Study of twinned dendrite growth stability

Under certain thermal conditions (G approximate to 1 x 10(4) K/m, v(s) approximate to 1 x 10(-3) m/s), 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. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

The influence of solid-liquid interfacial energy anisotropy on equilibrium shapes, nucleation, triple lines and growth morphologies

The anisotropy of the solid-liquid interfacial energy plays a key role during the formation of as-solidified microstructures. Using the xi-vector formalism of Cahn and Hoffman, this contribution presents the effect that anisotropy has on the equilibrium shapes of crystals and on surface tension equilibrium at triple lines. Consequences for heterogeneous nucleation of anisotropic crystals and for dendritic growth morphologies are detailed with specific examples related to Al-Zn and Zn-Al alloys. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Phase Field Modelling Of Twinned Dendrite Growth

Twinned dendrite growth has been found to occur in aluminum alloys when critical thermal conditions (G approximate to 100 K/cm, nu(s) approximate to 1 mm/s) and a slight convection in the melt are present during directional solidification. Split in their trunk by a coherent (111) twin plane, such dendrites grow along directions with a complex branch structure of , but also sometimes secondary arms. To explain the twinned dendrite growth kinetics advantage, Eady and Hogan suggested that the Young Laplace equation involving the solid-liquid interfacial energy gamma(sl) and the twin energy gamma(t) at the triple junction stabilizes a grooved tip(1). Wood et al proposed instead that torque terms associated with the anisotropy of gamma(sl) stabilize a sharp pointed tip(2). Finally, Henry suggested the possibility of the existence of a doublon, initiated precisely by a grooved tip, that would evolve depending on the solute content(3). In a recent experimental work, we have shown that the doublon conjecture is probably not valid for high solute content aluminum alloys, whereas it could be valid at low, composition. In the present work, the twinned dendrite tip morphology and growth kinetics have been investigated using a 3D phase field method implemented on a massively parallel computer. The twin boundary energy has been imposed via an appropriate boundary condition fixing the angle of the phase field gradient with respect to the boundary. Besides this angle, various experimental conditions such as thermal gradient, gradient direction, velocity of the isotherms and compositions have been investigated. The growth kinetics obtained under such conditions has been compared with that of regular dendrites

Study of the twinned dendrite tip shape I : Phase-field modeling

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 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 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. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Study of the twinned dendrite tip shape II: Experimental assessment

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 satisfying the Smith condition at the triple line; (ii) a doublon, 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, 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 μm 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. © 2011 Acta Materialia Inc

EBSD: a powerful microstructure analysis technique in the field of solidification

This paper presents a few examples of the application of electron back-scatter diffraction (EBSD) to solidification problems. For directionally solidified Al-Zn samples, this technique could reveal the change in dendrite growth directions from to as the composition of zinc increases from 5 to 90 wt%. The corresponding texture evolution and grain selection mechanisms were also examined. Twinned dendrites that form under certain solidification conditions in Al-X specimens (with X = Zn, Mg, Ni, Cu) were clearly identified as dendrite trunks split in their centre by a (111) twin plane. In Zn-0.2 wt% Al hot-dip galvanized coatings on steel sheets, EBSD clearly revealed the preferential basal orientation distribution of the nuclei as well as the reinforcement of this distribution by the faster growth of dendrites. Moreover, in Al-Zn-Si coatings, misorientations as large as 10 degrees mm(-1) have been measured within individual grains. Finally, the complex band and lamellae microstructures that form in the Cu-Sn peritectic system at low growth rate could be shown to constitute a continuous network initiated from a single nucleus. EBSD also showed that the alpha and beta phases had a Kurdjumov-Sachs crystallographic relationship

Study of the twinned dendrite tip shape II ::experimental assessment

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 μm 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