Effect of cubic equiaxed grains and its Ti-stabilizing performance in Nb₃Sn strands

Two kinds of multifilament internal-Sn Nb₃Sn superconducting strands were fabricated through the RRP method, one with 2wt% of Ti alloyed in an Sn core and the other just pure Sn. Four reaction temperatures of 650, 675, 700 and 725 °C and 128h duration were applied for A15 phase formation heat treatment after a Cu-Sn alloying procedure of 210 °C/50h+340 °C/25h. Through the standard four-probe technique the heat-treated coil samples were examined for the transport non-Cu J_C variation with applied field B which was then used to calculate the flux pinning force variation F _P-B. The samples' phase microstructure were also observed by means of FESEM. The obtained results demonstrate that for fully reacted Nb₃Sn superconductors the transport critical current density J_C is more importantly affected by the cubic equiaxed morphology than by grain dimension, due to its much stronger flux pinning performance of the morphology. Ti addition in Sn stabilizes the cubic equiaxed grains at lower reaction temperature so that the HT temperature is effectively reduced, the flux pinning strength is largely reinforced and thus the transport non-Cu J_C is substantially promoted.