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Two kinds of ITER internal-Sn Nb3Sn superconducting wires were chosen to investigate the effects of Sn core or Nb core alloyed with the third element on A15 phase formation and superconducting performances in this paper.In-situ neutron diffraction was used for measurement of phase formation progress during the heat treatment procedure.From the obtained results it can be concluded that Sn core alloyed with small amount of the third element Ti is more effective than Nb core alloyed with Ta in enhancing A15 phase formation and improving superconducting performances.The reason may be as follows:small amount of Ti addition will not only greatly increase the tin diffusion rate and A15 phase formation rate,but also effectively inhibit the Nb3Sn grain growth and elevate the pinning force,resulting in obvious improving of superconducting performances and the large increment of non-Cu critical current density Jcn.