Flux pinning effect of cubic equiaxed morphology and its Ti stabilizing in Nb₃Sn superconductors

Two sets of internal-Sn Nb₃Sn superconducting strands were fabricated through RRP method, one with 2 wt% of Ti alloyed in Sn core and the other just pure Sn. Four reaction temperatures of 650°C, 675°C, 700°C and 725°C and 128 h duration were applied for A15 phase formation heat treatment after Cu-Sn alloying procedure of 210°C/50 h + 340°C/25 h. For the heat-treated coil samples, transport non-Cu J _C was examined through standard 4-probe technique and phase microstructure was observed by means of Field Emission Scanning Electronic Microscope (FESEM). The obtained results demonstrate that the transport critical current density J _C of Nb₃Sn superconductors is more importantly determined by the cubic equiaxed crystalline morphology than by grain size. Ti addition in Sn stabilizes the cubic equiaxed phase at lower temperature so that heat reaction temperature is effectively reduced, the flux pinning performance is largely reinforced and the transport critical current density J _C is substantially promoted.