Nb3Sn strands are under development for the next generation of high field accelerator magnets (LHC Upgrade, ITER). They can provide high critical current (2500A/mm2) at high field (14T). The superconducting phase is obtained by solid state diffusion during heat treatments. The properties of Nb3Sn superconductors depend strongly on the microstructure (voids) and microchemistry which change during the production process. Void growth and phase transformations in Nb3Sn strands are commonly studied ex-situ by destructive, time consuming, metallographic techniques.

Variation of the diffraction patterns of the IT Nb3Sn strand during Cu–Sn mixing HT cycle between 120 and 540°C. Diffractograms have been acquired every 10min.

C. Scheuerlein et al. performed combined fast micro-tomography and x-ray angular dispersive diffraction measurements during in-situ heat treatment of Nb3Sn strand in order to obtain a quantitative description of the void growth and the phase transformations. Three different void growth mechanisms were found by correlating the quantitative void growth results with the quantitative description of the phase transformations during the strand. It was also found that long duration temperature ramps and isothermal holding steps neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting phase nucleation and growth.

C. Scheuerlein, M. Di Michiel, A. Haibel - On the formation of voids in internal tin Nb3Sn superconductor - Appl. Phys. Lett. 90, 132510 (2007)