Origin of intragranular crystallographic misorientations in hot-dip Al–Zn–Si coatings

The origin of intragranular variations of the crystallographic orientation in hot-dip Al–Zn–Si coatings is discussed based on new experimental results and modelling. The solidification microstructure in as-received 55Al–43.4Zn–1.6Si (in wt.%) coatings deposited on steel plates in an industrial production line was analyzed by electron backscattered diffraction, glow-discharge optical emission spectroscopy and atomic force microscopy (AFM). The results were compared with those obtained in coatings re-solidified under different cooling and mechanical loading conditions. Continuous variations of the crystallographic orientation as large as 35° were observed within individual grains of Al–Zn–Si, consistent with previous studies. However, the mechanisms previously proposed for the origin of intragranular crystallographic misorientations had to be revisited. The new experimental data acquired during this study indicate that the solidification shrinkage accumulating in the area of the grain envelope is the driving force for the formation of intragranular misorientations. The solidification shrinkage leads to the development of tensile stresses in the oxide film covering the coating while it solidifies. Estimations based on AFM profiles and phase field simulations of the dendritic structure indicate that the stresses applied on the dendrite network are sufficient to deform plastically the dendrite arms during solidification.