Microstructure and thermal stability of 'nanocrystalline' electrodeposited Au-Cu alloys

Gold and gold-copper foils were produced by electrodeposition with various baths. The process is relevant for jewellery, coloured coatings and metallization. Foils were obtained after stripping from the deposition substrate with thickness ranging from 30 to 90 mum The microstructure of foils is reported as revealed by XRD and TEM. In Au-Cu the substrate side presents two phases of different composition as shown by their lattice parameters: a Au-rich one and a Cu-Au solid solution, The composition changes across the foils and becomes uniform after a few microns. XRD reflections fi om the outer surface show broad peaks. Their profile analysis reveals that the material is heavily stressed. Stress relief occurs progressively on annealing both below and above the critical temperature for the order-disorder transformation. Although electrodeposited gold is usually reported as nanocrystalline, TEM shows areas containing very fine crystals next to larger ordered crystals containing twins. Thermal analysis shows disordering transitions at the expected temperatures and hydrogen desorption occurring in distinct temperature ranges. The latter phenomenon is due to either hydrogen absorbed during the deposition process (desorption around 300 degreesC) or to decomposition of chemicals incorporated in the foils (desorption above 650 degreesC). The annealed foils are ductile.