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 &#82