Effect of Cu4Ti Compound Formation on the Characteristics of NbTi Accelerator Magnet Wire

High critical current density, Jc > 2500 A/mm2, and small filament diameter, d ¿ 3 ¿m, are required in multifilamentary NbTi wire used for superconducting accelerator magnets. Wires obtained from various commercial sources had Jc´s in the range 1000 to 2800 A/mm2 and d´s in the range 1 to 23 pm. The filaments were examined by means of scanning electron microscopy in order to determine the reason for the variation in Jc. It was found that the filaments in high Jc wires had clean smooth surfaces and uniform cross section along-their lengths. Filaments in low Jc wires show formation of Cu4Ti compound particles on their surfaces and large variations in cross section. The lower critical current measured in these wires is believed to be largely due to this effect. The superconducting-normal state transition is relatively wide in these wires.