Observations of the effect of pre-reaction on the properties of Nb3Sn Bronze composites

The effect of varying the annealing temperature on the degree of pre-reaction of two Nb3Sn composites has been investigated. Annealing at 550, rather than 450 C produces noticeably more irregular growth and more uneven Nb3Sn layers when final reaction occurs. It is believed that this is due to the effect that pre-existing Nb3Sn nuclei have on subsequent Nb3Sn growth. In one case the use of four anneals at 550 C, rather than 450 C was found to reduce the Jcby 50%. Results are also presented on some samples of the HFTF conductor. It is concluded that over-annealing of the bronze can be a major cause of reduced Jcin bronze Nb3Sn conductors.