The formation of twinned dendrites (feathery grains) in binary Al-Zn, Al-Mg, Al-Cu and Al-Ni alloys has been studied in specimens directionally solidified under identical thermal conditions, i.e. G % 100 K cm À1, v % 1 mm s À1, and with slight natural convection in the melt. The influence of the solute element nature and content has been found to be of less importance than previously reported since feathery grains were formed in all four alloys, regardless whether the alloying elements are hexagonal close packed (Zn and Mg) or face-centered cubic with a high (Ni) or low (Cu) stacking fault energy. A detailed analysis confirmed that twinned dendrites grow along h1 1 0i directions in all four cases, with a complex branch morphology made of up to six to nine arms. Surprisingly, at high Zn or Mg compositions for which regular dendrites grow along h1 1 0i instead of h1 0 0i, [Gonzales F, Rappaz M. Metall Trans A 2006; 37: 2797. [1]] no twinned dendrites could be formed. In terms of both the growth kinetics advantage of twinned dendrites over regular ones and the associated tip shape, some experimental evidence seems to contradict the doublon conjecture suggested by Henry [Henry S. PhD thesis, Ecole Polytechnique Fédéral de Lausanne, 1999. [21]], at least for the solute compositions studied in the present work.
The growth kinetics advantage of twinned aluminum dendrites over regular ones is still an unsolved problem of solidification. Although it is linked to the tip geometry, the influence of a coherent (1 1 1) twin plane on a h1 1 0i twinned dendrite tip is unclear, despite several past experimental observations. In the present contribution, a three-dimensional phase field model implemented on a cluster of parallel computers has been used to simulate the growth of a twinned dendrite under various directional solidification conditions. Only half a dendrite was modeled by replacing the coherent twin plane by a boundary with an appropriate condition on the phase parameter that is equivalent to the Young-Laplace equilibrium condition along the triple line between twinned solid, untwinned solid and liquid. It is found that the small liquid cusp present at the tip rapidly evolves into a doublon-type morphology, i.e. a h1 1 0i dendrite split in its center by a deep and thin liquid pool with the triple line at the root. At high growth rates, the two sides of the doublon tend to coalescence and form small isolated liquid droplets. The positive concentration gradient near the doublon root appears to be rapidly smeared out by back-diffusion in the solid, thus making difficult its quantification through experimental methods. These simulation results are correlated with new experimental evidence presented in a companion paper.
Twinned dendrites appearing in an Al-26 wt pct Zn alloy have been quenched during growth using a specifically designed setup that is positioned on top of a directional solidification experiment. X-ray tomography performed at the Swiss Light Source (SLS-beamline TOMCAT) allowed us to reconstruct the 3D morphology of these structures and to confirm previous observations performed on single 2D sections (Henry et al., Metall Mater Trans A 35A:2495-2501 Salgado-Ordorica and Rappaz, Acta Mater 56:5708-5718, 2008). Further characterization of these quenched specimens led to a better description of the mechanisms involved in the in-plane and lateral growth propagation of twinned dendrites. These were then put into relation with the competition mechanisms taking place during simultaneous solidification of twinned and regular dendrites.
The favorable growth kinetics of twinned dendrites can be explained by their complex morphology, multiple side branching mechanisms, growth undercooling and tip morphology. Three models were proposed for the twinned dendrite tip shape: (i) a grooved tip [1] satisfying the Smith condition at the triple line; (ii) a doublon [2], i.e. a double-tip dendrite that grows with a narrow and deep liquid channel in its center; and (iii) a pointed (or edgy) tip [3], with consideration of the solid-liquid interfacial energy anisotropy. In the first part of this work, phase field simulations of half a twinned dendrite with an appropriate boundary condition to reproduce the Smith condition supported the doublon conjecture, with a narrow liquid channel ending its solidification with the formation of small liquid droplets. In this part, experimental observations of twinned dendrite tips reveal the presence of a small, but well-defined, groove, thus definitely eliminating the edged tip hypothesis. Focused ion beam nanotomography and energy-dispersive spectroscopy chemical analysis in a transmission electron microscope reveal the existence of a positive solute gradient in a region localized within 2 lm around the twin plane. In Al-Zn specimens, small particles aligned within the twin plane further support the doublon conjecture and the predicted formation of small liquid droplets below the doublon root.
Under certain thermal conditions (G % 1 Â 10 4 K m À1, m s % 1 Â 10 À3 m s À1), h1 1 0i twinned dendrites appear in aluminum alloys and can overgrow regular columnar dendrites, provided that some convection is also present in the melt. In order to check the stability of such morphologies, directionally solidified twinned samples of Al-Zn were partially remelted in a Bridgman furnace and then resolidified under controlled conditions, with minimal convection. It was found that, although twin planes remain stable during partial remelting, non-twinned dendrites regrow during solidification. They have a crystallographic orientation given by those of the twinned and untwinned ''seed" regions, and grow along preferred directions that tend to be those of normal specimens. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Keywords: Bridgman solidification; Dendrite growth; Twin stability; Aluminum alloys Feathery grains were found for the first time as a defect in semi-continuous casting of aluminum alloys almost 60 years ago [1]. Using electron backscattered diffraction (EBSD), Henry et al. [2][3][4] clearly showed that these fan-like structures are made of a lamellar sequence of twinned and untwinned regions with dendrites split in the center of their trunk by a coherent {1 1 1} twin plane. Twinned dendrite trunks always grow along h1 1 0i directions and have a highly complex branch morphology of h1 1 0i, and also sometimes h1 0 0i, secondary arms [5]. 1 Experience has shown that such morphologies form when the following twinning conditions are met: (i) a fairly high thermal gradient (G % 1 Â 10 4 K m À1 ); (ii) a fairly large solidification rate (m s % 1 Â 10 À3 m s À1 ); and (iii) some convection present in the melt [5,7]. Such conditions seem to give a kinetic advantage to twinned grains over ordinary columnar grains, i.e. under the same solidification conditions, the undercooling of the twinned dendrite tips, T tw , are lower than that of regular columnar dendrites, T rc [5,[8][9][10]. However, the morphology of the twinned dendrite tip is itself still an open question. A grooved tip [8], an edgy tip [9] and a deep-grooved tip (called in this case ''doublon") [10] have all been proposed by different authors. Furthermore, the influence of convection has been clearly identified [7], but the detailed mechanism by which this operates is still unclear. Does convection only act on the nucleation stage via an enhanced formation of stacking faults? Is it also necessary during growth? In order to answer such questions, specimens produced under twinning conditions were partially remelted and then resolidified under well-controlled Bridgman conditions, for which convection in the melt is minimal.With this goal in mind, a binary aluminum alloy of composition Al-23 wt.% Zn was selected for three reasons: (i) it has been shown recently that twins can form easily in this system with up to about 40 wt.% Zn [5]; (ii) zinc being heavier than aluminum and the partition coefficient k 0 being smaller than one, ...
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