MORPHOLOGY AND MICROSTRUCTURE OF ELECTRODEPOSITED ZINC-NICKEL BINARY-ALLOYS

Zinc-nickel electrodeposited sheet steels have widely been adopted for automobile bodies for their improvements in the corrosion resistance and formability. In principle, these properties depend on the crystal structure, morphology and microstructure of deposits. The deposits of various zinc contents have been prepared galvanostatically from sulfate baths and examined by scanning and transmission electron microscopies. The 96.8 at% zinc, eta-single phase deposit exhibits a collective morphology of hexagonal plates thin in the direction of c-axis. With the decrease of zinc content, the deposit changes to the hexagonal columnar crystals which have triangular pyramidal shape in morphology. These hexagonal columnar crystals are formed by the lateral growth of the hexagonal plate edge i.e. step along (00.1)eta-side surfaces. The 92.7 at% zinc hexagonal columnar crystals form steps on {10.0}eta-surfaces, and flatten the steps on {10.0}eta-surfaces with the decrease of zinc content. Further decrease of zinc content to 88.4 at% zinc, the hexagonal columnar crystals can no more absorb gamma-particles. These excess gamma-particles form granular crystals, and precipitate between the hexagonal columnar crystals.