Morphology and Growth Kinetic Advantage of Quenched Twinned Dendrites in Al-Zn Alloys
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TWINNED dendrites are the base microstructural feature of feathery grains, i.e., large fan-shaped grains that sometimes occur during the semi-continuous casting of industrial aluminum alloys. Owing to their abnormal size and shape, these grains induce the formation of segregation bands that modify the mechanical and fatigue properties of the ingot and thus are considered to be defects. In previous studies, it has been shown that twinned dendrites can nucleate during solidification when a strong thermal gradient and some convection currents are present in the melt near the liquidus isotherm.[1–5] Once nucleated, they rapidly overgrow columnar dendrites and even sometimes regular equiaxed dendrites.[6–8] Despite numerous studies which have contributed to the understanding of such morphologies since the first observations of Herenguel,[1,9] more than 60 years ago, many open questions remain. Optical and electronic microscopy observations combined with EBSD and EDX measurements made over the past fifteen years on such microstructures[6,7,10,11] have shown that (1) The primary trunks of twinned dendrites grow along h110i directions and are split in their centers by a straight boundary that corresponds to a coherent {111} twin plane; (2) Secondary arms grow not only along h110i but also sometimes along h100i directions and meet at wavy-like {111} incoherent MARIO A. SALGADO-ORDORICA, formerly PhD student with the Computational Materials Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland, is now Development Engineer, Novelis Switzerland, Sierre, Switzerland. Contact e-mail: mario. [email protected] ANDRE´ B. PHILLION, formerly Post-doc with the Computational Materials Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne, is now Assistant Professor, with the School of Engineering, University of British Columbia, Kelowna, BC, Canada, and MICHEL RAPPAZ, Director, is with the Computational Materials Laboratory, Ecole Polytechnique Fe´de´rale de Lausanne. Manuscript submitted March 25, 2012. Article published online February 21, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
boundaries; (3) These growth mechanisms create an alternated sequence of twinned and untwinned lamellae, separated by a sequence of coherent/planar and incoherent/wavy twin boundaries. Although a fairly good insight into the overall morphology of twinned dendrites has been gained thanks to these recent studies, the detailed growth mechanisms, the tip morphology, and the kinetics advantage that the twinned dendrites can have under certain circumstances over regular dendrites are still not fully clear. Henry et al.[10,11] made a conjecture that the twinned dendrite tip is in fact a doublon, i.e., a double-tip dendrite growing with a thin liquid channel in the center, the root of which contains the twin plane that solidifies at a solid composition close to C0. Recent phase-field simulations[12–14] as well as FIB observations[15] made by the present authors also strongly support this conjecture. On the other hand, if the propag
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