EFFECT OF LEAD AND ANTIMONY ON THE GRAIN STRUCTURE OF DIRECTIONALLY SOLIDIFIED ZN-AL ALLOYS

摘要

Zinc alloys with 0.2% Al are widely used to protect steel sheets against corrosion. The thin coatings (typically 20-30 μm), deposited by hot dipping, exhibit fine grains (typically 1 mm in diameter), but with small additions of lead or antimony, the grains become much larger (typically 1 cm). In order to investigate this phenomenon, bulk specimens have been solidified in a standard Bridgman furnace (BF) and in a water-cooled directional solidification (DS) experiment. In this last device, a pre-coated steel sheet was immersed in the melt parallel to the solidification direction in order to induce a competition between regular columnar grains and grains nucleated on the substrate and growing in a transverse direction. In the BF specimens, it was found that the structure changed from columnar to equiaxed when the Al content was increased from 0.2 wt% to 3 wt%. However, with 0.11 wt% Pb addition, it remained columnar over the entire Al concentration range. In Zn-0.2%Al DS alloys, the structure was columnar almost up to the top of the specimen, but "transverse" grains could form at the surface of the sheet. Their extension was analyzed using a model similar to that of Hunt for the columnar-to-equiaxed transition. With small additions of Sb, these "transverse" grains totally disappeared. From microstructure observations and EBSD analyses, it is concluded from both experiments that Pb and Sb most likely poison the nucleation sites.