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.

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