Applicability of Reel-to-Reel RHQ Treatments to $ \hbox{Nb}_{3}\hbox{Al}$ Precursors With a Ta/Cu/Ta Three-Layer Filament-Barrier Structure

Abstract

A tension-free rapid heating and quenching (RHQ) simulator was previously employed to demonstrate that a ductile and normal-conducting Cu layer can suppress a magnetic field instability and improve bending strain tolerance. However, a molten Cu barrier caused the filament to be shifted from the original location, and excess heating caused the Cu to locally corrode a Ta layer, through which it diffused into the filament, thereby embrittling the resulting wire. In some cases, the swelling molten-Cu tore the Ta skin. Fabrication of a long piece of Ta/Cu/Ta barrier Nb 3 Al wire is attempted using reelto-reel RHQ treatment of a modified precursor with a thicker Ta skin that would tolerate both tension forces in a section of the wire between electrodes and pressure from the swelling of molten-Cu. In the optimum RHQ condition (V RHQ : 9.5 V), a 50-m piece of Ta/Cu/Ta barrier precursor was successfully reelto-reel RHQ treated to form a bcc supersaturated-solid solution (Nb(Al) ss ) without bursting of the Ta skin. Microstructural analysis revealed that the molten Cu, into which Ta dissolved, diffused into the jelly-roll filament, mainly through a region of partially melted Al-rich Nb(Al) ss . The critical current density of the filament J c (18 T, 4.2 K) was optimized by the deformation of Nb(Al) ss and compared favorably with the conventional precursor.