Growth behavior of Nb3Sn layer during reactive diffusion between Cu–8.3Sn alloy and Nb

The growth behavior of the Nb3Sn compound during the reactive diffusion between Nb and a binary Cu–8.3 at.% Sn alloy was experimentally examined. In the experiment, (Cu–8.3Sn)/Nb/(Cu–8.3Sn) diffusion couples were isothermally annealed at temperatures of T = 923–1053 K for various times up to 1038 h. After annealing, a Nb3Sn compound layer was observed to form at each (Cu–Sn)/Nb interface in the diffusion couple. The Nb3Sn layer grows predominantly towards Nb but hardly towards the Cu–Sn alloy. Thus, the growth of the Nb3Sn layer is governed by the migration of the Nb3Sn/Nb interface. The thickness l of the Nb3Sn layer is mathematically described as a power function of the annealing time t as follows: l = k(t/t0)n, where t0 is unit time, 1 s. The exponent n is close to unity at T = 923 and 973 K and monotonically decreases from 0.96 to 0.77 with increasing annealing temperature from T = 973 to 1053 K. This means that the interface reaction in the Nb3Sn/Nb interface is the rate-controlling process for the growth of the Nb3Sn layer at T = 923–973 K and the interdiffusion contributes to the rate-controlling process at T = 973–1053 K. Furthermore, the volume diffusion may govern the interdiffusion at T = 1053 K, but the grain boundary diffusion will contribute to the interdiffusion at T = 923–1023 K.