Kinetics of isothermal reactive diffusion between solid Fe and liquid Al

The kinetics of the reactive diffusion between solid Fe and liquid Al was experimentally observed using Fe/Al diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed in the temperature range of T = 1053–1093 K for various times up to t = 600 s. Owing to annealing, an intermetallic layer with a rather uniform thickness is produced at the Fe/Al interface in the diffusion couple and grows into the solid Fe specimen. The intermetallic layer consists of Fe₂Al₅ and FeAl₃, and the thickness is much smaller for FeAl₃ than for Fe₂Al₅. Hence, the growth of the intermetallic layer is predominantly governed by Fe₂Al₅. The total thickness, l, of the intermetallic layer increases with increasing annealing time, t, according to the parabolic relationship l ² = Kt. This may mean that the growth of the intermetallic layer is controlled by volume diffusion. If the temperature dependence of the parabolic coefficient K is expressed by the equation K = K ₀exp(−Q _K /RT), K ₀ = 126 m²/s and Q _K  = 248 kJ/mol are obtained from the experimental values of K at T = 1053–1093 K by the least-squares method. A mathematical model was used to evaluate the interdiffusion coefficient, D, of Fe₂Al₅ from K. The evaluation provides D ₀ = 2.55 × 10³ m²/s and Q = 259 kJ/mol for the dependence of D on T described as D = D ₀exp(−Q/RT). Thus, K is one fifth of D at T = 1053–1093 K, and Q _K does not necessarily coincide with Q. D is about two orders of magnitude greater for the values evaluated in the present study than for those extrapolated from the previously reported result at T = 823–913 K. The microstructure observation in a previous study suggests that such large values of D are attributed to the boundary diffusion in the intermetallic layer as well as the crystallographic anisotropy of Fe₂Al₅.