Thermal transformations in mechanically alloyed Fe-Zn-Si materials
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I. INTRODUCTION
ZINC -based coatings are generally considered to be one of the most effective ways to protect steel against corrosive environments. The success of zinc coatings can further be attributed to their ease of application and cost-effectiveness.[1,2,3] The methods used in applying zinc alloy coatings to steel can vary depending on the specific product being coated and its intended use. Hot-dip galvanizing[4,5] and galvannealing[5,6,7] are two processes that are widely used for the application of an iron-zinc coating to steel. The principal method is galvanizing, where the article, after being pretreated, is immersed in a molten zinc bath at about 450 8C.[8] In galvannealing, as the name suggests, the article, after being galvanized, is subsequently heated to approximately 500 8C.[5] During this annealing cycle, the pure hzinc layer is completely transformed into an Fe-Zn alloy coating. The layers, as they appear above the steel surface, are G, G1, d, and z phases with decreasing iron contents, respectively. Examination of the Fe-Zn phase equilibrium diagram provided by to Reference 9 shows that these intermetallics form through invariant reactions. The presence of silicon in steel affects the formation of the coating layers during zinc applications. In fact, the abnormal increase in coating thickness associated with Sibearing steels was first observed by Sandelin.[10] He showed that, when silicon concentrations approached about 0.1 wt pct, the structure of the z layer was completely altered, whereby the uniform, continuous z layer, (which is found O. UWAKWEH, Assistant Professor of Metallurgical Engineering, and A. JORDAN, Graduate Student, are with the Materials, Science and Engineering Department, University of Cincinnati, Cincinnati, OH 452210012. P. MAZIASZ, Senior Development Engineer, Alloy Behavior and Design Group, Ceramic and Metals Division, is with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115. Manuscript submitted October 10, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
in a normal galvanized coating), disintegrates into individual z crystals surrounded by zinc.[10–13] Mackowiak and Short[13] suggest that this effect is attributed to the low solubility of Si in the solid Zn and, therefore, assume Si to also be low in the zinc-rich compounds. They also suggest that the Si is concentrated in pockets of liquid Zn near the iron surface, which, in turn, impedes the formation of a coherent z layer, thereby forming z crystals. In efforts to combat these problems, special galvanizing treatments, (i.e., high-temperature hot-dip galvanizing (HT-HDG)[14]) have been employed, in which the z layer is completely eliminated during the coating application. However, the layer may still form under equilibrium conditions, but without the thickness anomaly. The different coating applications are nonequilibrium processes, since the solidification of the molten Zn occurs simultaneously with reaction to the steel substrate. Given that the coating is usually thin and the application time is short,
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