Morphology and microstructure of electrodeposited zinc-iron binary alloys.

Abstract

Zinc-iron electrodeposited steel sheets have been adopted for automobile bodies for their improvements in the corrosion resistance and organic-coating capability. In principle, these properties depend on the crystal structure, morphology and microstructure of deposits. The deposits of various iron content were prepared galvanostatically from sulfate bath and examined by scanning and transmission electron microscopy. The single η-phase of 99.5 at% zinc exhibits a morphology of randomly oriented hexagonal plates which are thin in the direction of c-axis. With the increase of iron contentin the deposits, the Γ-phase forms in addition to the η-phase. The η-phase plates stack in the direction of c-axis and form the hexagonal columnar crystals having triangular pyramidal shape. The Γ-particles disperse within the grains and along grain boundaries of these η-phase plates. The Burgers relationship holds between the η- and Γ-particles. The hexagonal columnar crystals of 91.2 at% zinc form steps on {10·0}η surfaces. With an increase of iron content to 85.1 at% zinc, however, the steps on {10·0}η surfaces are flattened. With further increase of iron content to 73.0 at% zinc a large amount of Γ-particles are produced, and these Γ-particles can no more be absorbed within the η-phase. These excess Γ-particles form granular crystals, and precipitate among the hexagonal columnar crystals.