EBSD characterization of dendrite growth directions, texture and misorientations in hot-dipped Al-Zn-Si coatings

Al-Zn-Si coatings (55% Al-43.4% Zn-1.6% Si in weight pet) deposited on steel substrates by the hot-dipping process have been analyzed by Electron Back-Scattered Diffraction (EBSD) and optical microscopy. In these 20 mum-thick films, the main growth directions of the dendrites have been unambiguously identified as bring the closest to [320] directions. When the surface precisely corresponds to a (001) plane of the grain, the four usual [100] growth directions of fcc metals are replaced by eight [320] directions growing parallel to the coating (eight-fold symmetry dendritic pattern). When the surface nearly corresponds to (101) or (111) planes, six growth directions close to [320] projections result in a six-fold symmetry dendritic pattern. This morphology change is attributed to an intrinsic modification of the interfacial energy anisotropy. On the other hand, crystallographic orientation within a single grain is shown to vary substantially: in very large grains, misorientations as large as 35 deg. have been evidenced. Being spatially correlated to the location in the coating and to the dendritic pattern, these misorientations are explained in terms of systematic lattice spacing variations associated with the specific microsegregation pattern. However, thermal stresses induced upon cooling as a result of differential thermal contraction between the coating and substrate, can also contribute to these misorientations. Finally, it is shown that there is no relationship between the strongly marked texture of the cold-rolled substrate and the random orientation of the grains of the Al-Zn-Si coating. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.