Cracking behavior and formability of Zn-Al-Mg coatings:Understanding the influence of steel substrates

Zn-Al-Mg coatings are important materials for the corrosion protection of steel sheets. However, susceptibility towards cracking limits the formability performance of these coatings. In this study, we focus on the effect of the underlying steel substrate on cracking behavior in these coatings. In order to elucidate this, a high-strength low-alloy (HSLA) steel substrate and an interstitial-free (IF) steel substrate are coated with two different ZnAlMg coatings with and without binary eutectic microstructures. Meticulous in-situ tensile and bending tests are conducted in a scanning electron microscope. To quantify the strain distribution and damage incidents, micro and macro digital image correlation techniques are utilized in order to illuminate the associated cracking causes across length scales. Furthermore, electron backscatter diffraction method is applied to study the role of crystallographic orientation on the cracking tendency. Crack opening and crack area fractions are correlated with the applied strain and bending angles. The findings denote that the discontinuous yielding (Lüders banding) of the HSLA steel substrate generates substantial surface roughening and heterogeneous deformation in the coatings that facilitates cracking. In contrast, the IF steel induces a more uniform deformation within the coatings leading to much reduced crack size and crack area fraction. This study has resulted in a key element of a guideline towards crack-resistant and formable Mg-alloyed zinc coatings

Outstanding cracking resistance in Mg-alloyed zinc coatings achieved via crystallographic texture control

Cracking limits the formability of Mg-alloyed zinc coatings on steel substrates. Unfavorable crystal orientations and brittle microstructural components serve as the main sources of cracks in these coatings. In this study, we overcome the deformation-induced cracking and substantially enhance the formability of Zn-Al-Mg coatings by controlling their crystallographic texture. To demonstrate this, in-situ scanning electron microscopy uniaxial tensile tests and thorough orientation image microscopy have been employed. Ultimately, we validate our findings by implementing quantitative plastic deformation-based criteria, namely local strain hardening exponent and Schmid factor distributions within the examined coating microstructures. The approach and findings of the present work considerably resolve the long-lasting cracking problem of these coatings

The effect of grain refinement on the deformation and cracking resistance in Zn–Al–Mg coatings

The present study is dedicated to explore the effect of grain refinement on cracking resistance of hot-dip galvanized Zn–Al–Mg coatings on steel substrate. In this work, we demonstrate the enhancement of plastic deformation and cracking resistance by refining the microstructure (primary zinc grains) of the Zn–Al–Mg coatings. For this purpose, two types of Zn–Al–Mg coatings namely, fine grained and coarse grained microstructures are investigated utilizing in-situ scanning electron microscopy tensile tests. Electron backscatter diffraction technique is used to illuminate the deformation behavior at the scale of grains (and/or within grains). The results reveal that the coating with fine grained microstructure possesses higher ductility and cracking resistance, whereas the coating with coarse grain microstructure induces more transgranular cracking during deformation. Moreover, primary zinc grain refinement has been shown to decrease the fraction of coarse deformation twins that serve as undesirable sites of micro-cracking. In particular, both deformation mechanisms and cracking behavior are found to be grain size-dependent in these coatings

Mechanical properties and cracking behaviour of hot dip galvanized ZnAlMg coatings.

ZnAlMg coatings produced by hot-dip galvanization process have shown superior corrosion resistant, anti-galling and wear performances. Nevertheless, currently these coatings exhibit lower cracking resistance and ductility compared to conventional galvanized zinc (GI) coatings on steel sheets during forming processes. In this study, mechanical properties and cracking behavior of ZnAlMg galvanized steels have been investigated thoroughly. Microstructure, mechanical properties and key causes of cracking initiation and propagation have been scrutinized by utilizing scanning electron microscopy (SEM), orientation imaging microscopy, nanoindentation and in-situ SEM tensile/bending tests. Ultimately, effective plastic deformation-based factors are obtained to understand the cracking behavior and consequently link the microstructural features to cracking tendency of these coatings. The findings of this study are employed in designing new microstructure controlled ZnAlMg coatings with superb cracking resistance

Monitoring of the oxide chemistry-controlled surface ion mobility

Die Oberflächen- und Grenzflächenmobilität der Ladungsträger spielt eine zentrale Rolle für die Stabilität und die Dauerhaftigkeit der Metall/Polymer Grenzflächen. Das Verständnis dieser Zusammenhänge bildet die Grundlagen für die Entwicklung von neuartigen Polymerbeschichtungen und Metallvorbehandlungsprozessen. Trotz dieser bedeutender Rolle, blieben bis heute viele Aspekte der Oberflächen- und Grenzflächenmobilität unaufgeklärt. Einer der zugrunde liegenden Gründe ist der Mangel an Methoden die eine zerstörungsfreie Analyse der inneren Polymer/Metall Grenzfläche ermöglichen. Das Ziel dieser Arbeit ist eine Erweiterung der Anwendbarkeit der Raster Kelvin Sonde (SKP) für die Untersuchung der Ladungsträgermobilität auf Oxidoberflächen. Unterschiedlich vorbehandelte Aluminiumoxidoberflächen wurden daher mittels Röntgen Photoelektronenspektroskopie und Spektroskopische Ellipsometrie charakterisiert und mittels SKP bezüglich der Mobilität der Gegenionen untersucht