Grain Selection and Texture Evolution in Directionally Solidified Al-Zn Alloys
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UM-ZINC alloys, which constitute the basis of the 7000 series alloys, appear to be also very interesting from a fundamental point of view of solidification. Indeed, in a recent contribution,[1] we have shown that Al-Zn dendrites produced by directional or Bridgman solidification do not necessarily grow along the usual h100i directions characteristic of cubic metals.[2] Below 25 wt pct of zinc, fcc aluminum dendrites do grow along h100i, but beyond 60 wt pct, their growth direction was clearly identified as h110i by electron backscattered diffraction (EBSD). In between these two concentrations, a gradual dendrite orientation transition (DOT) from h100i to h110i occurs in a (001) plane as the zinc concentration CZn increases. At the start and end of this DOT, i.e., for CZn 30 and 55 wt pct, textured seaweeds were observed instead of dendrites. The change of dendrite growth direction is not new for organic alloys or salty solutions.[3,4] For example, Akamatsu et al.[3] have observed in situ the various growth morphologies in the organic transparent CBr4-C2Cl6 system confined in between two glass plates, i.e., nearly two-dimensional (2-D) growth. When the FRE´DE´RIC GONZALES, Postdoctoral Student, and MICHEL RAPPAZ, Professor, are with the Computational Materials Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, CH-1015, Switzerland. Contact e-mail: michel.rappaz@epfl.ch Manuscript submitted November 12, 2007. Article published online May 22, 2008 2148—VOLUME 39A, SEPTEMBER 2008
alloy was solidified with a [001] direction perpendicular to the glass plates, the weak anisotropy in the (001) plane produced dendrites with trunks growing along a [100] direction with [010] side arms. When the seed crystal was oriented in such a way that 2-D growth occurred in the (111) plane, the anisotropy normally vanished and the solid grew as an ‘‘oriented seaweed.’’ Finally, when growth occurred again in the (001) plane but at higher speed, a ‘‘doublon’’ morphology was observed. This morphology is similar to a dendrite growing along [100] with [010] side arms, but with a deep liquid channel at the center. Although very carefully conducted, such experiments present two inherent difficulties in their interpretation: first, the influence of the glass plates on the 2-D growth is difficult to assess and second, the contribution of the attachment kinetics of molecules to the solidliquid interface can compete with that related to the solid-liquid interfacial energy csl. The second remark also applies to observations made by Chan et al.[4] in the NH4Cl-H2O system. In this case, for an increasing supersaturation (or velocity), h100i dendrites were first observed, then h110i trunks with h100i side arms, and finally h111i trunks. The advantage of metallic alloys is that the contribution of the attachment kinetics to the dendrite growth kinetics and directions is very small for a velocity of the interface in the range of mm/s and below. Indeed, in the case of Al-Zn, the same dendrite orientations were identified in specimens directionally solidifie
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