Residual Stress in Nb3Sn Superconductor Strand Introduced by Structure and Stoichiometric Distribution after Heat Treatment

Nb₃ Sn, a widely adopted superconducting material, suffers from fragility and sensitivity to strain. Many studies have anticipated these drawbacks to be greatly influenced by thermal expansion mismatch between the Nb₃ Sn superconducting phase and matrix. We conducted X-ray diffraction (XRD) stress analysis on a heat treated bronze strand at room temperature and found that its average axial residual stress was -135.7 MPa with fluctuation of about 300 MPa. We developed an elastic analytical model and explained the overall compression. The considerable stress fluctuation phenomenon may originate from diffusion, which was supported by electron probe microanalysis results. This work highlights the importance of volume transmutation and component distribution on the stress status of a superconducting strand and can supplement the thermal mismatch model.