Electrodeposition of Metastable Au–Ni Alloys

The electrodeposition of Au–Ni alloys from near-neutral, sulfite-based electrolytes derived from a commercial bath for soft gold plating is investigated. Alloy compositions ranging from 0 to 90 atom % Ni were obtained by varying the deposition potential, with Ni content increasing with overpotential. Cathodic efficiency was lower than 50% due to concurrent parasitic reactions, including the reduction of products from the decomposition of sulfites and the hydrogen evolution reaction. As-deposited films form a continuous series of metastable solid solutions and exhibit a nanocrystalline morphology, with grain size decreasing with increasing Ni content and a possible Ni enrichment at the grain boundaries. Thermal annealing at was sufficient to start the relaxation of the metastable solid solution toward the thermodynamically stable biphasic configuration of pure Au and Ni phases; however, was necessary to complete the phase separation process within . The formation of a metastable structure is interpreted in terms of the limited surface diffusivities of adatoms at the growing interface and atomic volume differences. The excess free energy of the as-deposited alloys with respect to the stable, phase separated configuration is estimated between 6 and 18 kJ/mol, consistent with what can be expected in electrochemical processing.