Dendritic Array Research Articles

Primary dendrite array morphology in Al-7 wt% Si alloy samples directionally solidified aboard the International Space Station

Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7 wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified aboard the International Space Station (ISS) to determine the effect of mitigating convection on the primary dendrite array morphology. MICAST-6 was processed with a step-increase in the growth speed from 5 to 50 µm s−1, MICAST-7 with a step-decrease from 20 to 10 µm s−1, and MICAST2-12 with constant speed of 40 µm s−1. Nearest-neighbor primary dendrite arm spacings and primary dendrite trunk diameters were measured on transverse sections taken at approximately 5 mm intervals along the length of the MICAST samples. In MICAST samples, especially in the steady-state growth regime, the observed primary dendrite nearest-neighbor spacings show a good agreement with predictions from the Hunt-Lu numerical model. The maximum to minimum (highest 10% to lowest 10%) nearest-neighbor spacing ratio in MICAST samples is 2.10 ± 0.27 in agreement with Hunt-Lu model, which assumes pure diffusive transport. Trunk diameters in MICAST samples also agree well with predictions from a coarsening based model. The terrestrial-grown equivalent samples, on the other hand, display smaller nearest-neighbor spacing, and larger primary dendrite trunk diameters than those predicted from the models. Thermosolutal convection appears to increase the dendrite tip radius, causing the trunk diameter to increase, while simultaneously reducing the primary dendrite spacing. This requires verification-experiments involving directional solidification followed by rapid quenching of the liquid-solid interface in metallic alloys in a low gravity environment. The International Space Station currently does not have on-board hardware to conduct such experiments.

Multiscale prediction of microstructure length scales in metallic alloy casting

Microstructural length scales, such as dendritic spacings in cast metallic alloys, play an essential role in the properties of structural components. Therefore, quantitative prediction of such length scales through simulations is important to design novel alloys and optimize processing conditions through integrated computational materials engineering (ICME). Thus far, quantitative comparisons between experiments and simulations of primary dendrite arms spacings (PDAS) selection in metallic alloys have been mainly limited to directional solidification of thin samples and quantitative phase-field simulations of dilute alloys. In this article, we combine casting experiments and quantitative simulations to present a novel multiscale modeling approach to predict local primary dendritic spacings in metallic alloys solidified in conditions relevant to industrial casting processes. To this end, primary dendritic spacings were measured in instrumented casting experiments in Al–Cu alloys containing 1 wt.% and 4 wt.% of Cu, and they were compared to spacing stability ranges and average spacings in dendritic arrays simulated using phase-field (PF) and dendritic needle network (DNN) models. It is first shown that PF and DNN lead to similar results for the Al-1 wt.%Cu alloy, using a dendrite tip selection constant calculated with PF in the DNN simulations. PF simulations cannot achieve quantitative predictions for the Al-4 wt.%Cu alloy because they are too computationally demanding due to the large separation of scale between tip radius and diffusion length, a characteristic feature of non-dilute alloys. Nevertheless, the results of DNN simulations for non-dilute Al–Cu alloys are in overall good agreement with our experimental results as well as with those of an extensive literature review. Simulations consistently suggest a widening of the PDAS stability range with a decrease of the temperature gradient as the microstructure goes from cellular-dendrites to well-developed hierarchical dendrites.

Heterogeneity of the Dendrite Array Created in the Root of Cored SX Turbine Blades during Initial Stage of Crystallization.

The roots of cored single-crystalline turbine blades made of a nickel-based CMSX-4 superalloy were studied. The casts were solidified by the vertical Bridgman method in an industrial ALD furnace using the spiral selector and selector continuer situated asymmetrically in the blade root transverse section. Scanning electron microscopy, the Laue diffraction and X-ray diffraction topography were used to visualize the dendrite array and the local crystal misorientation of the roots. It has been stated that heterogeneity of the dendrite array and creation of low-angle boundaries (LABs) are mostly related to the lateral dendrite branching and rapid growth of the secondary and tertiary dendrites near the surface of the continuer–root connection. These processes have an unsteady character. Additionally, the influence of the mould walls on the dendrite array heterogeneity was studied. The processes of the lateral growth of the secondary dendrites and competitive longitudinal growth of the tertiary dendrites are discussed and a method of reducing the heterogeneity of the root dendrite array is proposed.

Effect of sub-boundaries on primary spacing dynamics during 3D directional solidification conducted on DECLIC-DSI

To characterize the dynamical formation of three-dimensional (3D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile – 0.24 wt% camphor model alloy was carried out under low gravity in the DECLIC Directional Solidification Insert onboard the International Space Station. The continuous interface observation enables to construct space-time evolution maps of cell location and primary spacing. Both convergent and divergent sub-boundaries are identified and new insights on their effects on the spatiotemporal evolution of the pattern are thus evidenced. 3D phase-field simulations that reproduce the experimental sub-boundary configurations are performed to support the analyses. Even for the low angle sub-boundaries studied, the primary spacing increases or decreases in the vicinity of the boundary respectively for divergent and convergent sub-boundary. This effect may extend on a long distance within the different sub-grains and its magnitude depends on the average primary spacing and its positioning relative to the limits of the stability band. On the sample scale, the primary spacing profile is also influenced by the presence of sources and sinks at the crucible wall due to the pattern drift. Their type and distance from the sub-boundaries give rise to complex spatial distributions of primary spacing over the entire sample.

Spherical Gaussians: An intuitive method for creating complex anisotropies in interface energies for the phase field method

The Phase Field Method requires continuous functions to control interface energies between phases. We have developed a method for phase field modeling where interface energies are described by spherical gaussians, a set of continuous functions which can simulate a wide variety of complex anisotropies. We demonstrate the use of the technique to produce a wide array of crystal geometries, including rods, hexagonal rods, cubes, and cubic dendrites, using the native properties of spherical gaussians to create specific interface energies throughout the full solid angle of an energy surface. Parameters controlling the sharpness of minima are shown to have a significant effect on morphology.

One-pot synthesis of three-dimensional Pt nanodendrites with enhanced methanol oxidation reaction and oxygen reduction reaction activities

The high cost of Pt-based catalysts impedes the practical large-scale commercial application of fuel cells. Controlling the morphology of Pt nanostructure is one of the most promising approaches to promote the electrocatalytical activity and stability toward methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). Herein, we report a facile method for the synthesis of Pt nanodendrites consisting a dense array of dendrites using K2PtCl4 as metal resource, L-ascorbic acid as reducing agent and hexadecyltrimethylammonium chloride as capping agent in aqueous solution at 96 °C for 30 min. Owing to the novel structure, the obtained Pt nanodendritic electrocatalyst exhibites superior MOR catalytic performance and particularly long-term stability both in acid and alkaline conditions. the Pt nanodendrites also exhibited a higher half wave potential of 0.86 V and lower tafel slope of 51.69 mV dec–1 when compared with 20% Pt/C (0.83 V, 93.55 mV dec–1) in acid conditions toward ORR. This work provides a promising way for the rational design of efficient and robust catalysts for sustainable energy conversion.

Phase-field simulation of microstructure evolution in electron beam additive manufacturing.

Electron beam additive manufacturing (EBAM) is an emerging additive manufacturing technology with extremely high energy beam. The rapid solidification in the molten pool is of interest but not fully understood. In EBAM, with both large thermal gradient and cooling rate, the microstructure evolution during solidification is difficult to be described. The quantitative multi-phase-field model provides an effective way to reveal the dynamic evolution of dendrites in the molten pool of EBAM. In this study, the thermal profile is interpolated from the macroscale simulation at each time-step, to couple the realistic thermal evolution in the molten pool. The microstructure evolution and competitive growth have been investigated in details. Simulations of dendrite arrays with the same orientation showed how the growth velocity and the primary spacing of columnar dendrites depend on thermal gradient and cooling rate. The results are in agreement with theoretical models qualitatively. Moreover, the Gaussian nucleation model was introduced so as to give a better prediction of the microstructure in EBAM.

Mushy zone solidification in a static thermal gradient: The effect of dendrite orientation

The solidification dynamics of a mushy zone, i.e. an array of tilted dendrites, kept in a static thermal gradient has been investigated by phase-field simulations. In comparison to our previous study of a well-aligned, i.e. non-tilted dendritic mushy zone [Acta Mat. 122, 310 (2017)], we show that for the general case of dendrites with growth orientations tilted to the thermal gradient, the ”Temperature Gradient Zone Melting” phenomenon also affects the primary trunks, yielding a similar solidification kinetic but different microsegregation pattern in the solid. We briefly discuss the relevance of the presented observations for the microstructure formation in additive manufacturing processes.

Phase-field modeling of complex dendritic structures in constrained growth of hexagonal close-packed crystals.

We perform the phase-field modeling to investigate the growth pattern selections of the complex dendritic structures in constrained growth with different solidification and orientation conditions. The results show that hexagonal close-packed (hcp) crystals emerge as dendritic and cellular arrays in different planes, originating from the specific hcp anisotropy that allows different growth preferences between the basal and cylindrical planes. A morphological transition of the titled dendrites to tip-splitting dendrites arises reflecting the competition between the preferred orientation induced primary growth and the misorientation induced sidebranching formation. Furthermore, the dendritic patterns exhibit sharper tips and the more significant sidebranches, while the cellular pattern is changed from the symmetric cells to the tip-splitting cells, and to seaweeds with the increase of anisotropy strength, indicating the competitive mechanism of the in-plane anisotropy induced growth promotion and the out-plane anisotropy induced growth restriction. We expect to understand the growth competition, the morphology selection, as well as the orientation dependence of the complex dendritic structures in the three-dimensional (3D) constrained growth.

Competitive dendrite growth during directional solidification of a transparent alloy: Modeling and experiment.

A two-dimensional (2-D) cellular automaton-finite difference method (CA-FDM) model and in situ observation experiments of directional solidification using a transparent alloy of SCN-2wt.% ACE are employed to investigate various microstructural evolution of columnar dendrites during directional solidification. In the present model, the growth of columnar dendrites is simulated using a CA technique. The solute diffusion is solved using the FDM. The model is capable of visualizing the interaction between the formation of dendrite arrays with identical or different growth orientations, and the evolving solute concentration field. Several dendritic competitive growth modes between two converging and diverging dendrite arrays are reproduced. The simulation results agree well with the experimental observations. The simulations are also performed to study the effects of temperature gradient and cooling rate on the growth morphology of diverging dendrites. It is found that with the increase of temperature gradient and cooling rate, the tertiary branches produced from the well-developed side branches of the unfavorably oriented grain at the divergent grain boundaries are more likely to become the new primary dendrite arms.

Microstructure and mechanical properties of wire and arc additive manufactured AZ31 magnesium alloy using cold metal transfer process

Wire and arc additive manufacturing (WAAM), using cold metal transfer (CMT) as heat source, exhibits a great potential for additive manufacturing of magnesium alloys due to low heat input. With the purpose of revealing the relationship between the microstructure and mechanical properties of WAAMed AZ31 material, the present study has been carried out. The manufactured AZ31 thin-walled deposit is mainly composed of columnar dendrite arrays, including dendritic α-Mg matrix, interdendritic eutectics (α-Mg and β-Mg17Al12) and some dispersive η-Al8Mn5 phases. The average primary dendrite arm spacing increases from 17 μm at the bottom to 39 μm at the top of the deposit, and the volume fraction of the interdendritic eutectic decreases from 52.1% to 39.3%. The microstructure of each layer except the top layer consists of vertical columnar dendrites and direction-changed columnar dendrites in sequence. The top layer appears equiaxed dendrites due to columnar to equiaxed transition (CET). The tensile properties present obvious anisotropic characteristics because of the epitaxial columnar dendritic growth along the building direction. The tensile properties also show obvious variation from the bottom to the top of the deposit because of the differing microstructures in different regions. The results are further analyzed in detail through the microstructure evolution resulted from the new manufacturing method.

Phase-Field Modeling of Microstructure Evolution in the Presence of Bubble During Solidification

Simulation of the solid–liquid–gas interaction during solidification is challenging due to the presence of complex phase interfaces, bubble deformation, and high liquid/gas density ratio. In this work, a hybrid phase-field lattice-Boltzmann (PFLB) approach, together with a parallel and adaptive-mesh-refinement (Para-AMR) algorithm, is developed to model interactions between the gas bubble and solid growth front during solidification. The solid growth and bubble evolution are solved by the phase-field method. Both melt flow and bubble movement are determined by a kinetic-based lattice-Boltzmann model. Bubble dynamics during alloy solidification is modeled and compared with experiments, and a good agreement is achieved for various solid/liquid interfaces including planar, cellular, and dendritic interfaces. Results show that the effect of the bubble on solid array is dependent on the solid/liquid interface morphology, bubble size, and relative position between the bubble center and dendritic tip. Two interaction mechanisms, including engulfment and entrapment, are compared, and the difference is caused mainly by the redistribution of solute. The interaction mechanism between the rising multibubbles with large deformation and the dendritic array is also discussed.

Variation of Crystal Orientation and Dendrite Array Generated in the Root of SX Turbine Blades.

The variation of the crystal orientation and the dendrite array generated in the root of the single-crystalline (SX) turbine blades made of CMSX-4 superalloy were studied. The blades with an axial orientation of the [001] type were solidified by the industrial Bridgman technique using a spiral selector at a withdrawal rate of 3 mm/min. The analysis of the crystal orientation and dendrite arrangement was carried out using scanning electron microscopy, X-ray diffraction topography, and Laue diffraction. It was found that the lateral growth of such secondary dendrite arms, which are defined as “leading” and grow in the root at first, is related to the rotation of their crystal lattice, which is the reason for creation of the low-angle boundary (LAB) type defects. The primary crystal orientation of the selector extension (SE) area determines the areas and directions of the lateral growth of the leading arms. Additionally, it was found that in the SE areas of the root, near the connection with the selector, the spatial distribution of the [001]γ′ crystallographic direction has a complex wave-like character and may be related to the shape of the crystallization front.

Stable Eutectic Formation in Spray-Formed Cast Iron

Spray forming is an advanced casting process that produces refined and homogenous microstructure directly from the liquid metal regardless of the alloy system. However, the microstructure evolution during spray forming is complex because the process comprises two sequential steps with very different cooling rates, i.e., atomization and deposition. It is well known that the microstructure of cast irons is highly dependent on the chemical composition and the cooling rate imposed to the liquid. In order to better understand the microstructural evolution during solidification by spray forming, this study investigated the solidification of two cast irons with different stable–metastable eutectic temperature interval (ΔTES−M), 30 K and 17 K. The microstructures of both overspray powders presented a dendritic array of austenite with cementite in the inter-dendritic spacing. Despite the high cooling rates imposed to the alloys during the atomization step, the final microstructure was defined by the cooling conditions prevailing during the final step of deposit solidification and stable eutectic was formed. This was ascribed to the dynamic process involving heating and remelting of the low melting temperature phases present in the droplets that arrives completely or partially solid in the deposition zone.

3D metal dendrite-derived petaloid shaped NiFe2O4@ NFM as binderless electrode for oxygen evolution reaction and electrochemical energy storage

Two-dimensional nanosheets materials anchored on the surface of ordered dendritic array substrate not only realizes the multiple scale assembly of materials, but also enables the construction of self-supporting electrodes. In this work, we fabricate the Ni–Fe alloy dendritic array matrix by the facile electrodeposition process for the first time. It can provide a large number of vertical paths for electrolyte ions, electron transfer and gas overflow, which further helps to improve the electrochemical performance of electrode. Whereafter, the metal on the surface of dendrite is converted into NiFe2O4 via in situ calcination oxidation method. The NiFe2O4@NiFe dendrite matrix electrode is characterized by Field emission scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy analysis. During the oxygen evolution reaction measurement, the as-prepared NiFe2O4@NFM-3 requires a low overpotential of 234 mV (vs.RHE) to arrive 10 mA cm−2 current density. Furthermore, the NiFe2O4@NFM-3 electrode reveals the high area capacitance about 1560 mF cm−2 at current density of 1 mA cm−2. The electrochemical performances test results demonstrate that this fiber electrode can serve as the bifunctional electrode in oxygen evolution reaction (OER) and energy storage.

Influence of Microstructure Length Scale on the Tensile Properties and Superplasticity of Cu-Doped Sn-34Bi TIM Alloy

Sn-Bi alloys are candidates for use as thermal interface materials, TIM. Such materials are often used to conduct heat away from a temperature sensitive device. Sn-Bi alloys are considered competitive in cost and cost volatility, being particularly interesting with doping of ternary elements. Among various attended pre-requisites, high manufacturability when utilizing rolling or pressing is a particular benefit. To take advantage of this property, Sn-Bi alloys should be designed to ensure high ductility, maximizing manufacturability. Suitable microstructures capable of improving the alloy's plasticity are very desirable. The present investigation aims to evaluate the effects of starting as-cast microstructures on the strength and ductility of the Sn-34 wt.%Bi-0.1 wt.%Cu alloy. Specimens with very different length scales of the dendritic array were subjected to tensile tests at three temperatures: − 50°C, 25°C and 60°C. It was demonstrated that the samples with more refined microstructure are related to slightly higher tensile properties at room temperature if compared to the results observed for the coarser microstructure specimens. On the other hand, much higher ductility was observed for specimens having more refined microstructure tested at 60°C. Strain was three times higher than those characterizing coarse related specimens. This disparity in ductility has been investigated. The principal mechanism of flow in superplasticity was found to be the grain boundary displacement. Various contributing factors were recognized for the Sn-Bi-Cu alloy sample with initial fine-dendritic form, which are: grain size lower than 10 μm, complete destruction of the dendritic array, significant rotation of grains during loading at 60°C and incidence of cavities.

Sol-gel synthesis of cubic Nb/Ta-doped SrCoO3−δ with mixed nano-micro morphology

SrCo0.90M0.10O3−δ (M = Nb, Ta) perovskite powders were synthesized by sol-gel, using tartrate-precursor decomposition. The crystal structure of these materials was analyzed by x-ray powder diffraction. Rietveld refinements showed that both samples achieved the cubic crystal structure (Pm-3m), which was stabilized by the incorporation of highly-charged transition-metal cations at the octahedral sites. This synthesis method successfully lowered the calcination temperature relative to other methods mentioned in literature. The concentration of oxygen vacancies (δ value) was determined by thermogravimetric analysis. Microstructural analysis revealed micrometric scale particles with a superficial nanometric dendritic array with an average grain size less than 100 nm. The present work provides a new strategy to synthesize nanostructured perovskite materials that could have better electrical and electrochemical properties.

Effect of pouring temperature on microstructure and microsegregation of as-cast aluminum alloy

An experimental investigation of pouring temperature effects on the thermal parameters, microstructure, and microsegregation during directional solidification of the hypoeutectic Al–Cu alloy is presented and discussed. The hypoeutectic Al–4.0 wt.% Cu alloy with superheat temperature is poured at temperatures 682.5, 747.5, and 812.5 °C. The thermal parameters (cooling rate, solidification speed, and local solidification time) are affected by pouring temperature. These, in turn, affects the microstructure arrangement and microsegregation profiles. Experimental growth laws of tertiary dendrite arm spacing relating to the cooling rate have been determined, indicating that increase in thermal parameter was responsible for the refinement effect on dendritic morphology. Results showed that pouring temperatures nearer to the liquidus temperature produced a dendritic array refined, i.e., pouring temperatures few degrees Celsius higher than the liquidus temperature favored the reduction of the tertiary dendrite arm spacings. The results of chemical composition obtained by fluorescence X-ray spectrometry technique confirmed that microsegregation profiles were influenced by pouring temperature, i.e., the microsegregation profiles were shown to move upward with the decrease in the pouring temperature.

Primary spacing selection and phase transformation in directional solidified Cu-21%Ce alloy

In this study, a hyper-peritectic Cu-21 wt% Ce alloy was prepared by directional solidification method at various withdrawal rates of 100 μm s−1, 25 μm s−1, 6 μm s−1, 5 μm s−1, 4 μm s−1 and 2 μm s−1. The microstructure evolution was investigated by optical microscope. The phase constitutions were analysed by the x-ray diffraction, differential scanning calorimetry and transmission electron microscope. Further, the minimum spanning tree method was selected to investigate the spacing characters of dendritic arrays. The experimental results demonstrate that primary phase microstructure transforms from dendritic to island structure with decreasing the withdrawal rates, and the maximum volume fraction of the primary phase reached 45.16% at the withdrawal rate of 6 μm s−1. It was found that when reducing the withdrawal rate from 100 μm s−1 to 2 μm s−1, the primary dendrite spacing increased first and then decreased, with a maximum value of 144.64 μm presenting at 6 μm s−1. The phase constitutions in the directional solidified samples were confirmed to be Cu6Ce, Cu4Ce and Cu4Ce + Cu2Ce eutectics. Moreover, the maximum compressive force was determined to be 10.3 kN and the maximum compressive strain was 3.73% respectively, corresponding to the maximum primary phase volume fraction at a withdrawal rate of 6 μm s−1. Directional solidification method has been proved to be an efficient way to improve the mechanical properties of hyper-peritectic Cu-21%Ce alloy.

Primary Dendrite Trunk Diameter in Al-7wt% Si Alloy Directionally Solidified Aboard the International Space Station

Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified at growth speeds varying from 10 to 50 µm s-1 aboard the International Space Station to determine the effect of mitigating convection on the primary dendrite array. The observed primary dendrite trunk diameters during steady-state growth of MICAST samples show a good agreement with predictions from a coarsening based model developed by the authors. The trunk diameters in the terrestrial-grown equivalent samples were larger than those predicted from the model. This suggest that thermosolutal convection increases the trunk diameter of primary dendrites, perhaps by increasing their tip radius due to compositional changes.