The influence of the reaction heat-treatment process on the mechanical properties of multi-filamentary composite Nb_3Sn superconducting wires at 77 and 300 K

摘要

Stress-strain curves have been obtained for a 1.5 mm diameter bronze-process multi-filamentary composite Nb_3Sn wire at 77 and 300 K. Tensile test specimens were prepared with a variety of heat-treatment conditions, ranging from no heat-treatment to one with a duration exceeding that of the wire manufacturer's recommended full heat-treatment. The results show that for each heat-treatment condition, the mechanical properties are significantly different at 77 and 300 K: at the lower temperature, specimens have higher proof stress, are stronger and more ductile. The elastic modulus is not significantly different at 77 and 300 K, but rises from 102 GPa for specimens that were not heat-treated to 110 GPa for fully reacted ones. The proof stress is significantly higher for specimens that were not heat-treated compared to those that were. This is true even for heat-treatment durations that were too short to convert much of the niobium into Nb_3Sn. Extending the heat-treatment beyond these short durations, and thus increasing the Nb_3Sn volume fraction, makes the wires more brittle. However, despite having a dramatic effect on the composition, extending the heat-treatment duration does not have much of an effect on the stress-strain curve up to the point of failure.