Growth behavior of compound layers during reactive diffusion between solid Cu and liquid Al

In order to examine experimentally the kinetics of the reactive diffusion between solid Cu and liquid Al, Cu/Al diffusion couples were isothermally annealed in the temperature range between T = 973 and 1073 K. Owing to annealing, compound layers consisting of the β, γ1 and ɛ2 phases are produced between the Cu-rich solid (α) and Al-rich liquid (L) phases. The β, γ1 and ɛ2 phases are the only stable compounds at T = 973–1073 K. The mean thickness of each compound layer is proportional to a power function of the annealing time. For the β layer, the exponent of the power function is close to 0.5 at T = 1023–1073 K, but nearly equal to 0.25 at T = 973 K. On the other hand, for the γ1 layer, the exponent takes values between 0.25 and 0.5 at T = 1023–1073 K, but that smaller than 0.25 at T = 973 K. The exponent smaller than 0.5 indicates that grain boundary diffusion predominantly controls the growth of the compound layer and grain growth occurs at a certain rate. In contrast, the ɛ2/L interface migrates towards the α phase. The migration distance of the ɛ2/L interface is much greater than the total thickness of the compound layers, and proportional to the square root of the annealing time. Consequently, the migration of the ɛ2/L interface is governed by interdiffusion in the L phase. According to estimation, the interdiffusion coefficient is much greater for the L phase than for the solid phases. As a result, the ɛ2/L interface migrates towards the α phase, and the migration rate of the interface is much greater than the overall growth rate of the compound layers.