Characterization of the A15 Layer Growth and Microstructure for Varying Heat Treatments in ${\rm Nb}_{3}{\rm Sn}$ Tube Type Composites

Tube type composites containing up to 167 subelements were studied in terms of their reaction systematics in an attempt to enhance their properties. These conductors had transport J_c values in excess of 2200 A/mm² at 12 T and 4.2 K. Samples were heat treated at temperatures between 650-800degC for varying amounts of time. The phase formations as well as the growth kinetics of the A15 layer during reactive diffusion between the Nb and a liquid Sn-rich bronze alloy were experimentally investigated at 650degC and 725degC. The A15 layer growth exponent ´n´, proportionality constant ´k´, and the rate controlling process were determined at these temperatures. Compositional analysis was performed using EDS on samples with various initial Cu-Sn ratios. The variation of grain size and morphology with temperature and composition were studied in detail. The activation energy for grain growth in the fine and coarse grain A15 regions were determined.