Unraveling dislocation mediated plasticity and strengthening in crack-resistant ZnAlMg coatings

Abstract

Mg-alloyed zinc coatings suffer from early cracking during deformation due to the combined influences of detrimental constituents (e.g., Zn/MgZn2 binary eutectic), unfavorable crystal orientations and incompatible local plastic deformation. This study addresses the crack resistance and deformation mechanisms of a ZnAlMg coating, in the absence of binary eutectic, by using correlative in-situ scanning electron microscopy tensile tests, electron backscatter diffraction, scanning transmission electron microscopy and micro digital image correlation methods. The plasticity and strengthening mechanisms of the binary eutectic-free ZnAlMg coating are thoroughly unravelled via theoretical and experimental approaches including dislocation slip trace analysis, Taylor factor calculations, quantitative dislocation density determination and slip/strain transfer across the grain boundaries. It has been found that the microstructure of the coating, comprising of primary zinc and ternary eutectic, effectively accommodates the applied strain without cracking till fracture of steel substrate. The absence of binary eutectic and the activation of abundant deformation mechanisms in the coating microstructure, promote local compatible plastic deformation and inhibit early damage. Effective slip transfer is unveiled by activation of pyramidal <c+a> slip in addition to primary dislocation slip systems. The present findings deliver significant insights and solution for improvement in the formability of Mg-alloyed zinc coatings on steel sheets by tailoring the microstructure.