Analysis of inhomogeneities in Nb3Sn wires by combined SEM and SHPM and their impact on J (c) and T (c)

We demonstrate the combined use of scanning electron microscopy (SEM) and scanning Hall probe microscopy (SHPM) to analyse inhomogeneities in Nb3Sn wires. Inhomogeneities of the A15 phase in Nb3Sn sub-elements of a Ti-alloyed Restacked Rod Process wire and a Ta-alloyed Powder-In-Tube wire are investigated. Microstructural features are examined by SEM, elemental concentration gradients by energy dispersive x-ray spectroscopy (EDX) and the superconducting properties by SHPM. Correlations between the results are analysed to gain information about the impact of inhomogeneities in the microstructure on the superconducting properties. We find considerable differences in geometry and performance between sub-elements, as well as compositional and geometric inhomogeneities of the A15 phase inside single sub-elements. Additionally, simulations of the influence of Sn concentration gradients on the critical current density J (c) are performed. We also demonstrate the viability of SHPM and EDX for determining the dependence of the critical temperature T (c) on the Sn concentration and discuss possible performance gains by a reduction of inhomogeneities in Nb3Sn wires.