Dendrite Orientation Transition Research Articles

Atomistic insight into dendrite growth orientation transition in Al-Cu alloy

In this study, we investigate the impact of solid-melt interfacial energy anisotropy on the microstructure of components. Our focus is on the Al-Cu system's Al-rich side, where we compute the solid-melt interfacial free energy and its anisotropy using Molecular Dynamics (MD) simulations. Our findings reveal that, as the Cu concentration in the melt increases, the preferred dendrite growth orientation shifts from <100> to <110>. Furthermore, atomic density profile analysis shows an improvement in ordering in the (110) plane at higher Cu concentrations. Our results suggest that the transition in dendrite growth orientation is a consequence of the weakened anisotropy between (100) and (110) interfacial energies at higher Cu concentrations and undercooling temperatures. We also observe displaced atomic density profiles between Al and Cu atoms in proximity to the interface. We propose that a displaced atomic density profile between solute and solvent atoms could be a characteristic feature of systems undergoing a dendrite orientation transition.

Experimental determination of solid-liquid interfacial free energy anisotropy in Al-Zn via equilibrium droplet method

The increasing Zn content causes dendrite orientation transition during solidification of Al-Zn alloys, which is supposed to be related to the change of solid-liquid interfacial energy anisotropy. In this paper, experimental measurements of the interfacial energy anisotropy were performed in Al-17, 30 and 75wt.%Zn alloys with equilibrium droplet method. Droplets shape in relation to crystal orientation and droplets morphology are obtained by Electron Back-Scatter Diffraction and X-ray Computed Tomography, respectively. The result indicates that anisotropic parameters ε1 and ε2 both decreases with increasing Zn concentration and the directions of maximum interfacial energy transform from 〈100〉 directions in Al-17wt.%Zn and Al-30wt.%Zn to 〈110〉 directions in Al-75wt.%Zn. In addition, the Wulff plot and normalized interfacial stiffness are quantified and compared with the corresponding Al-Zn dendrites orientation selection.

Role of interfacial energy anisotropy in dendrite orientation in Al-Zn alloys: A phase field study

The influence of interfacial energy anisotropy on dendrite morphology is still not well understood. We investigated the dendrite morphology and solute segregation in Al-Zn alloys using the phase-field method. We observed typical and atypical dendrite orientation transition and three types of dendrite morphology “Christmas tree dendrite”, “seaweed dendrite” and “butterfly dendrite” depending on the anisotropic interfacial energy. Two types of dendrite may undergo the so called-“smooth dendrite orientation transition”, which produces a large amount of solute segregation holes in 〈100〉 dendrites, and liquid grooves in 〈110〉 seaweed and butterfly patterns, thus may significantly impact the mechanical properties of castings. We identified a strong correlation between supersaturation and the formation of liquid holes and liquid grooves as well as the degree of solute segregation. We also proposed that the interfacial energy anisotropy can be related with Zn content by discussing the anisotropic Young’s modulus and experimental pattern of Al-Zn alloys. The inducement of anisotropy and the quantitative relationship between interfacial energy anisotropy, Zn concentration, elastic anisotropy and dendrite orientation deserves attention.

Controlling solid-liquid interfacial energy anisotropy through the isotropic liquid

Although the anisotropy of the solid-liquid interfacial free energy for most alloy systems is very small, it plays a crucial role in the growth rate, morphology and crystallographic growth direction of dendrites. Previous work posited a dendrite orientation transition via compositional additions. In this work we examine experimentally the change in dendrite growth behaviour in the Al-Sm (Samarium) system as a function of solute concentration and study its interfacial properties using molecular dynamics simulations. We observe a dendrite growth direction which changes from langle 100rangle to langle 110rangle as Sm content increases. The observed change in dendrite orientation is consistent with the simulation results for the variation of the interfacial free energy anisotropy and thus provides definitive confirmation of a conjecture in previous works. In addition, our results provide physical insight into the atomic structural origin of the concentration dependent anisotropy, and deepen our fundamental understanding of solid-liquid interfaces in binary alloys.

Influence of Minor Cr-Additions to the Growth of Columnar Dendrites in Al-Zn Alloys: Influence of Icosahedral Short Range Order in the Liquid

The effect of minute Cr additions on dendritic growth directions in Al-20 wt pct Zn alloy has been investigated. As first evidenced by Gonzales and Rappaz (Metall Mater Trans A 37:2797–2806, 2006) Al-Zn alloys exhibit a Dendrite Orientation Transition (DOT), from 〈100〉 below 25 wt pct Zn to 〈110〉 above 60 wt pct, regardless of the solidification speed. The DOT has been interpreted as a modification of the solid–liquid interfacial energy anisotropy (Gonzales and Rappaz, 2006; Haxhimali et al. in Nat Mater 5:660–664, 2006). However, 0.02 to 0.1 wt pct addition of Cr in Al-20 wt pct Zn drastically modifies the dendrite trunk direction from 〈100〉 to 〈110〉 at low solidification speed, typically 200 μm/s. Even more surprising, 〈100〉 dendrite trunks are retrieved in Al-20 wt pct Zn-0.1 wt pct Cr when the speed is increased to 1000 μm/s, but with the concurrent formation of twinned dendrites. Minute additions of Cr have been reported recently to promote icosahedral short range order (ISRO) in liquid Al-Zn, thus reducing the atomic mobility (Kurtuldu et al. in Acta Mater 115:423–433, 2016) and modifying the nucleation kinetics (Kurtuldu et al. in Acta Mater 61:7098–7108, 2013). The present results indicate that ISRO also modifies the attachment kinetics of icosahedral clusters rather than individual atoms, and is most likely responsible of growth direction change in Al-Zn-Cr alloy and of twinned dendrites formation.

Three dimensional dendritic morphology and orientation transition induced by high static magnetic field in directionally solidified Al-10 wt.%Zn alloy

Effect of high static magnetic field on the dendritic morphology and growth direction in directionally solidified Al-10 wt.%Zn alloy were studied by three-dimensional (3D) X-ray micro-computed tomography, Electron Back-scattered Diffraction (EBSD) and X-ray Diffraction (XRD). The application of high static axial magnetic field (5T) during directional solidification was found to destabilize the solid/liquid interface and cause the growth direction of dendrite deviate from thermal gradient, leading to irregular solid/liquid interfacial shape and cellular to dendritic morphology transition. The thermoelectric magnetic convection (TEMC) caused by the interaction of thermoelectric effect and magnetic field was supposed to be responsible for the transition. In addition, the EBSD and XRD results confirm that the preferred growth direction of α-Al was found to transform from the traditionally expected <100> to <110>. The dendrite orientation transition (DOT) in Al-10 wt.%Zn alloy can be attributed to the effect of applied magnetic field on the anisotropy of crystal during solidification. The result indicates the potential application of high static magnetic field in altering the morphology and preferred growth direction of dendrite during directional solidification.

Dendrite orientation transition in Al[sbnd]Ge alloys

We report on a dendrite orientation transition (DOT) discovered in the AlGe system by conducting isothermal solidification experiments in thin samples under slow cooling conditions. The DOT was revealed using a combination of in situ X-radiography imaging and post-mortem electron backscatter diffraction. In Al-20 wt% Ge the primary arms grow along 〈100〉, whereas in Al-46 wt% Ge the primary arms grow along 〈110〉. At an intermediate composition of Al-29 wt% Ge, we observe both directions growing simultaneously. Phase-field simulations in which the solid-liquid interfacial energy anisotropy was systematically varied to correspond with the composition changes reproduced the experimentally observed microstructures. The remarkable agreement between the experimental observations and the simulations provides strong evidence that the DOT in this alloy system is caused by modification of surface energy anisotropy with increasing Ge content. Furthermore, we show that the confinement due to the thin sample geometry influences the selection of the primary growth directions, so that different growth morphologies develop depending on the orientation of the crystal in the sample.

Synchrotron tomographic quantification of the influence of Zn concentration on dendritic growth in Mg-Zn alloys

Dendritic microstructural evolution during the solidification of Mg-Zn alloys was investigated as a function of Zn concentration using in situ synchrotron X-ray tomography. We reveal that increasing Zn content from 25 wt% to 50 wt% causes a Dendrite Orientation Transition (DOT) from a six-fold snow-flake structure to a hyper-branched morphology and then back to a six-fold structure. This transition was attributed to changes in the anisotropy of the solid-liquid interfacial energy caused by the increase in Zn concentration. Further, doublon, triplon and quadruplon tip splitting mechanisms were shown to be active in the Mg-38 wt%Zn alloy, creating a hyper-branched structure. Using the synchrotron tomography datasets, we quantify, for the first time, the evolution of grain structures during the solidification of these alloys, including dendrite tip velocity in the mushy zone, solid fraction, and specific surface area. The results are also compared to existing models. The results demonstrate the complexity in dendritic pattern formation in hcp systems, providing critical input data for the microstructural models used for integrated computational materials engineering of Mg alloys.

Phase-field simulation of tip splitting in dendritic growth of Fe-C alloy

Two types of dendrite tip splitting including dendrite orientation transition and twinned-like dendrites in Fe-C alloys were investigated by phase-field method. In equiaxed growth, the possible dendrite growth directions and the effect of supersaturation on tip splitting were discussed; the dendrite orientation transition was observed, and it was found that the orientation regions of anisotropy parameters were reduced from three to two with increasing the supersaturation, which was due to the effect of interfacial anisotropy controlled by the solute in front of S/L interface changing with the increase of supersaturation. In directional solidification, it was found that the twinned-like dendrites were formed with the fixed anisotropy couples and no seaweed dendrites were observed; these were concluded from the results of competition between process anisotropy and inherent anisotropy. The formation process of twinned-like dendrite was investigated by tip splitting phenomenon, which was related to the changes of dendrite tips growth velocity. Then, the critical speed of tips splitting and solute concentration of twinned-like dendrites were investigated, and a new type of microsegregation in Fe-C alloys was proposed to supplement the dendrite growth theories.

Effect of different solute additions on dendrite morphology and orientation selection in cast binary magnesium alloys

In solidification, dendritic morphology was observed to change accordingly if either the type or quantity of the additional element was modified. To gain insight into this phenomenon, the 3D dendrite morphology of different binary magnesium alloys, including MgAl, MgBa, MgCa and MgZn alloys was characterized using synchrotron X-ray tomography and electron backscattered diffraction. Results showed that for most Mg-based alloys, the dendrite exhibited a typical 18-branch morphology with preferred growth orientations along 〈112¯0〉 and 〈112¯3〉, whereas for MgZn alloys, the dendrite morphology would change from the 18-branch pattern to 12-branch if the Zn content increased, i.e. the so-called dendrite orientation transition (DOT) took place. This DOT behaviour of the Mg alloy dendrite was then successfully modelled using the 3D phase field method by changing the magnitude of related parameters in the specially formulated anisotropy function based on spherical harmonics.

Mechanism selection for spontaneous grain refinement in undercooled metallic melts

In a previous paper (Castle et al., 2014) [17], we employed a melt fluxing technique to study the velocity–undercooling relationship and microstructural development of a Cu–8.9wt.% Ni alloy in order to investigate the fundamental mechanism behind the rapid solidification phenomenon of “spontaneous grain refinement”. A number of growth transitions were identified with increasing undercooling, including an extended dendrite orientation transition through: fully 〈100〉-oriented→multiply twinned, mixed 〈100〉/〈111〉-oriented→fully 〈111〉-oriented. Here, we present the results of an identical study of a Cu–Ni alloy of lower Ni content and observe a similar set of growth transitions, with some notable differences. In particular, three distinct grain refinement mechanisms have been observed between the two alloys, including: recrystallization, dendrite fragmentation and dendritic seaweed fragmentation. It appears that the mechanism selected depends strongly upon the original growth structure present, which we suggest is dictated by the balance between the capillary and kinetic anisotropies; with the addition of Ni to Cu either increasing the strength of the kinetic anisotropy, decreasing the strength of the capillary anisotropy, or a combination of both. The unambiguous identification of three different spontaneous grain refinement mechanisms in two closely related alloys is significant as it may help to resolve some of the debate surrounding the “true” grain refinement mechanism.

Dendritic Growth Morphologies in Al-Zn Alloys—Part II: Phase-Field Computations

In Part I of this article, the role of the Zn content in the development of solidification microstructures in Al-Zn alloys was investigated experimentally using X-ray tomographic microscopy. The transition region between $${\langle{100}\rangle}$$ dendrites found at low Zn content and $${\langle{110}\rangle}$$ dendrites found at high Zn content was characterized by textured seaweed-type structures. This Dendrite Orientation Transition (DOT) was explained by the effect of the Zn content on the weak anisotropy of the solid–liquid interfacial energy of Al. In order to further support this interpretation and to elucidate the growth mechanisms of the complex structures that form in the DOT region, a detailed phase-field study exploring anisotropy parameters’ space is presented in this paper. For equiaxed growth, our results essentially recapitulate those of Haxhimali et al.[1] in simulations for pure materials. We find distinct regions of the parameter space associated with $${\langle{100}\rangle}$$ and $${\langle{110}\rangle}$$ dendrites, separated by a region where hyperbranched dendrites are observed. In simulations of directional solidification, we find similar behavior at the extrema, but in this case, the anisotropy parameters corresponding to the hyperbranched region produce textured seaweeds. As noted in the experimental work reported in Part I, these structures are actually dendrites that prefer to grow misaligned with respect to the thermal gradient direction. We also show that in this region, the dendrites grow with a blunted tip that oscillates and splits, resulting in an oriented trunk that continuously emits side branches in other directions. We conclude by making a correlation between the alloy composition and surface energy anisotropy parameters.

Dendritic Growth Morphologies in Al-Zn Alloys—Part I: X-ray Tomographic Microscopy

Upon solidification, most metallic alloys form dendritic structures that grow along directions corresponding to low index crystal axes, e.g., $$\langle100\rangle$$ directions in fcc aluminum. However, recent findings[1,2] have shown that an increase in the zinc content in Al-Zn alloys continuously changes the dendrite growth direction from $$\langle100\rangle$$ to $$\langle110\rangle$$ in {100} planes. At intermediate compositions, between 25 wt pct and 55 wt pct Zn, $$\langle320\rangle$$ dendrites and textured seaweeds were reported. The reason for this dendrite orientation transition is that this system exhibits a large solubility of zinc, a hexagonal metal, in the primary fcc aluminum phase, thus modifying its weak solid–liquid interfacial energy anisotropy. Owing to the complexity of the phenomenology, there is still no satisfactory theory that predicts all the observed microstructures. The current study is thus aimed at better understanding the formation of these structures. This is provided by the access to their 3D morphologies via synchrotron-based X-ray tomographic microscopy of quenched Bridgman solidified specimens in combination with the determination of the crystal orientation of the dendrites by electron-backscattered diffraction. Most interestingly, all alloys with intermediate compositions were shown to grow as seaweeds, constrained to grow mostly in a (001) symmetry plane, by an alternating growth direction mechanism. Thus, these structures are far from random and are considered less hierarchically ordered than common dendrites.

Phase-field modeling of the dendrite orientation transition in Al-Zn alloys

With a few exceptions, phase-field simulations of dendritic growth in cubic materials have been modeled using simple expressions for the interfacial energy anisotropy and with strong anisotropy. However, recent experimental results show that the Dendrite Orientation Transition (DOT) observed in Al-Zn alloys by Gonzales and Rappaz [Met. Mat. Trans. A37 (2006) 2797] occurs at weak anisotropy, and modeling these results requires at least two anisotropy parameters. In the present work, we solve the corresponding phase-field model on an adaptive grid, after measuring and compensating for the grid anisotropy. A systematic scan of equiaxed growth simulations was performed in the range of the anisotropy parameter space where the transition is expected. We find separate domains of existence of ⟨100⟩ and ⟨110⟩ dendrites, similar to those previously reported by Haxhimali et al. [Nat. Mat. 5 (2006) 660] for pure materials. In the so-called hyperbranched regime, lying between the ⟨100⟩ and ⟨110⟩ regions, we observe a competition between ⟨100⟩ and ⟨110⟩ growth directions, but no seaweed structures. Directional solidification simulations showed the stabilizing effect of the thermal gradient on the twofold splitting of ⟨110⟩ dendrites, and the importance of the choice of anisotropy parameters. We also found a strong dependence between the orientation of the crystal axes with respect to the thermal gradient and the actual growth direction. Finally, 3-dimensional seaweed microstructures were modeled for the first time, demonstrating that this pattern is a result of not only the values of anisotropy parameters, but also a consequence of directional solidification.

X-ray tomographic microscopy analysis of the dendrite orientation transition in Al-Zn

Recently, Gonzales and Rappaz [Met. Mat. Trans. A37:2797, 2006] showed the influence of an increasing zinc content on the growth directions of aluminum dendrites. ⟨100⟩ and ⟨110⟩ dendrites were observed below 25wt.% and above 55wt.% zinc, respectively, whereas textured seaweeds and ⟨320⟩ dendrites were observed at intermediate compositions. Considering the complexity of these structures, it is necessary to first characterize them in further details and second, to model them using the phase field method. The so-called Dendrite Orientation Transition (DOT) was thus reinvestigated in quenched Bridgman solidification samples. The combination of X-ray tomographic microscopy and electron backscattered diffraction (EBSD) analysis on a whole range of compositions, from 5 to 90wt.% Zn, allowed insights with unprecedented details about texture, growth directions and mechanisms of the aforementioned structures. We show that seaweeds rather than dendrites are found at all intermediate compositions. Their growth was confirmed to be constrained within a (100) symmetry plane. However, new findings indicate that the observed macroscopic texture does not necessarily correspond to the actual growth directions of the microstructure. Further, it seems to operate by an alternating growth direction mechanism and could be linked to the competition between the ⟨100⟩ and ⟨110⟩ characters of regular dendrites observed at the limits of the DOT. These characters, as well as 3D seaweeds, are observed in phase-field simulations of equiaxed growth and directional solidification, respectively. This study emphasizes the importance of accurate experimental data to validate numerical models and details the progress that such combinations provide for the understanding of growth mechanisms.

Grain Selection and Texture Evolution in Directionally Solidified Al-Zn Alloys

In a recent contribution, we have reported that dendrites in the aluminum-zinc binary system can change their growth direction continuously from $$ {\left\langle {{\text{100}}} \right\rangle } $$ to $$ {\left\langle {{\text{110}}} \right\rangle } $$ as the concentration of zinc C Zn increases from 5 to 90 wt pct.[1] This was interpreted as an influence of the hcp Zn element on the low anisotropy of the solid-liquid interfacial energy of aluminum. Seaweed morphologies were even observed at the start and end of this so-called “dendrite orientation transition” (DOT), i.e., for C Zn ≈ 30 and 55 wt pct. In the present work, we analyze how this DOT influences the selection of grains and the evolution of the texture in directionally solidified (DS) specimens. The solidification texturing of $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ dendritic specimens is shown to be similar to that occurring in normal $$ {\left\langle {{\text{100}}} \right\rangle } $$ specimens. It is analyzed on a similar basis by considering a random orientation distribution of nuclei at the surface of the chill plate and a minimum undercooling criterion. This produces $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ textures with grains that look fairly “equiaxed” in transverse sections. For seaweed morphologies, which also exhibit an $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ texture parallel to the thermal gradient, the grain selection is slower and the grains appear more elongated in transverse sections. Their elongation occurs along a (001) plane, i.e., along a $$ {\left\langle {1\ifmmode\expandafter\bar\else\expandafter\=\fi{2}0} \right\rangle } $$ direction for $$ {\left\langle {210} \right\rangle } $$ seaweeds (C Zn = 30 wt pct) and along a $$ {\left\langle {1\ifmmode\expandafter\bar\else\expandafter\=\fi{1}0} \right\rangle } $$ direction for $$ {\left\langle {{\text{110}}} \right\rangle } $$ seaweeds (C Zn = 55 wt pct). The scanning electron microscopy (SEM) observations reveal that this elongation is accompanied by a microsegregation pattern that is mainly parallel to (001) planes. This indicates that seaweeds at the onset and end of the DOT grow with some type of layered structures, but their detailed growth and grain selection mechanisms are still unknown.