Orientation Dependence of Cracking in Hot-Dip Zn-Al-Mg Alloy Coatings on a Sheet Steel
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INTRODUCTION
HOT-DIP Zn and Zn alloy coatings on steel sheets have evolved to meet demands such as stronger corrosion resistance and higher formability in the automotive and construction industries.[1] For the simultaneous satisfaction of these two properties, numerous studies have been conducted to determine appropriate alloying elements to add to Zn and to control the phases formed from them.[2–6] Different problems have both appeared and been solved in regard to the development of different alloying systems of Zn alloy coatings, but among them, the cracking that occurs during the deformation of coated steel sheets is a persisting issue awaiting an appropriate countermeasure. Previous studies[7–20] on the cracking of Zn and Zn alloy coatings have reported many influencing factors, such as alloying elements and composition, phase distribution, grain size, texture, bonding condition between the coating and substrate, coating thickness, and deformation mode. There have been a lot of models to explain the cracking behavior in hot-dip Zn and Zn alloy coatings.[1] Jaffrey et al[21] demonstrated that non-basal grains were associated with a higher density of spangle cracks than other oriented grains. Parisot et al[22] reported that cleavage cracking depends on the preferred orientations of the grains in the coatings, and the normal stress component for cleavage to occur is maximal in grains having their c-axis along the tensile direction. Tzimas and Papadimitriou[23] noted possible Y.B. PARK and I.G. KIM are with the Department of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, 540-742, Korea. Contact e-mail: [email protected] S.G. KIM is with the Department of Materials Science and Engineering, Kunsan National University, Kunsan, 573-701, Korea. W.T. KIM is with the Department of Optical Engineering, Cheongju University, Cheongju, 360-764, Korea. T.C. KIM, M.S. OH, and J.S. KIM are with POSCO Technical Research Laboratories, Gwangyang, 545-090, Korea. Manuscript submitted June 23, 2016. Article published online January 9, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
effects of residual stresses generated due to difference in the coefficients of thermal expansions (CTEs) between the coating phases and the substrate steel. They[23] claimed that the microcracks observed in the as-deposited state of the coatings must form due to tensile residual stresses. However, in reality, cracking phenomena occurring in Zn and Zn alloy coatings seem not to be completely understood using the prevailing models, and uncertainty still remains with regard to the basic causes of cracking, especially for texture-related cracking. The current work was aimed at investigating cracking phenomena in Zn-Al-Mg alloy coatings applied to automotive steel sheets. A bending test was devised to simulate practical deformation, and cracks generated at the initial stage of bending deformation were observed using electron backscattering diffraction (EBSD). The crystallographic orientations of the cracking planes observed in
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