Using Cu/Al diffusion couples initially composed of pure Cu and Al, the reactive diffusion in the binary Cu–Al system had previously been experimentally studied. The diffusion couple was isothermally annealed at 973 to 1073K. During annealing, compound layers of the β, γ and ε phases formed between the Cu-rich solid α-phase and the Al-rich liquid L-phase, and the L/ε interface migrated towards the ε-phase. For each annealing time, the migration distance of the L/ε interface was much greater than the total thickness of the compound layers. There was a parabolic relationship between the migration distance and the annealing time. This implied that the migration of the interface was controlled by volume diffusion in the L-phase. A mathematical model for interface migration controlled by volume diffusion was used to analyze quantitatively the migration rate of the interface. From the analysis, the interdiffusion coefficient of the L-phase was found to be 1.24 x 10-9, 2.91 x 10-9 and 3.62 x 10-9m2/s at 973, 1023 and 1073K, respectively. The relationship:

D (m2/s) = 1.42 x 10-4 exp[-93.5(kJ/mol)/RT]

was obtained by using the least-squares method. According to the analysis, the interdiffusion coefficient was much greater for the L-phase than for the solid phases. Therefore, the L/ε interface migrated towards the ε-phase and the migration rate of the interface was much greater than the overall growth rate of the compound layers.

Evaluation of Interdiffusion in Liquid Phase during Reactive Diffusion between Cu and Al. Y.Tanaka, M.Kajihara: Materials Transactions, 2006, 47[10], 2480-8. See also: Materials Science and Engineering A, 2007, 445-446, 355-63