12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Hot-dip galvanized Zn-rich coatings on steel sheets often exhibit dendritic morphologies formed by the primary Zn phase during solidification. The corrosion resistance and surface quality of the galvanized coatings depend not only on the coating chemistry and grain size, but also on the dendritic morphologies of individual coating grains. Therefore, it is of fundamental importance to understand how the dendritic morphology depends on the crystallographic orientations of coating grains. In this study, the dendritic morphologies of hot-dip galvanized Zn-0.2 wt.% Al coatings on steel sheets 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 <1-210>, the dendrite changed its morphology by following the sequence of six-fold --> eight-fold --> elongated X --> elongated X+C patterns with increasing inclination angle from 0 degree to 90 degrees. When the inclination axis was <10-10>, the sequence of morphological patterns was six-fold --> eight-fold --> two-fold --> four-fold. These morphological changes of dendrites could not be explained based on the two previously known families of preferred growth directions, <10-10> and <0001>, but they suggested the existence of a third family of weakly preferred growth directions, which were close the normal directions of the {1-211} planes. Based on the morphological observations of Zn dendrites, we proposed an anisotropy function of the growth kinetics as a function of the polar angle and the azimuthal angle. In the presentation, we will demonstrate using three-dimensional phase-field simulations that most of the morphological characteristics, such as the sequences of the morphological changes depending on the inclination axis and the existence of the two-fold, eight-fold and elongated X patterns, can be reproduced only when the third preferred growth directions are taken into account.