Selection of Phase Formation in Electroplated Ag-Cu Alloys

Abstract

Ag-Cu alloys were electrodeposited from a sulfate-based, non-complexing solution and a thiourea complexing electrolyte. The former solution exhibits independent reduction of the two elements, resulting in a sharp change in composition vs. potential and in the formation of a eutectic structure, with limited or none reciprocal solubility. The complexing solution on the other hand shows a more gradual change of the composition with potential, and X-Ray diffraction suggests the formation of a continuous series of solid solutions. TEM on the other hand was unable to distinguish between solid solution or eutectic formation. The different phase constitution observed from the two electrolytes can be rationalized in terms of the more negative applied potential that need to be applied to the complexing solution to achieve similar growth rates, leading to a lower surface mobility for the diffusing species and a higher tendency to form metastable structures.