Effects of gold addition on recrystallization and grain growth of zone-refined aluminum

Abstract

The effects of addition of a small amount of gold on the recrystallization and grain growth of zone-refined. aluminum were kinetically studied.At first, dependence of recrystallization behavior of pure aluminum on the purity (number of passes for zone refining) was examined and it was shown that the recrystallization temperature of pure aluminum was lowered with the rise of purity. It was further shown that the rate-controlling mechanism of grain boundary migration in high purity aluminum depended upon the driving force for the migration.In the primary recrystallization where the driving force was great, the activation energy for grain boundary migration (Q) was determined to be 13.6 kcal/mol; while, in the grain growth where the driving force was small, the energy (Q) was determined to be 29.1 kcal/mol.The low value of Q in the primary recrystallization strongly suggested that the grain boundary migration was closely connected with the fundamental process of atomic transfer across the boundaries. However, the higher value of Q in the grain growth indicated that it was controlled by the diffusion of foreign atoms even if their residual amount was as low as the order of a few ppm.Secondly, the effects of addition of gold on the recrystallization and grain boundary migretion of high purity aluminum were investigated. It was found that the addition of a small amount of gold remarkably raised the recrystallization temperature and that the activation energy for the grain growth attained to 44.052.0 kcal/mol when it contained a small amount of gold (0.0015 0.0065 at.%).According to the theory of Lücke et al., the results of the present experiments suggest that the binding energy between the gold atom and the grain boundary of pure aluminum is at least 12 kcal/mol. Such a high value of the energy cannot be explained by the elastic model taking into account of the difference in atomic radius between solvent and solute atoms. A chemical interaction between the gold atoms and the grain boundary was proposed on the assumption that the grain boundary was in a liquid state and the binding energy with the gold atoms was estimated to be about 5 kcal/mol from the temperature dependence of the distribution coefficient.