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.