On the diffusion of ⁵⁹Fe into aluminium and Al Mn solid solutions

Abstract

The diffusion of ⁵⁹Fe into pure Al and Al Mn solid solutions has been measured by a tracer-sectioning technique. The penetration plots are fitted by a solution of the diffusion equation according to which the surface flux is controlled by a surface barrier (characterizing the dissolution of tracer atoms from the intermetallic phases formed in the near-surface layer) higher than the diffusion activation energy in the bulk. For annealing times t << τ(τ is the time required to reach the solubility limit just inside the solvent) the solution reduces to that of constant surface concentration, and at much longer t (when the surface layer is depleted) the thin-film solution can also be valid. Detailed measurements have been carried out to investigate the time dependence of the penetration plots; the diffusion coefficients calculated from the above solution were the same (within experimental error) at fixed temperature. The effect of increased grain-boundary and dislocation density in the near-surface layer was weak; however, several measurements yielded D values higher than those obtained on samples containing large grains and having low dislocation density. The temperature dependence of D can be expressed by

This result is in good agreement with the work of Alexander and Slifkin (1970). However, the activation energy, for example, differs by 0.78 eV from Hood's (1970) result which is generally accepted in the literature. The concentration dependence of the diffusion of ⁵⁹Fe on the Mn concentration was also measured in Al Mn solid solutions at 908 K, and the enhancement factor is b = −43 ± 38. The presence of Mn thus decreases the D ^Fe values.