Dendritic Growth Morphologies in Al-Zn Alloys-Part II: Phase-Field Computations

In Part I of this article, the role of the Zn content in the development of solidification microstructures in Al-Zn alloys was investigated experimentally using X-ray tomographic microscopy. The transition region between dendrites found at low Zn content and dendrites found at high Zn content was characterized by textured seaweed-type structures. This Dendrite Orientation Transition (DOT) was explained by the effect of the Zn content on the weak anisotropy of the solid-liquid interfacial energy of Al. In order to further support this interpretation and to elucidate the growth mechanisms of the complex structures that form in the DOT region, a detailed phase-field study exploring anisotropy parameters' space is presented in this paper. For equiaxed growth, our results essentially recapitulate those of Haxhimali et al.[1] in simulations for pure materials. We find distinct regions of the parameter space associated with and dendrites, separated by a region where hyperbranched dendrites are observed. In simulations of directional solidification, we find similar behavior at the extrema, but in this case, the anisotropy parameters corresponding to the hyperbranched region produce textured seaweeds. As noted in the experimental work reported in Part I, these structures are actually dendrites that prefer to grow misaligned with respect to the thermal gradient direction. We also show that in this region, the dendrites grow with a blunted tip that oscillates and splits, resulting in an oriented trunk that continuously emits side branches in other directions. We conclude by making a correlation between the alloy composition and surface energy anisotropy parameters.

Texture Formation in Hot-Dip Galvanized Coatings

Aurèle Mariaux

Continuous hot-dip galvanization is a process during which steel sheets are coated with a layer of Zn-Al alloy to protect them against corrosion. As the name indicates, this is achieved by dipping the preheated sheet into a bath of molten alloy. The sheet is then withdrawn and cooled down in air. The solidification microstructure of such coatings is made of large zinc grains (spangles), which preferentially have their hexagonal ‹0001› axis perpendicular to the sheet plane (basal texture). The mechanisms leading to this preferred orientation were investigated in this study, with a particular focus on nucleation and dendritic growth. An experiment was designed to reproduce the solidification stage of hot-dip galvanization on a laboratory scale. Industrial coatings were remelted in an infrared image furnace and solidified under controlled cooling conditions. Thanks to the wide range of possible cooling rates, the influence of this parameter on the size and orientation of the zinc grains could be investigated. The thermal plateau induced by the solidification of the thin coating could be detected, even for fairly thick steel sheets. However, the signal was better for larger coating/steel thickness ratio. A minimum coating thickness and Al content are needed if Fe-Zn intermetallic outbursts are to be avoided during remelting. Additionally, skin-pass, as well as any deformation of the coated sheets, must be avoided prior to these laboratory experiments. A coupled cellular automaton — finite volume model was concurrently developed to simulate solidification. It was then used to obtain the nucleation parameters of the spangles by inverse modeling based on the remelting experiments. The model accounts for the effects of zinc crystal symmetry and of the confined coating geometry on nucleation and growth phenomena. In particular, the preferred orientation of the grains is included in the nucleation site distribution. Its basal part is still an estimate, but it could be refined from new experimental measurements with the infrared remelting device. On a more fundamental side, a mathematical formulation of the anisotropic solid-liquid interfacial energy of Zn-Al alloys was derived from the equilibrium shape of liquid droplets in a solid matrix. This property changes by more than 40% between the c axis and the basal plane. This very large variation has important effects on the solidification mechanisms. The interfacial energy, as well as the wetting conditions for an anisotropic grain, were then implemented in a phase field model, which was used to simulate the growth of zinc dendrites in coatings. It revealed that the growth directions and velocities are strongly influenced by the film thickness and by anisotropic wetting effects. The possibility that the preferred zinc orientation could be induced by the textured steel substrate was investigated by comparing the orientation of zinc grains with that of the ferrite grains underneath. It was observed that spangles with a random orientation distribution are independent of the substrate. Unfortunately, the observed specimen did not exhibit a basal coating texture and thus, the existence of a relation between basal grains and ferrite texture could not be investigated. The anisotropy formulation was also used to extend the classical theory of heterogeneous nucleation to anisotropic metals. These computations showed that a basal orientation of the nuclei is strongly favored from an energy point of view. Such developments can explain the occurrence of basal grains around holes that form during infrared remelting, so-called rose structures. Preferential basal nucleation and the wetting influence on dendrite growth are also responsible for the morphology transition observed in Galfan, a Zn-Al coating of eutectic composition: when appropriate cooling conditions are applied, a shiny layer of basal grains forms at the top surface before the onset of eutectic solidification. Finally, these mechanisms could also explain the preferential basal orientation of regular hot-dip galvanized coatings. However, the conditions on a galvanizing line do not meet the criteria that are requested to activate them in Galfan. Additionally, the texture of hot-dip coatings is reported to depend on the surface preparation of the sheet, although the anisotropic nucleation theory does not consider the steel substrate. Further investigations on these contradictions are needed before these mechanisms can be used to explain the preferred orientation in regular galvanized coatings.

EPFL2010

Phase-field modeling of the dendrite orientation transition in Al-Zn alloys

Jonathan Dantzig, Paolo Di Napoli, Jonathan Friedli, Michel Rappaz

With a few exceptions, phase-field simulations of dendritic growth in cubic materials have been modeled using simple expressions for the interfacial energy anisotropy and with strong anisotropy. However, recent experimental results show that the Dendrite Orientation Transition (DOT) observed in Al-Zn alloys by Gonzales and Rappaz [Met. Mat. Trans. A37 (2006) 2797] occurs at weak anisotropy, and modeling these results requires at least two anisotropy parameters. In the present work, we solve the corresponding phase-field model on an adaptive grid, after measuring and compensating for the grid anisotropy. A systematic scan of equiaxed growth simulations was performed in the range of the anisotropy parameter space where the transition is expected. We find separate domains of existence of < 100 > and < 110 > dendrites, similar to those previously reported by Haxhimali et al. [Nat. Mat. 5 (2006) 660] for pure materials. In the so-called hyperbranched regime, lying between the < 100 > and < 110 > regions, we observe a competition between < 100 > and < 110 > growth directions, but no seaweed structures. Directional solidification simulations showed the stabilizing effect of the thermal gradient on the twofold splitting of < 110 > dendrites, and the importance of the choice of anisotropy parameters. We also found a strong dependence between the orientation of the crystal axes with respect to the thermal gradient and the actual growth direction. Finally, 3-dimensional seaweed microstructures were modeled for the first time, demonstrating that this pattern is a result of not only the values of anisotropy parameters, but also a consequence of directional solidification.

Iop Publishing Ltd2012

X-ray tomographic microscopy analysis of the dendrite orientation transition in Al-Zn

Paolo Di Napoli, Jonathan Friedli, Michel Rappaz

Recently, Gonzales and Rappaz [Met. Mat. Trans. A37:2797, 2006] showed the influence of an increasing zinc content on the growth directions of aluminum dendrites. < 100 > and < 110 > dendrites were observed below 25wt.% and above 55wt.% zinc, respectively, whereas textured seaweeds and < 320 > dendrites were observed at intermediate compositions. Considering the complexity of these structures, it is necessary to first characterize them in further details and second, to model them using the phase field method. The so-called Dendrite Orientation Transition (DOT) was thus reinvestigated in quenched Bridgman solidification samples. The combination of X-ray tomographic microscopy and electron back scattered diffraction (EBSD) analysis on a whole range of compositions, from 5 to 90wt.% Zn, allowed insights with unprecedented details about texture, growth directions and mechanisms of the aforementioned structures. We show that seaweeds rather than dendrites are found at all intermediate compositions. Their growth was confirmed to be constrained within a (100) symmetry plane. However, new findings indicate that the observed macroscopic texture does not necessarily correspond to the actual growth directions of the microstructure. Further, it seems to operate by an alternating growth direction mechanism and could be linked to the competition between the < 100 > and < 110 > characters of regular dendrites observed at the limits of the DOT. These characters, as well as 3D seaweeds, are observed in phase-field simulations of equiaxed growth and directional solidification, respectively. This study emphasizes the importance of accurate experimental data to validate numerical models and details the progress that such combinations provide for the understanding of growth mechanisms.

Iop Publishing Ltd2012

Texture Formation in Hot-Dip Galvanized Coatings

Aurèle Mariaux

EPFL2010