A novel experiment for the study of substrate-induced nucleation in metallic alloys: Application to Zn-Al
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I. INTRODUCTION
BECAUSE of their good ductility and corrosion resistance properties, galvanized steel sheets produced by the hot-dipping process are widely used by automotive, household, and construction industries. In this process, the coldrolled steel sheet is first cleaned and then pre-heated before entering the Zn bath at about 450 °C. Typical galvanized baths contain Zn-0.2 wt pct Al and are saturated with Fe. Aluminum is added to form an inhibition layer,* which *The inhibition layer is first made of Fe2Al5.[1]
avoids the undesirable reaction of Fe and Zn. At the exit of the bath, the sheet is coated with a thin Zn liquid layer of about 10 to 20 m, the thickness of which is adjusted by air knives, located just above the bath. The coating then solidifies in an air-cooling tower. The growth morphology of Zn grains, termed “spangles” in coatings, is fairly well understood and has been the subject of many studies.[1–4] In many cases, the spangles exhibit a typical sixfold symmetry with six primary dendrite arms (Figure 1). This occurs when their basal plane is nearly parallel to the coating surface. As their misorientation increases, their appearance changes. For small misorientations, they keep a sixfold symmetry, but one-half of each spangle is “shiny” and the other is “dull,” according to the terminology proposed by Strutzenberger.[1] This appearance, induced by the location of Pb precipitates, has been very well explained by phase-field simulation.[5] When the c-axis is in the plane of the coating, the shape of the spangle is that of a rhombus, the extension of which along the two directions reflect the slower dendrite growth kinetics along the 0001 direc-
A. QUIROGA, Doctoral Student, and M. RAPPAZ, Professor, are with the Computational Materials Laboratory, Institute of Materials, Ecole Polytechnique Fédérale de Lausanne, CH-1015, Lausanne, Switzerland. Contact e-mail:[email protected] S. CLAESSENS, Department Manager, is with the Arcelor Group, Ocas, B-9060, Zelzate, Belgium. B. GAY, Project Leader, is with Umicore Research, B-2250, Olen, Belgium. Manuscript submitted December 5, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
tions as compared with the 1010 directions.[1,2,4] In summary, as already understood by Cameron and Harvey,[2] the spangle morphology of Zn-Al-Pb can be explained by the interactions of six fairly fast 1010 and two slower 0001 dendrites interacting with the surfaces of the coating. If the spangle growth is more or less clear, their nucleation is the subject of controversy and studies, in particular, regarding the effect of elements such as Pb, Bi, or Sb. It is commonly admitted that heterogeneous nucleation of Zn grains occurs at the Fe2Al5 intermetallic layer formed at the steel sheet/Zn interface.[1,2] However, the oxide layer formed by the air knives at the free surface may probably also act as a nucleation surface. Cameron and Harvey[2] explained the drastic increase of the spangle size with the addition of Pb or Bi, via a poisoning mechanism.[2,3] Fasoyi
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