Systematic Changes of the Nb-Sn Reaction With Time, Temperature, and Alloying in Restacked-Rod-Process (RRP) Nb₃Sn Strands

Reaction heat treatments spanning 605 to 750degC and 24 to ~400 hours were applied to several sets of RRP strands. Magnetization and transport measurements were used to track the changes of superconducting properties and the amounts of Nb3Sn formed. The experiments showed that temperature increases of 15degC produced equivalent properties in half the time within the temperature range 620-680degC. This result was the same whether Ta or Ti was used to alloy the Nb3Sn. The bulk pinning forceFpfor Ta-alloyed wires displayed a significant drop for temperatures outside the range above, due to tin gradients at low temperatures and grain growth at high temperatures. TheFpdrop at high reaction temperatures prevents wire technology from taking advantage of the significantly higher Kramer-plot interceptHKfor high-temperature reactions. On the other hand, Ti alloying provides a quick and potent means to increaseHKat reaction temperatures for whichFpremains high.