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.