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

Abstract

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.

Experimental study and phase diagram calculation in Al–Zn–Mg–Si quaternary system

“…Microalloying elements in aluminum-zinc based alloys alter properties by changing the morphology, chemistry, structure, spatial distribution and size of precipitates. It was found that Mg, Si and RE (rare-earth) as alloying elements can significantly improve characteristics of the Al-Zn based alloy, such as AZ alloy [1][2][3] and alloy coating [2,4]. The solid solution phases (Al) and (Si) and the intermetallics Mg 2 Si and MgZn 2 are able to improve the wear resistance of Al-Zn based alloy and the corrosion resistance of the hot-dip Al-Zn alloy coating.…”

Galvanic corrosion behaviour of hot-dipped zinc-aluminum alloy coated titanium-aluminum couples

“…It can be concluded that the content of the aluminum gradually decreases from the depth of 30 µm to the substrate, which is benefit to the improvement of the interfacial bonding strength. Results suggest that the maxima of Al is observed in the vicinity of the two boundaries due to the preferred areas for the growth of the primary Al dendrites after solidification starts . X‐ray diffraction pattern in Fig.…”

Microstructural Development in a Laser-Remelted Al-Zn-Si-Mg Coating

“…The misorientation value from the centre of the dendrites to the grain boundary is usually in the range of 1 to 2 degrees, sometimes even 5 degrees. The misorientation phenomenon is known in the literature and is explained by the thermal stresses induced by differential thermal expansion of the coatings and the steel substrate or micro-segregation during solidification of the coating 5 , 6 .

Figure 6 Top-view EBSD mapping IPF in Z direction; ( a ) as-received and ( b ) and ( c ) laser-remelted.

…”

“…The solidification process of the coating overlay is initiated by the formation of aluminium nuclei on the intermetallic layer and the growth of the primary phase through the coating thickness and then laterally in two dimensions, parallel to the steel sheet 5 . It has been shown that there is no texture correlation between the substrate and the coating overlay, and the crystallographic misorientation within a single grain is relatively high, especially in the large grains 5 , 6 .…”