Dendritic Morphologies of Hot-Dip Galvanized Zn-0.2 Wt Pct Al Coatings on Steel Sheets

The dendritic morphologies of hot-dip galvanized Zn-0.2 wt pct Al coatings on steel sheets with two different surface roughness conditions were systematically examined as functions of the inclination angle and axis of the Zn basal plane with respect to the sheet surface by using electron backscattered diffraction. When the inclination axis of the basal plane was \( \langle 1\bar{2}10 \rangle \), the dendrite changed its morphology by following the sequence of six-fold → eight-fold → elongated X → elongated X + C patterns with increasing inclination angle. When the inclination axis was \( \langle 10\bar{1}0 \rangle \), the sequence of morphological patterns on the mirror-polished steel substrate was six-fold → eight-fold → two-fold → four-fold, whereas, for the steel substrate with a rough surface, the eight-fold pattern was missing in the sequence. These sequences of morphological changes indicated the presence of a third family of preferred dendritic growth directions, in addition to the two previously known families of \( \langle 10\bar{1}0 \rangle \) and \( \langle 0001 \rangle \). The third growth direction family is proposed to be those normal to the \( \left\{ {1\bar{2}11} \right\} \) planes. The absence of the eight-fold pattern on the rough steel surface was attributed to the surface undulation that perturbed the growth in the six weakly preferred directions toward the two strongly preferred \( \langle 10\bar{1}0 \rangle \) directions.