of modern bronze process conductors. The electrical properties of these conductors have been reported previously. These conductors have now been examined specifically to study the characteristics of the bronze as part of a general understanding of mechanical properties. Five types of conductor are examined for residual tin content and grain size in the bronze. Using energy dispersive X-ray spectroscopy, the residual tin content of the bronze in the central regions of the conductors was measured to be in the range of 2 to 4 wt%. Given the high temperature of the heat treatment, relatively large grains would generally result in such bronze materials. For the bronze in a multifilamentary conductor, however, the filament spacing was expected to provide a constraint on grain growth. There are a number of identifiable bronze regions in a composite conductor, including between filaments, between sub-bundles and beneath the barrier. It is observed that the grain size in each region is comparable to the space available between the boundaries of the filament structure, and that the grains are regular and not highly elongated in either a transverse or longitudinal section. As the characteristic dimensions of the bronze regions vary widely, the grain sizes in these regions are quite different as well. As the yield stress of the bronze satisfies the Hall-Petch relation, the small grain size between filaments is estimated to contribute significantly to the yield stress of the bronze in this region.