Nucleation of secondary defects in quenched gold and zinc-doped gold

The nucleation of stacking-fault tetrahedra in pure gold and in gold-0·65 a/o Zn has been quantitatively studied by transmission electron microscopy over the temperature range −22°C to 120°C. The activation energy for nucleation is 0·29±0·03 ev for pure gold and 0·54±0·04 ev for the alloy. Nucleation appears to be heterogeneous in both cases. At temperatures higher than 60°C the final density of tetrahedra and the rate of nucleation both decrease as the temperature increases. The nucleation kinetics in this upper temperature range have been studied in detail and the existence of a ‘negative reaction’ above 60°C established. The negative reaction is a counter process to nucleation and is a direct result of the instability of embryonic clusters, possibly trivacancies, above 60°C. The analysis of the negative reaction results in a physical definition of two characteristic temperatures, T 1∗ = 60°C for pure and zinc-doped gold and T 2∗ = 112°C for pure gold and 144°C for zinc-doped gold. Between T 1∗ and T 2∗ the negative reaction is appreciable and its rate must be subtracted from the extrapolated nucleation rate in order to obtain the observed nucleation rate. Above T 2∗ the negative rate is higher than the extrapolated rate and the formation of tetrahedra is therefore improbable. The proportion of Frank loops increases with temperature above T 1∗. The kinetics of their formation were studied in the temperature range 80–120°C, and the activation energy found to be 0·10±0·03 ev. Their formation is a nucleation and growth process.