Initial stages of Nb₃Sn phase formation in Nb-bronze matrix composite wires investigated by means of electron microscopy

Abstract

At present, Nb₃Sn superconductors are becoming more popular in high magnetic fields. The growth law of Nb₃Sn phase in a planar CuSn/Nb diffusion couple has been studied, whereas the formation mechanism of Nb₃Sn phase in a cylindrical CuSn/Nb diffusion couple is still controversial. The purpose of this work is to investigate the growth exponent of Nb₃Sn phase at the initial stage of annealing by use of scanning electron microscopy (SEM) through which the thickness of Nb₃Sn layer can be obtained. In this study, bronze-processed Nb₃Sn multifilamentary wires with different annealing time were investigated. The Nb₃Sn phase was formed during isothermal annealing at 670 °C by solid-state diffusion, which was accomplished by the movement of Sn atoms from the CuSn/Nb₃Sn interface to Nb₃Sn/Nb interface. However, the formation mechanism of Nb₃Sn phase at the initial stage of annealing is still not well understood. Microstructural evolution of Nb₃Sn phase during isothermal annealing was studied by SEM. The mean thickness of the Nb₃Sn layer ($\Delta x_{{{\text{Nb}}_{ 3} {\text{Sn}}}}$) is expressed as a power function of the annealing time (t) by the equation $\Delta x_{{{\text{Nb}}_{ 3} {\text{Sn}}}}^{2} = k(t/t_{0} )^{n}$, 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 Nb₃Sn phase is both governed by the interface reaction and the grain boundary diffusion.