The diffusion of iron in aluminium

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Abstract
The diffusion of radioactive 59Fe has been measured in both single crystals and polycrystalline specimens of aluminium in the temperature range 520 to 660°c. The material for the diffusion samples was between 99·99 and 99·999% pure aluminium. The temperature dependence of the diffusion coefficient for single crystal specimens could be expressed by in the temperature range 580 to 660°c. The results are not in agreement with values previously published for this system by Hirano, Agarwala and Cohen (1962). The earlier results are thought to be in error as a result of tracer hold-up at the surface-oxide film on the aluminium specimens. This effect was overcome in the present work by implanting the 59Fe tracer atoms below the surface-oxide layers using a 40 kev 59Fe ion beam. Analysis of the data indicates the activation energy for diffusion is dominated by an iron-vacancy repulsive interaction term. Calculations of the difference between the activation energies for self and solute diffusion in aluminium, following the procedure adopted by LeClaire (1962), but using an oscillating form of the inter-ion potential, have been made for Cr, Mn, Fe and Cu. In each case remarkable agreement between calculated and experimental values is obtained, in spite of the many assumptions involved. The calculations indicate that the solute-vacancy interaction energies for Fe at the nearest-neighbour distance and at the saddle-point configuration are of opposite sign with the repulsive interaction at the saddle-point dominating. Due to a combination of low solubility and low lattice diffusion rates, extrinsic effects, ascribable to dislocations and grain boundaries, have been found to occur within 100°c of the melting point.