Phase Formation and Critical Currents in PM Nb₃Al Multifilamentary Wires

Abstract

It has been demonstrated that multicore Nb₃Al wires with stabilizing copper matrix can be prepared by powder metallurgy. High critical current densities of j_c = 1.2·10⁵ A/cm² at 10 T were achieved for single core wires with a high areal reduction ratio (ARR) of q = 9·10⁵. For high ARR the A15 phase is formed at relatively low temperatures of about 650°C, bypassing the a phase. Phase formation has been studied by DSC, T_c measurements and XRD. The DSC results are quantitatively described by a layer model. Prior to the A15 phase NbAl3 and bcc NbAl are formed. The amount and formation temperature of these preliminary stages depends on the ARR, indicating that the absolute layer thickness of Nb and Al influences the phase formation. Phase formation in Nb-Al multilayers of submicron scale does not follow the reaction sequence according to the equilibrium phase diagram.