The microstructure and microchemistry of high critical current Nb3Sn strands manufactured by the bronze, internal-Sn and PIT techniques

Recent advances in Nb₃Sn conductor development have advanced the non-Cu critical current density, J_c, from 2000 A/mm² to almost 3000 A/mm² (12 T, 4.2 K). We have quantified a variety of state of the art composites for their microstructures using the fracture/field emission scanning electron microscope, FESEM, technique and their microchemistry using energy dispersive X-ray spectroscopy (EDS)/FESEM. The results of the measurements increasingly point to the importance of A15 composition in determining the critical current density as well as grain size. The highest critical current densities, however, are being attained by the internal Sn process which has yet to achieve as high a level of Sn (23-24.5 at.% Sn) in the A15 as for powder-in-tube (PIT) in which we measure as high as 25-26 at.% Sn. When Sn diffuses into the Cu stabilizer, it is found to have a great affinity for Nb₃Sn formation than dissolution into the Cu. A15 forms at the Nb-stabilizer surface with local Cu concentrations within the grains of the stabilizer of less than 0.1 at.% Cu. Elevated levels of Sn, however, were observed at the Cu grain boundaries. Both the quantified variations in composition and the peak levels of Sn indicate that further increases in performance should be expected.