Stacking fault formation and Ag precipitation in Cu-Ag-Sc alloys

We studied nucleation and growth of Ag precipitates in a Cu-6 wt%Ag-0.15 wt%Sc alloy and found that the early nucleation of Ag precipitates was similar to continuous precipitation in that it occurred only on dislocations within Cu grains, not on any grain boundaries. In aged samples, we observed stacking faults that extended into nanotwins. These planar defects formed during early nucleation of Ag precipitates and extended into the surrounding Cu matrix. The formation of planar defects released misfit strain on Cu/Ag interfaces, enhancing subsequent nucleation and growth of Ag precipitates. Unlike the intrinsic defects found in previous research, these defects were clearly extrinsic. The planar defects provided a row of additional sites aligned along twin boundaries for the nucleation of Ag precipitates. The formation of new Ag precipitates, by reducing dissolved Ag in the Cu matrix, increased conductivity significantly. Planar defects reduced electrical conductivity somewhat, but the synergy between Ag precipitation combined with planar defects had the effect of substantially increasing both hardness and conductivity.