Formation of Fe2Al5-xZnx intermetallic crystals at the Fe-Zn interface in hot-dip galvanizing

The formation of Fe2Al5-xZnx inhibition layer at the Fe-Zn interface has been characterized by scanning electron microscopy/electron backscatter diffraction and transmission electron microscopy for interstitial-free steels galvanized in Zn baths containing 0.14-0.28 wt% Al. The inhibition layer formed in the 0.14 wt% Al bath is composed of fine, equiaxed Fe2Al5-xZnx crystals without preferred orientations. The inhibition layers formed in the baths containing 0.18 wt% Al or more are composed of orientated domains and each domain contains 4-12 Fe2Al5-xZnx crystal variants of specific orientations. All the variants follow the same Fe2Al5-xZnx/Fe orientation relationship of [110](Fe2Al5-xZnx)//[1 (1) over bar1](Fe) and (001)(Fe2Al5-xZnx)//(110)(Fe). Therefore, each domain is formed on a single alpha-Fe grain and the number of variants in the domain is determined by the orientation of the alpha-Fe grain. The fraction of the area of epitaxial growth increases with increasing Al content in bath. When the Al content reaches 0.28 wt%, a layer of a single variant (SV) crystal can grow epitaxially on the alpha-Fe grains having orientations distributed from < 110 > to < 113 > parallel to the normal direction of the sheet surface. The formation of a dense SV inhibition layer is the major reason for the drastic reductions in both the Al-pickup rate and the inhibition layer thickness at high Al content baths.