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Micropillar Compression Deformation of Fe-Zn Intermetallic Compounds in the Coating Layer of Galvannealed Steel Norihiko L. Okamoto, Daisuke Kashioka and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT The deformation behavior of two of the five Fe-Zn intermetallic phases (ī, ī1, į1k, į1p and ȗ), which are formed in the coating layer of galvannealed steel, has been investigated through uniaxial compression tests for single-phase polycrystalline micropillars. The ȗ phase is ductile to some extent while the ī1 phase is brittle. These results are consistent with the Peierls stress estimated from the crystal structures by assuming the primitive Peierls-Nabarro model. INTRODUCTION Zinc-coated (galvanized) steel is widely used in applications in automotive and building industries. Zinc is coated to improve the aqueous corrosion resistance of steel by a shielding mechanism called galvanic protection, in which the substrate steel is cathodically protected by the sacrificial corrosion of the zinc coating because zinc is less noble (more electronegative) than iron. The galvanized steel is sometimes further heat-treated (galvannealed: GA) to alloy the zinc coating with the substrate iron by diffusion, resulting in improved coating adhesion and weldability. The coating layer of galvannealed steel consists of a lamellar series of intermetallic compounds in the Fe-Zn system; ī (Fe3Zn10), ī1 (Fe11Zn40), į1k (FeZn7), į1p (FeZn10) and ȗ (FeZn13) in decreasing order of the iron content (see figures 1(a,b)) [1]. The deformation and fracture behavior of these intermetallic compounds influences the press formability response of the galvannealed steel. During press forming, zinc coating occasionally fails as a result of decohesion at the coating/substrate interface (flaking) and/or particle formation by intracoating failure (powdering). The coating failure occurs more significantly with both increasing Fe content and thickness of the coating layer. It has been reported in review articles that the į1 (į1k/į1p) phase is the most ductile and the ī (ī/ī1) and ȗ are brittle [2], whereas the ȗ phase is the most ductile and the į1 and ī are brittle [3]. However, the coating failure has been only phenomenologically understood and still under discussion. No mechanical properties of each phase have been investigated except for micro-Vickers hardness, partly because of difficulties in preparing single-phase specimens in bulk form. If any, the neighboring phases (į1k/į1p as well as ī/ī1) have not been always distinguished because the layers are as thin as a single micrometer [4-7]. However, recent advances in fabrication processes with precise control of material dimensions down to nanometer level have made it possible to investigate mechanical properties at these scales [8-10]. In the present paper, we investigate compression deformation behavior of two of the five intermetallic phases through compression tests of micrometer-sized specimens prepared via the focused ion beam (