Microstructural factors controlling crack resistance of Zn-Al-Mg alloy coatings prepared via hot-dip galvanizing process: Combined approach of in-situ SEM observation with digital image correlation analysis

To understand the microstructural factors controlling deformability during the forming process, a comprehensive analysis using in -situ scanning electron microscopy (SEM) with digital image correlation (DIC) for hot-dip Zn-6 %Al-3 %Mg alloy coatings on steel sheets in bending deformation was performed. The Zn-Al-Mg alloy coating exhibited two major microstructural constituents: the primary solidified Al (fcc) phase with dendritic morphologies and a fine ternary eutectic (TE) microstructure of the Zn (hcp), Al, and Zn2Mg phases. Major cracks were initiated on the tensile-strained coating surface, where the strain was localized in the TE regions, whereas the softer dendritic Al phases prevented the crack propagation. Microcracks were initiated in the binary eutectic (BE) Zn/Zn2Mg phases (Al-poor region) localized around the dendritic Al phases, whereas they were arrested in the TE region containing fine granular Al phases. The dendritic Al and fine granular Al phases embedded in the TE microstructure contributed to the high crack resistance.