Discontinuous precipitation in Cu-5.7wt%Ag single crystals

The formation and growth of disk-shaped aggregates consisting of rod-shaped Ag precipitates and a Cu matrix on {001} planes of single crystals of a Cu-5.7wt%Ag alloy have been investigated. The precipitation starts on dislocations and proceeds by means of a discontinuous process without existing dislocations. The aggregates grow at a constant velocity under an isothermal growth condition. A kinetic analysis of the aggregate growth using the model of Zener has yielded values of the diffusivity of Ag in Cu. The activation energy for diffusion is estimated to be 185 kJ/mol. The stress-orienting formation of the disc-shaped aggregates has been found. A tensile stress applied in the [001] direction favors the formation of the aggregates perpendicular to the [001] axis, while a compressive stress results in the preferential formation of the aggregates parallel to the stress axis. The relation between the sense of applied stress and the stress-oriented morphology is understood in terms of the misfit strain of the aggregates in a direction perpendicular to the plate plane being larger than that in the parallel direction.