Etude des morphologies de solidification dans les alliages aluminium-zinc

Aluminum zinc alloys constitute an important class of wrought alloys, the so-called 7000 series, and are also widely used as anti-corrosion coatings of steel sheets. They are also interesting from a more fundamental point of view because zinc, a hexagonal close-packed (hcp) element, can be added to aluminum up to very large amounts (up to 94 wt pct) while keeping the face-centered cubic (fcc) structure. Dendrites are very common to most metallic alloys produced by solidification. Resulting from an instability of the solid-liquid interface, they have a tendency to develop along directions corresponding to convexities of the equilibrium shape crystal. The equilibrium shape of a crystal in contact with its liquid phase is dictated itself by the anisotropy of the solid-liquid interfacial energy γsl. Metallic alloys such as fcc nickel exhibit dendrites which are very well constrained to grow along ‹100› directions, thus indicating that γsl in this case has a fairly large anisotropy. The first unusual dendrite morphology observed in aluminium alloys more than half a century ago is the so-called feathery grains or twinned dendrites [1]. When trying to reproduce such morphologies in the laboratory using directional or Bridgman solidification conditions, Henry et al. [2] obtained various dendrite growth morphologies, depending on the composition of the alloy, speed of solidification and thermal gradient. Using Electron Back-Scattered Diffraction (EBSD), they clearly showed that such morphologies are ‹110› dendrites split in their center by a (111) twin plane and extending on both sides ‹110› arms. At about the same time, Sémoroz et al. [3] made careful observations of dendrite growth directions in Al-45 wt pct Zn coatings deposited on steel sheets by the hot dipping process. Although dendrites in this case interact with the free (oxidised) surface and with the thin intermetallic layer formed at the steel-coating interface, these authors could clearly identify ‹320› dendrite growth directions in grains having various orientations with respect to the coating surface. These observations were interpreted by Henry et al. [2] and Sémoroz et al. [3] as a modification of the weak anisotropy of the solid-liquid interfacial energy γsl due to the addition of solute element (such as zinc), having a strong anisotropy of the interfacial energy γsl. From these observations, a natural question was raised : what is the in uence of solute element on the weak anisotropy of the aluminium interfacial energy ? Can zinc progressively modify this anisotropy, and thus dendrite growth morphologies as its content is increased ? In order to answer this question, two kinds of solidification experiments were performed Bridgman and directionnal solidification on Al-Zn alloys over a large composition range. Results show that aluminum-zinc binary alloys exhibit a continuous change of dendrite growth direction, from ‹100› to ‹110›, as the concentration of zinc increases from 5 to 90 wt% [5]. Textured seaweed morphologies were even observed at the start and end of this so-called "Dendrite Orientation Transition" (DOT), i.e., for C0 ≈ 30 and 55 wt%. 3D reconstructions of conventional ‹110› dendrite and seaweed structures allowed a better comprehension and precise characterization of such growth morphologies. This transition was interpreted as due to a modification of the anisotropy of the interfacial anisotropy γsl by an increasing amount of zinc in aluminum. The first terms of the development of γsl into spherical harmonics for a cubic symmetry could be linked to the zinc concentration in a semi-quantitative way. The same development used in 3D phase field simulation allowed to reproduce these experimental dendritic growth morphologies, in particular the DOT, but not the seaweed morphologies. Growth directions and crystallographic orientations of solidification microstructure have been measured in Al-Zn and Zn-Al alloys close to the eutectic composition. If Al dendrites in Al-92 wt% Zn alloy were found to grow along ‹110› directions, Zn dendrites in Al-96 and 98 wt% Zn have ‹1010› trunks. This showed that aluminium could not in uence at such concentrations the weak anisotropy of zinc in the basal plane. In the lamellar eutectic, a crystallographic relationship has been found between the dense plane of each phase, i.e., {111}fcc ‖ {0001}hcp, and the dense directions, i.e., ‹110›fcc ‖ ‹1210›hcp. On the other hand, the selection of grains and the evolution of the texture in Directionally Solidified (DS) specimens was analysed. The solidification texturing of ‹hk0› dendritic specimens was shown to be similar to that occurring in normal ‹100› specimens. It was explained on a similar basis by considering a random orientation distribution of nuclei at the surface of the chill plate and a minimum undercooling criterion. This produces ‹hk0› textures with grains that look fairly "equiaxed" in transverse sections. For seaweed morphologies, which exhibit also a ‹hk0› texture parallel to the thermal gradient, the grain selection is slower and the grains appear more elongated in transverse sections. Their elongation occurs along a (001) plane, i.e., along a ‹120› direction for ‹210› seaweeds (CΖn = 30 wt%) and along a ‹110› direction for ‹110› seaweeds (CΖn = 55 wt%). SEM observations reveal that this elongation is accompanied by a microsegregation pattern that is mainly parallel to a (001) planes. This indicates that seaweeds at the onset and end of the DOT grow with some type of layered structures, but their detailed growth and grain selection mechanisms are still unknown.

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

Interfacial Energy Anisotropy and Growth Morphologies in Aluminium-Zinc Alloys

Jonathan Friedli

This thesis presents an investigation aimed at the understanding of the effect of interfacial energy anisotropy on growth morphologies in Al-Zn alloys. Most low alloyed metals form dendritic structures that grow along directions that correspond to low index crystal axes, e.g., 〈100〉 directions in fcc aluminum. However, recent findings [Gon06, Gon08] have shown that an increase in the zinc content of Al-Zn continuously changes the dendrite growth direction from 〈100〉 to 〈110〉 in the (100) plane. At intermediate compositions, 〈320〉 dendrites and textured seaweeds were even observed. The reason for this dendrite orientation transition (DOT) is that this system exhibits a large solubility of the element zinc, which forms hexagonal crystals, in the primary fcc aluminum phase, thus modifying its weak solid-liquid interfacial energy anisotropy γsl. Due to the complexity of the phenomenology, there is still no satisfactory theory that predicts all the observed microstructures. The goal of this work was to measure the anisotropy of γsl, to gain further insight in the dendrite growth morphologies and mechanisms in Al-Zn via X-ray tomography and investigate the relationship between dendrite growth directions and the anisotropy of γsl via phase-field modeling. The values of the anisotropy in Al-82 wt.% Zn, obtained by equilibrium shape measurement, suggests a DOT in a regime with very low anisotropy, favoring the formation of seaweed structures. The enhanced experimental procedures and observation methods allowed an unprecedented characterization of such complex morphologies. Observations showed that seaweeds are far from random structures. They grow in a (100) symmetry plane by an alternating growth direction mechanism and are therefore less hierarchically ordered than common dendrites. Phase-field simulations of equiaxed growth of a dilute alloy showed that the DOT is not a continuous transition of growth directions, but rather a competition between the 〈100〉 and the 〈110〉 character of dentrites. However, crystallographic orientation measurements showed that the macroscopic texture evolves as evidenced by Gonzales and Rappaz. This showed that the preferred growth direction is not the same as the final texture of a sample, which is uncommon. Simulations of directional growth showed that seaweeds, which do not appear in equiaxed growth, are in fact produced by the interplay of low or degenerate crystalline anisotropy and the misorientation of the crystal with respect to the thermal gradient.

EPFL2011

Dendritic Growth Morphologies in Al-Zn Alloys-Part I: X-ray Tomographic Microscopy

Paolo Di Napoli, Jonathan Friedli, Michel Rappaz

Upon solidification, most metallic alloys form dendritic structures that grow along directions corresponding to low index crystal axes, e.g., directions in fcc aluminum. However, recent findings[1,2] have shown that an increase in the zinc content in Al-Zn alloys continuously changes the dendrite growth direction from to in {100} planes. At intermediate compositions, between 25 wt pct and 55 wt pct Zn, dendrites and textured seaweeds were reported. The reason for this dendrite orientation transition is that this system exhibits a large solubility of zinc, a hexagonal metal, in the primary fcc aluminum phase, thus modifying its weak solid-liquid interfacial energy anisotropy. Owing to the complexity of the phenomenology, there is still no satisfactory theory that predicts all the observed microstructures. The current study is thus aimed at better understanding the formation of these structures. This is provided by the access to their 3D morphologies via synchrotron-based X-ray tomographic microscopy of quenched Bridgman solidified specimens in combination with the determination of the crystal orientation of the dendrites by electron-backscattered diffraction. Most interestingly, all alloys with intermediate compositions were shown to grow as seaweeds, constrained to grow mostly in a (001) symmetry plane, by an alternating growth direction mechanism. Thus, these structures are far from random and are considered less hierarchically ordered than common dendrites.