The kinetics of reactive diffusion were experimentally examined for the binary Cu-Al system, using Cu/Al diffusion couples which initially consisted of pure Cu and Al. The diffusion couple was isothermally annealed at 973 to 1073K. During annealing, compound layers of the β, γ and ε phases were formed between the Cu-rich solid (α) and Al-rich liquid (L) phases, and the L/ε interface migrated towards the ε-phase. The migration rate of the L/ε interface was much greater than the overall growth rate of the compound layers, and a parabolic relationship existed between the migration distance and the annealing time. This showed that interdiffusion in the solid phases was negligible and that migration of the interface was controlled by interdiffusion in the L phase. The parabolic relationship possibly applied only to a semi-infinite diffusion couple. Within the experimental annealing times, overlap of the diffusion field occurred in the L-phase, and thus the diffusion couple became finite. In order to account for the experimental results, the migration behavior of the L/ε interface in the finite Cu/Al diffusion couple was numerically analyzed by using a mathematical model. The analysis showed that the parabolic relationship held to within experimental uncertainty, even in a finite diffusion couple, provided that volume diffusion was the rate-controlling process and that slight overlap of the diffusion field occurred in each phase.

Numerical Analysis for Migration of Interface between Liquid and Solid Phases during Reactive Diffusion in the Binary Cu-Al System. Y.Tanaka, M.Kajihara: Materials Science and Engineering A, 2007, 459[1-2], 101-10