Role of α precipitates in flux pinning in a superconducting Ti-Nb-Ta-Zr quaternary alloy

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Abstract
The precipitation behaviour of the a phase in a Ti-27 at. %Nb-6 at. %Ta-6 at. %Zralloy has been investigated mainly by means of small-angle X-ray scattering measurements, by which the average size and number density of a precipitates were determined. The alloy was isothermally aged at 643 K after cold-rolling to various thicknesses. During ageing the average size of α precipitates increased but the number density decreased. The effect of cold-rolling was to increase the volume fraction of a precipitates. The superconducting critical current density was measured for the same specimens after the metallographical investigation. The specific pinning force produced by α precipitates, which corresponds to the global pinning force density divided by the number density of precipitates, was found to be proportional to the cube of the particle radius. It was found that the global pinning force density can be described using a scaling rule in terms of the volume fraction of α precipitates and the reduced magnetic field. The dominant global pinning force in the present foil specimens, as well as in commercial multifilamentary wires, is attributed to α precipitates. Dislocations and their secondary substructure introduced by cold-working also contributed to flux pinning, and could offer nucleation sites for the α phase.