Quantitative description of a high J c Nb-Ti superconductor during its final optimization strain. I. Microstructure, T c, H c2, and resistivity

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Abstract
A most important step in the critical current density (Jc) optimization of Nb‐Ti is the large final drawing strain, in which α‐Ti precipitates, initially approximately equiaxed and 100–200 nm in diameter, are drawn into ribbons, whose thickness (1–2 nm) is less than the superconducting coherence length [ξ (4.2 K)∼5 nm]. Using transmission electron microscopy, the precipitate thickness, spacing, cross‐sectional area, and circumference were measured over the whole final drawing strain range. Each of these parameters was found to have a simple power dependence on the wire diameter. Tc, Hc2, and the resistivity (ρn) were also change considerably during the refinement of the precipitates. Directly after precipitation, Tc increased, and (dHc2/dT)Tc and ρn were reduced from the single‐phase values. Drawing the wire returned these parameters to their single‐phase values, as the precipitate thickness was reduced to less than ξ. This observation explains a long‐standing peculiarity in this system, namely that the optimum Hc2 of high Jc conductors occurs for a composition close to Nb 46 wt.% Ti, even when the precipitation of 18 vol % of α‐Ti shifts the matrix composition to a Nb‐rich composition of theoretically lower Hc2.