Initial stages of Nb_(3)Sn phase formation in Nb-bronze matrix composite wires investigated by means of electron microscopy

At present,Nb_(3) Sn superconductors are becoming more popular in high magnetic fields.The growth law of Nb_(3) Sn phase in a planar CuSn/Nb diffusion couple has been studied,whereas the formation mechanism of Nb_(3) Sn phase in a cylindrical CuSn/Nb diffusion couple is still controversial.The purpose of this work is to investigate the growth exponent of Nb_(3) Sn phase at the initial stage of annealing by use of scanning electron microscopy(SEM)through which the thickness of Nb_(3) Sn layer can be obtained.In this study,bronze-processed Nb_(3) Sn multifilamentary wires with different annealing time were investigated.The Nb_(3) Sn phase was formed during isothermal annealing at 670 ℃ by solid-state diffusion,which was accomplished by the movement of Sn atoms from the CuSn/Nb_(3) Sn interface to Nb_(3) Sn/Nb interface.However,the formation mechanism of Nb_(3) Sn phase at the initial stage of annealing is still not well understood.Microstructural evolution of Nb_(3) Sn phase during isothermal annealing was studied by SEM.The mean thickness of the Nb_(3) Sn layer(Δ(xNn3 Sn)) is expressed as a power function of the annealing time(t) by the equation A_(x^(2)Nb_(3) Sn)=k(t/t0)^(n),where t0 is the unit time,k is the reaction rate constant and n is the growth exponent.The growth exponent has the average value of 0.82,which means that the formation of the Nb_(3) Sn phase is both governed by the interface reaction and the grain boundary diffusion.