Regular Dendrites Research Articles

Formation of seaweed morphology and enhancing mechanical properties of an A356 alloy by directional solidification

In the present work, regular dendrite and seaweed pattern are produced by controlling the crystal orientation during directional solidification of an A356 aluminium alloy. These morphologies and growth orientation were investigated by electron backscatter diffraction (EBSD), and the results showed that the seaweed pattern mainly prevailed under the (100)[011] orientation. At the same time, the effects of seaweed pattern in the properties of A356 alloy were analysed by room temperature tensile test. It was found that the elongation and toughness of the samples reached 20 % and 36 J/m3, respectively, for the seaweed; 13 % and 24 J/m3, respectively, for the regular dendrites; and 8 % and 14 J/m3, respectively, for the cast ingot. Compared with the regular dendrite samples, the seaweed pattern obtained relatively excellent elongation and toughness. By using scanning electron microscopy (SEM), it is found that the distribution of eutectic silicon particles in seaweed morphology sample is more refined and uniform, which is the main reason for these enhancements.

Introducing high-density growth twins in aluminum alloys by laser surface remelting via templated nucleation of grains

It is difficult to generate coherent twin boundaries in bulk Al alloys due to their high intrinsic stacking fault energy. Here, we report a strategy to induce high-density growth twins in aluminum alloys through the heterogeneous nucleation of twinned Al grains on twin-structured TiC nucleants and the preferred growth of twinned dendrites by laser surface remelting of bulk metals. The solidification structure at the surface shows a mixture of lamellar twinned dendrites with ultra-fine twin boundary spacing (∼2 μm), isolated twinned dendrites, and regular dendrites. EBSD analysis and finite element method (FEM) simulations have been used to understand the competitive growth between twinned and regular dendrites, and the solidification conditions for the preferred growth of twinned dendrites during laser remelting and subsequent rapid solidification are established. It is shown that the reduction in the ratio of temperature gradient G to solidification rate V promotes the formation of lamellar twinned dendrites. The primary trunk spacing of lamellar twinned dendrites is refined by the high thermal gradient and solidification rate. The present work paves a new way to generate high-density growth twins in additive-manufactured Al alloys.

Dendritic flux avalanches in superconducting hybrid structures

Magneto-optical imaging was employed to study dendritic flux avalanches in metal/superconductor and superconductor/superconductor hybrid structures over an extended range of magnetic field ramping rates. Our results in Cu/NbN show that the previously reported suppression of dendritic flux avalanches in metal coated superconducting films is limited to low ramping rates; as the ramping rate increases, the metal coating becomes less and less effective. A more complex behavior is exhibited in superconductor/superconductor hybrid structures. Our measurement in NbN partially coated with Nb, reveal three distinctive types of dendritic avalanches: those propagating in only one layer, either as regular dendrites in the uncoated NbN or as surface dendrites in the Nb layer, and hybrid dendrites that propagate in both the Nb and NbN layers simultaneously. These three types of dendrites are distinguished by their morphology, temperature dependence and instability threshold field. The overall stability of the hybrid structure significantly exceeds that of its weak component.

Effects of deposition paramaters on the microstructure evolution of wire arc additive manufactured Al–Zn–Mg–Cu alloy

Wire arc additive manufacturing (WAAM) is a suitable method for fabricating large high-strength components made of the Al–Zn–Mg–Cu alloy. However, the solidification behavior of this alloy is complex due to its composition. The inhomogeneous solidification microstructure results in numerous defects during the deposition process, hindering the application of the deposited Al–Zn–Mg–Cu alloy. This study aims to analyze the microstructure evolution under different process parameters and reveal the relationship between the solidification behavior of the molten pool, pore defects, and grain morphology. Thin-walled components based on Al–Zn–Mg–Cu alloy were fabricated using WAAM under different parameters. The results showed that increasing deposition current initially decreased and subsequently increased pore defects, while the proportion of twinned dendrites exhibited the opposite trend. A lower deposition speed reduced pore defects by promoting gas escape, whereas a higher deposition speed led to the precipitation of Al3Zr, grain refinement, and the suppression of twin dendrite growth. A faster wire feeding speed had a better suppression effect on defects, while a slower wire feeding speed inhibited the growth of twinned dendrites, promoting the directional growth of regular dendrites.

Twinned dendrites growth in wire arc directed energy deposition of Al-Zn-Mg-Cu alloy

Twinned dendrites have attracted worldwide attention in the past half century due to their unusual orientation and crystallographic symmetry. Successive research has established a basic understanding of them in relatively steady processes like direct chill casting and directional solidification. This work, for the first time, investigated the growth behaviors of twinned dendrites in the wire arc directed energy deposition. The crystallographic characteristics, formation, and evolution laws of the twinned dendrites in the unsteady localized solidification condition are explored via experiments and numerical simulations. Results show that stacking faults are vital in the longitudinal growth of twinned dendrites by adjusting intermittent twin boundaries. During deposition, Mg, Zn, and Cu, which have low stacking fault energy, can promote the growth of twinned dendrites. The molten pool tail has large temperature gradients, crystallization velocities, and sufficient convection, providing advantageous growth conditions for twinned dendrites. Competitive relationships form between twinned and regular dendrites at the molten pool center. However, the twinned dendrites also cooperate with regular dendrites in epitaxial growth via their 〈100〉 lateral branch. With the remelting depth increasing, the twinned dendrite morphology changes from lamellar to isolated unilateral feather shape, then to unchained islands, and finally disappear.

Non-regular dendritic pattern selection induced by interface energy anisotropy in the directional solidification of Al-3 wt% Mg single crystals

• Finding plate-like and hyperbranched seaweed in 3D conditions of metallic alloy. • Constructing a non-regular dendritic pattern selection map. • Well understanding effect of interface energy anisotropy on the dendritic pattern. Dendritic morphology and pattern selection are fundamentally important issues in solidification cycles. Here, a well-designed experiment was employed to investigate the solid–liquid interface energy anisotropy on the directionally solidified microstructures of Al-3 wt% Mg single crystals. Various dendritic morphologies, including regular dendrite, plate-like seaweed and hyperbranched seaweed, were found along the <100>, <110> and <111> orientations, respectively. The primary dendritic arm spacing (PDAS) of the seaweed pattern is closely related to the interface energy anisotropy and deviates from the current PDAS model. A non-regular dendritic pattern selection map is constructed to interpret the different dendrites occurring in alloy solidification.

Enhanced mechanical properties of Al-4.5 wt.% Cu single crystals with seaweed morphology

While the seaweed pattern is one the most important classes of interfacial morphologies, their mechanical properties are not well understood. Here we produce axial seaweed (AS), tilted seaweed (TS) and degenerate seaweed (DS) by controlling the orientation-dependent interfacial energy anisotropy during the directional solidification of an Al-4.5 wt.% Cu alloy. Characterization of the disordered morphology by electron backscattered diffraction (EBSD) showed that the seaweed growth was oriented in the <110> direction, which was significantly different from that of a <100> regular dendrite. The mechanical properties of seaweed morphology specimens were evaluated using a tensile-testing machine at room temperature. The ultimate tensile strength (σb), elongation (δf) and toughness of the seaweed morphology specimens reach 186.1 MPa, 45.5% and 72.9 MJ m−3, respectively, for AS; 151.6 MPa, 50.9% and 67.9 MJ m−3, respectively, for TS; and 164.3 MPa, 62.2% and 89.4 MJ m−3, respectively, for DS. With respect to specimens with regular dendrite, the DS specimen exhibited a significant improvement in the elongation and toughness of 109% and 96%, respectively. Additionally, the seaweed morphology specimens show a stronger work hardening effect (n = 0.33, 0.30 and 0.38 for AS, TS and DS specimens, respectively) than the dendrite specimen (n = 0.22). Continuous tip-splitting of the seaweed morphology specimen produced refined and uniformly distributed second-phase particles that contributed to the aforementioned enhancement. Our results provide the underlying insight of the mechanical properties of seaweed morphology specimens, a topic crucial to the design of aluminum alloys.

Coupling effect of undercooling and cooling on Ti–Al–V alloy solidification

Rapid solidification is one of the most significant studies for titanium alloys. In this paper, we investigated the rapid solidification of Ti–Al–V alloy micro-droplets by a drop tube. The generally ignored coupling effect of undercooling and cooling on the solidification was explored, which associated with inherent solute contents. According to the results of thermal analysis, as well as the calculation of undercooling and cooling rate, a valid model was proposed to discuss the β → α phase transition. It suggested that the microstructures of supersaturated Ti–Al–V alloys hardly preserved primary β phase in the rapid solidification. Moreover, the formation energy calculated from first principle complementally indicated that excess addition of vanadium was against to the stability of β phase. Accordingly, the rapid solidification paths and microstructure evolutions were summarized, which was explained by the calculation results. The final microstructures were all composed of α phase with various grain configurations, including lamellar and dendritic crystals, evolving with the decrease of droplet diameter. The excess solutes changed the way of dendrite growth from the regular dendrite to worm-like dendrite. Note that the competitive nucleation behavior between β phase and α phase was clarified in the rapid solidification. In addition, the mechanical properties of master alloys and solidified droplets were also studied.

Competitive Growth of Degenerate Pattern and Dendrites During Directional Solidification of a Bicrystal Metallic Alloy

A degenerate pattern is seaweed-like morphology with its tip splitting continuously, differing from that the regular dendrite has a stable tip. Here, we carried out bicrystal (BC)-assembled experiments to investigate the competitive growth between a degenerate pattern and regular dendrites in a directionally solidified Al-4.5 wt pct Cu alloy. Using the electron backscattered diffraction (EBSD) technique, we characterized a degenerate pattern that solidified with a (100)[001]45 deg orientation and a dendrite growth comprising (100)[001]0 deg or (100)[001]15 deg orientations. The experimental results for the competitive growth show that the dendrites could overgrow the degenerate pattern completely at V = 15 and 25 µm/s. The grain boundary (GB) in between these two morphologies is not smooth, and its inclination angle θGB is slightly increased with an increase in the growth velocity. For the converging GBs, we find that the dendrites overgrow the degenerate pattern either by generating new primary arm dendrites through tertiary branching or being blocked by the growth of the existing primary arm dendrites, depending on the misorientation arrangement between these two morphologies. The limited growth stability of the degenerate pattern in comparison with the dendrite contributes to deepening the understanding of why the degenerate pattern is not widely prevailing in metallic alloy solidification microstructures.

Probing the degenerate pattern growth of {100} orientation in a directionally solidified Al-4.5 wt% Cu alloy

Degenerate pattern is a seemingly disordered morphology but it exhibits the inherently ordered crystal connected with tip-splitting and limited stability which makes it difficult to observe in the metallic system. Here we employ (100)[011] orientated planar-front seeds using directional solidification and reveal the fundamental origins of the degenerate pattern growth in an Al-4.5 wt% Cu alloy. We find that the spacing of the tip-splitting (λ) in the degenerate of the alloys followed a power law, λ∝V−0.5, and the frequency (f) of the splitting was related to the growth velocity (V) by ƒ∝V1.5. The dimensionless growth direction (θ/θ0) increased monotonously and approached 0.6 with faster velocity, attributed to its anisotropy in the interface kinetics. Once growth velocity exceeded a threshold, two types of pattern transitions from degenerate to regular dendrites were proposed. One of them exhibited a random and chaotic mode and the other underwent a rotation in growth direction.

Microstructure and properties of twinned dendrites in directionally solidified A356 alloy

Twinned dendrites and regular dendrites have been respectively produced in A356 alloy by Bridgman solidification. The primary trunks of twinned dendrites grew along <110> direction and were split by a coherent (111) twin plane were characterized by electron backscattered diffraction (EBSD), which is different from the <100> regular dendrites. The growth of twinned dendrites was found to have a rotation relationship around a common <110> direction. With respect to the specimens with regular dendrites, the mean tensile strength, elongation and toughness of the specimens with twinned dendrites were increased from 230 MPa to 239 MPa, 16.69–18.99%, 34.45 J/m3 to 40.88 J/m3, respectively. Meanwhile, the coefficient of thermal expansion (CTE) of the specimen with twinned dendrites increase monotonically up to 2.28×10−5K−1 at temperature of 473 K. This CTE was successfully lowered than that of specimen with regular dendrites at the temperature above 323 K, and reaches the maximum decrease around 4.2% at 473 K. The fractures of two types of specimens were both originated from the Si-rich regions where dimples formed. In addition, detailed analyses on coherent twin boundary (CTB) of twinned dendrites by transmission electron microscope (TEM) and high-resolution TEM (HRTEM) indicate that the mechanisms of twin and dislocation-CTB interactions can enhance the strength and ductility of the A356 alloy with twinned dendrites.

Microstructure and orientation evolution in unidirectional solidified Al–Zn alloys

Morphological instability and growth orientation evolution during unidirectional solidification of Al–Zn alloys with different pulling speeds were investigated by X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) in scanning electron microscope (SEM). The experimental results show that, as the pulling speed increases, the primary dendrite spacing becomes smaller gradually and dendrite trunks incline to the heat flow direction perfectly in unidirectional solidified Al–9.8wt%Zn and Al–89wt%Zn alloys. However, regardless of the pulling speed in unidirectional solidified Al–Zn alloys under fixed thermal gradient, the regular dendrites with <100> directions of primary trunks and secondary arms in 9.8wt% Zn composition are replaced by <110> dendrites of primary trunks and secondary arms in 89wt% Zn composition. In unidirectional solidified Al–32wt% Zn alloy, cellular, fractal seaweed, and stabilized seaweed structures were observed at high pulling speeds. At a high pulling speed of 1000µm/s, seaweed structures transform to the columnar dendrites with <110> trunks and <100> arms. The above orientation evolution can be attributed to low anisotropy of solid-liquid interface energy and the seaweed structure is responsible for isotropy of {111} planes.

In situ synchrotron X-ray studies of the coupled effects of thermal and solutal supercoolings on the instability of dendrite growth

Special growth pattern representing unique growth conditions is a vital clue to investigate the morphology evolution mechanism in metallic alloy solidification. The dendrite pattern and growth orientation of dendritic doublons in the hypoeutectic Al-Cu alloy have been studied by synchrotron radiation imaging technology and electron back scatter diffraction. The results show that this kind of doublon is two-dimensional and the secondary arms grow perpendicularly to the primary stem. This doublon morphology can appear as an equiaxed grain, columnar dendrite, or coexist with a regular dendrite. The growth directions of the dendritic doublon tips and the secondary arms are <110> and <001>, respectively. Rising cooling rate or Cu concentration in the alloys facilitates the formation of the doublonic structure. According to the kinetic morphology diagram of Al-Cu alloys (pattern-dimensionless supercooling-anisotropy) obtained from the experimental data, the dendritic doublon forming region was above the boundary of fractal dendrite and compact dendrite.

Axon-Carrying Dendrites Convey Privileged Synaptic Input in Hippocampal Neurons

Neuronal processing is classically conceptualized as dendritic input, somatic integration, and axonal output. The axon initial segment, the proposed site of action potential generation, usually emanates directly from the soma. However, we found that axons of hippocampal pyramidal cells frequently derive from a basal dendrite rather than from the soma. This morphology is particularly enriched in central CA1, the principal hippocampal output area. Multiphoton glutamate uncaging revealed that input onto the axon-carrying dendrites (AcDs) was more efficient in eliciting action potential output than input onto regular basal dendrites. First, synaptic input onto AcDs generates action potentials with lower activation thresholds compared with regular dendrites. Second, AcDs are intrinsically more excitable, generating dendritic spikes with higher probability and greater strength. Thus, axon-carrying dendrites constitute a privileged channel for excitatory synaptic input in a subset of cortical pyramidal cells.

A Vegetable Garden-like Zinc Dendritic Patterning by Electrodeposition

Abstract A highly regular dendrite pattern surface was fabricated using a dot-pattern zinc sheet by electrodeposition. The relationship between pretreatment conditions and morphology of the dendrite pattern was investigated. The dendrite growth position could be controlled with high regularity depending on the height of the relief structure formed by electrodeposition.

Morphology and Growth Kinetic Advantage of Quenched Twinned Dendrites in Al-Zn Alloys

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, 2004; 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.

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.

Experimental Study on Surface Characteristics of Laser Cladding Layer Regulated by High-Frequency Microforging

High-frequency microforging technology is used to produce micrometer-scale plastic deformation on the surface of material out of the vibration impact of a forging punch, and the cumulative effect of its various frequencies on micrometer-scale plastic deformation can cause changes of surface microstructure and mechanical properties. This study used (1) a self-made machine to treat NiCrBSi alloy, (2) a mechanical comparator and optical microscopy (OM) to study the geometric characteristics of plastic deformation, (3) OM and scanning electric microscopy (SEM) to observe influence on surface microstructure and cracking behavior of the laser cladding layer under microforging, (4) x-ray diffractometer (XRD) to measure the surface residual stress of laser cladding layer before and after forging, and (5) microhardness tester and wearing experimental machine to study changes of microhardness, friction coefficient, and wear characteristics of laser cladding layer after microforging. The results have shown that high-frequency microforging could produce plastic deformation about 150 μm deep on the surface of NiCrBSi alloy clad by laser. Regular dendrite and eutectic crystallization microstructure, which is a peculiar characteristic of the laser cladding layer, was broken into pieces and formed residual compression residual stress on the surface. Resistance to cracking of laser cladding layer improved greatly, microhardness and wearability increased, and the friction coefficient did not under go a noticeable change.

Twinned dendrite growth in binary aluminum alloys

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 −1, v ≈ 1 mm s −1, 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 〈1 1 0〉 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 〈1 1 0〉 instead of 〈1 0 0〉, [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.

Universal completely regular dendrites

Universal completely regular dendrites