Kinetics of reactive diffusion between Cu–8.1Sn–0.3Ti alloy and Nb

The kinetics of the reactive diffusion between a ternary Cu–8.1 at.% Sn–0.3 at.% Ti alloy and pure Nb was experimentally examined in a metallographical manner. In the experiment, sandwich (Cu–Sn–Ti)/Nb/(Cu–Sn–Ti) diffusion couples were isothermally annealed in the temperature range between T = 923 and 1053 K for various periods up to 604 h. During annealing, the Nb3Sn compound is produced as a layer along each (Cu–Sn–Ti)/Nb interface in the diffusion couple. The Nb3Sn layer grows predominantly towards Nb but scarcely towards the Cu–Sn–Ti alloy. This means that the growth of the Nb3Sn layer is governed by the migration of the Nb3Sn/Nb interface. The mean thickness l of the Nb3Sn layer is expressed as a power function of the annealing time t by the equation l = k(t/t0)n. Here, t0 is unit time, 1 s. The exponent n is nearly equal to unity at T = 923–973 K and monotonically decreases from 0.9 to 0.7 with increasing annealing temperature from T = 973 to 1053 K. Thus, the interface reaction at the migrating Nb3Sn/Nb interface is the rate-controlling process for the growth of the Nb3Sn layer at T = 923–973 K and the interdiffusion across the Nb3Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973–1053 K. The volume diffusion in the Nb3Sn layer may govern the interdiffusion at T = 1053 K, but the grain boundary diffusion along grain boundaries in the Nb3Sn layer will contribute to the interdiffusion at T = 923–1023 K.