Orientation domains and texture in hot-dipped galvanized coatings

The crystallographic orientation of galvanized coatings (Zn-0.2Al-0.15Sb in wt pct) has been characterized by Electron-Backscattered Diffraction (EBSD) and optical microscopy. While 80 pct of the nucleation spots in the coating give rise to single-crystal Zn grains, it has been found that about 20 pct of them give rise to two or more orientation domains, each having a specific crystallographic orientation. For such “polycrystalline Zn grains,” the orientations of the domains are crystallographically related: they have a dense crystallographic direction (〈1010〉, 〈1120〉, or 〈0001〉) in common. Moreover, the crystallographic relationships are similar to those observed in snowflakes and can be partially explained by the concept of a coincidence-site lattice (CSL). The EBSD measurements were also used in order to measure quantitatively the crystallographic texture. In particular, it has been evidenced that the (0001) texture of galvanized coatings is the result of two contributions: (1) the nuclei are preferentially oriented with the basal plane parallel to the coating plane (33 pct of the grains have an angle between the basal plane and the coating plane smaller than 22.5 deg), and (2) the grains having the basal plane parallel to the coating plane grow faster (these grains represent 43 pct of the coating surface). This reinforcement of the texture during growth is in agreement with that predicted by growth models, which take into account the effect of the interfaces.