Solute-vacancy interaction energies and the effect of 0.009 at.% Mg on the ageing kinetics of an Al-4.01 at.% Zn alloy

Abstract

The ageing kinetics of an Al-4.01 at.% Zn alloy have been studied at 0°C as a function of quenching temperature using electrical resistance measurements. The extent of clustering during the quenching operation was shown to be significant and a correction has been applied to the experimental data to allow for it. An apparent vacancy formation energy of about 0.70–0.71 eV was derived by considering both the initial ageing rate and also the time taken to achieve fractions of the resistance peak as a function of the quenching temperature. The ageing process was found to be slowed down by the addition of 0.009 at.% Mg but the general pattern of the clustering was unaltered. The results have been interpreted to give the effective vacancy binding energy of zinc as about 0.05 eV per atom. This value, which is much lower than that reported from studies of dilute alloys, is thought to reflect a reduction in the vacancy binding energy per atom resulting from association of solute atoms before quenching. Some experimental evidence of association is presented. The magnitude of the binding energy of the zinc is particularly significant because the slowing of the ageing process by magnesium could not be explained simply were this not the case. In the model used here, the added atoms act initially as vacancy traps so that the concentration of vacancies available to transport zinc atoms is reduced. It is thought that these trapped vacancies are subsequently released during the ageing process and give rise to the apparently anomalous behaviour reported by earlier researchers. A magnesiumvacancy binding free energy of 0.17 eV at 0°C was derived on the basis of the model.