The structure of quenched and aged gold, gold—cadmium, and gold—zinc alloys

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Abstract
The structure of quenched and aged zone-refined gold and gold doped with 0·087, 0·17 and 0·35 at. % Cd, and 0·15 at. % Zn has been studied by transmission electron microscopy. Thin foils of the materials were quenched from 980°C and pre-aged at temperatures between −30°C and 20°C prior to ageing at 165°C for one hour. Thus the kinetics of defect nucleation, a fast reaction occurring during the pre-ageing treatment, have been studied and the activation energy for the process found to be 0·28 ev for zone-refined gold, and 0·34 ev and 0·42 ev for the Cd- and Zn-doped gold respectively. The structure of zone-refined gold consists of stacking-fault tetrahedra with 10 to 20% dislocation loops. Both cadmium and zinc present in gold in amounts of ∼0·1 at. % strongly induce heterogeneous nucleation of tetrahedra so that the density of tetrahedra is higher in the doped gold. Zinc atoms also nucleate dislocation loops in the early stage of pre-ageing. The activation energy for the process resulting in loops in the zinc-doped gold is 0·7 ev. The results are discussed in terms of a nucleation mechanism proposed by Chik for the stacking-fault tetrahedra involving collisions between single, di and trivacancies. The absence of trivacancies in the early stage of preageing is most probably the cause of loops in the zinc-doped gold.