Influence of Minor Cr-Additions to the Growth of Columnar Dendrites in Al-Zn Alloys: Influence of Icosahedral Short Rang
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ic metals, it is commonly admitted that dendrites grow along h100i directions. For many alloys such as steel, Ni-base and Cu-base alloys, h100i directions indeed correspond to minima of the solid–liquid interfacial stiffness, or maxima of the solid–liquid interfacial energy cs‘.[1] However, Al-base alloys seem to behave differently. Henry et al.[2] and later Salgado et al.[3,4] have made detailed analyses of twinned dendrites in various Al alloys, a morphology considered as a defect since its discovery in the 1940’s by Herenguel.[5] These authors have clearly shown that twinned dendrites grow along h110i directions and are split in their center by a (111) twin plane, with a mixture of h100i and h110i side arms. Under various directional solidification conditions
GU¨VEN KURTULDU is with the Institute of Materials, Ecole Polytechnique Fe´de´rale de Lausanne, Station 12, 1015, Lausanne, Switzerland and also with the Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland. Contact e-mail: [email protected] PHILIPPE JARRY is with Constellium C-TEC, ZI Centr’alp, 0725 rue Aristide Berge´s, BP 27, Voreppe, 38341, France. MICHEL RAPPAZ is with the Institute of Materials, Ecole Polytechnique Fe´de´rale de Lausanne. Manuscript submitted July 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(high-thermal gradient and solidification speed), Henry et al.[2] could also produce for the first time in Al-Mg alloys ‘‘regular’’ (i.e., untwinned) h110i dendrite trunks, h100i dendrite trunks with seaweed-type side arms at about 45 degree with respect to the trunk, and curly h112i dendrite trunks. In careful observations of dendrite growth directions in Al-43.4 wt pct Zn-1.6 wt pct Si coatings deposited on steel sheets by hot dipping, Se´moroz et al.[6] identified h320i dendrites in grains having various orientations. Bedel et al.[7] identified various dendrite morphologies including h111i dendrites in rapidly solidified Al-Cu droplets. This diversity of microstructures is attributed to the low solid–liquid interfacial energy anisotropy of Al[8,9] that can be easily perturbed by solute element additions, e.g., zinc which is a non-compact hexagonal-type element with a very large anisotropy. The influence of an alloying element on Al microstructure has been studied by Gonzales and Rappaz.[10,11] These authors demonstrated that addition of Zn gradually changes the growth direction of dendrite trunks, from h100i below 25 wt pct Zn to h110i above 60 wt pct Zn. This so-called dendrite orientation transition (DOT) does not depend on the type of experiments (Bridgman (BS) or Directional (DS) Solidification), i.e., it is independent of the solidification speed. This indicates that the DOT in Al-Zn is not the result of a
competition between attachment kinetics and solid–liquid interfacial anisotropy, and is rather the result of a modification of the anisotropy of cs‘ by solute elements. Indeed, considering first- and second-order terms in the development of cs‘, Haxhimali et al.[12] showed by p
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