Flux Pinning by Grain Boundaries in Superconducting Niobium-Zirconium Alloys

The influence of grain, subgrain, or dislocation cell size d or superconducting critical flux gradient dB/dr was investigated in a single phase Nb-33 at. % Zr alloy. By appropriate heat treatments, d was varied from 0.25 to 1500 μ. A monotonic decrease of more than one hundred-fold in dB/dr accompanied this thousand-fold increase in d. At a given d, the maximum increase in dB/dr obtained from precipitation of a second phase does not exceed a factor of about 30.