Dendritic Crystals Research Articles

Preparation of composite gradient coating on the surface of 20Cr steel by scanning electron beam surface alloying

Scanning electron beam surface alloying can effectively improve the microstructure of the surface layer of the material. Plasma thermal spraying and scanning electron beam technology were used to conduct molten alloy treatment on the surface of 20Cr steel to study the formation mechanism of the gradient structure of the sample surface. The study found that after scanning electron beam surface alloying treatment, the surface coating of the sample is composed of alloying layer and heat-affected zone. The microstructure of the alloying layer is acicular martensite, lath martensite, retained austenite and the second phase of the mesh structure, the crystal grains in this region are equiaxed, columnar and dendritic crystal. The heat-affected zone is composed of a complete phase transformation zone and a partial phase transformation zone, the microstructure of the complete transformation zone is acicular martensite and lath martensite, and the microstructure of the partial transformation zone is acicular martensite and ferrite.

Microstructure and enhanced electromagnetic wave absorbing performance of Zn0.6Ni0.3Cu0.1Fe2O4 ferrite glass-ceramic

Here we introduce a controllable route for the efficient synthesis of Zn0.6Ni0.3Cu0.1Fe2O4 ferrite glass-ceramic with enhanced electromagnetic wave (EMW) absorbing performance. By adding a certain amount of Zn, Ni, Cu and Fe oxides into the SiO2–Al2O3–B2O3–CaO-R2O glass system, the microstructure of three-dimensional dendritic ferrites combined with amorphous SiO2-rich phase is constructed through a high-temperature melt and quenching route. The good EMW absorption performance is attributed to the unique combination of amorphous glass and spinel ferrite, which improves the impedance matching of the material and absorbs EMW by the dielectric loss and magnetic loss. Moreover, the dendritic ferrite crystal phase is compounded with the SiO2-rich amorphous phase to form grain boundaries and crystal-amorphous interfaces, which enhances the interfacial polarization and builds multiple transmission-absorption mechanisms. The results show that the reflection loss peak value of the glass-ceramics containing 60 wt% Zn0.6Ni0.3Cu0.1Fe2O4 spinel is −42.16 dB with the sample thickness of 2 mm, and the effective absorption band range (reflection loss ≤ -10 dB) is 3.76 GHz (13.6–17.36 GHz) at 1.5 mm. This approach presents a scalable and low-cost solution that may be applied to the design of high-efficiency EMW consumption components in the future.

Structural phase variations in high-entropy alloy at irradiation by pulsed electron beam

The high-entropy alloy (HEA) of Al - Co - Cr - Fe - Ni system of nonequiatomic composition is obtained by the technology of wire-arc additive manufacturing (WAAM) in atmosphere of pure nitrogen. By the methods of modern physical materials science it is shown that in the initial state the alloy has dendritic structure indicating nonhomogeneous distribution of alloying elements. It is a multiphase material whose main phases are Al3NCr3C2 , (Ni, Co)3Al4 . Nonadimensional particles (Ni, Co)3Al4 of cubic shape are located along interfaces of submicron phases Al3Ni and Cr3C2 . The HEA irradiation by pulsed electron beams with energy density Es = 10 + 30 J/cm2, pulse duration of 50 is, frequency of 3 Hz and pulse number of 3 leads to high-velocity melting and subsequent crystallization of surface layer. If Es = 10 J/cm2, no failure of dendritic crystallization structure happens. Interdendritic spaces are enriched in chemical elements Al, Ni and Fe, and dendrites themselves - in chromium atoms. The most liquating element of the alloy is Al, the least one is Co. If Es = 20 J/cm2, a nanocrystalline structure is formed in the layer 15 inn thick in bulk of grains. Size of crystallization cells amounts to 100 - 200 nm, size of inclusions in cell junctions is 20 - 25 nm, and along cell boundaries it is 10 - 15 nm. Cells of high-velocity crystallization are enriched in Al and Ni. The Co atoms are homogeneously distributed along the surface layer volume. The most liquating element is Cr, the least liquating one is Co. The increase in energy density of electron beam to 30 J/cm2 doesn't lead to substantial (as compared to Es = 20 J/cm2 ) variations in surface layer structure. The irradiation mode (Es = 20 J/cm2, 50 is, 3 pulses, 0.3 Hz) is detected that allows formation of the surface layer with the highest level of homogeneity of chemical element distribution in the alloy.

Microstructure and high temperature tribological performance of Co-based laser cladded coatings reinforced with in situ TiN-VC

Co-based laser cladded coatings reinforced with in situ TiN-VC were prepared. The microstructure and high temperature tribological performance of the cladded coatings were researched. Results revealed that TiN and VC were identified in the cladded coatings with in situ TiN-VC besides γ-Co and Cr23C6 phases. In situ TiN and VC promoted the formation of many short rod-shaped dendritic crystals and equiaxed crystals, refining the microstructure. Compared with the Co-based cladded coatings, the microhardness of the cladded coatings with in situ TiN-VC at 20 °C was improved by 30.0%, the mass loss was reduced by 51.3%, and the friction coefficient was obviously decreased. The mass loss and friction coefficient of the cladded coatings with in situ TiN-VC were both initially increased and subsequently decreased as the temperature was increased from 20 to 600 °C and were up to 12.2 mg and 0.256 at 400 °C, respectively. The abrasion mechanism was translated from the abrasive wear to the abrasive wear, adhesive wear, and oxidation wear.

Formation and kinds of boride on microstructure evolution and mechanical properties in Ti47.5Al2.0Cr2.0NbxB alloys

To adjust the kinds of borides and improve the mechanical properties, different concentrations of boron were added to the Ti47.5Al2.0Cr2.0Nb alloy by arc melting, and the phase composition, lamellar colony, elemental distribution and compressive properties were investigated. Results show that when the boron concentration increases to 2.0 at.%, dendritic crystals are transformed into equiaxed crystals. The size of the lamellar colony decreases from 232.9 to 92.3 μm with the boron content increases from 0 to 3.0 at.%. After adding boron, borides with two distinct morphologies appear (curved and bar borides), and then the B2 phase disappears. With the increase of boron content, the percentages of curved boride and bar boride are changed, the former decreases and the latter increases. During the solidification process, the constitutional undercooling, at the solidification front and the nucleation particles of borides increase with increasing boron content. The refined microstructure and formation of boride improve the uniformity of the elemental distribution. Compressive results show that the maximum strength is 2228.2 MPa with 1.5 at.% B, and the maximum strain is 29.1% with 2.0 at.% B. The grain refinement strengthening, solid-solution strengthening of Nb and Cr, and the precipitation strengthening of borides are the main reasons for the improved compressive properties. The fracture morphology is a mixed fracture mode which contains both transbar fracture and inter-bar fracture.

Analytical solutions describing the oblique flow of a viscous incompressible fluid around a dendritic crystal

This article considers the hydrodynamic problem of an oblique flow of a viscous incompressible fluid around the tip of a dendritic crystal. Approximate analytical solutions of Oseen's hydrodynamic equations are obtained in 2D and 3D cases using special curvilinear coordinates. It is shown that the projections of the fluid velocity change significantly with a change in the flow slope and Reynolds number. The theory developed in this work has a limiting transition to the previously known solutions for the rectilinear (without tilt) fluid flow around a dendrite.

Structure modulation and properties of NiCo2O4 nanothorn electrode materials prepared by a self-sacrificial template method

Herein, we report the preparation of an effective and novel NiCo2O4 dendritic structure with nanothorn arrays, using a stepwise, bottom-up hydrothermal and self-sacrificial template method. Rod-like ZnO is synthesized on Ni foam as self-sacrificing template, and Ni-Co-based precursor is grown on it through a hydrothermal method. NiCo2O4 dendritic crystals are obtained by alkali etching of the template and calcination, which significantly enhance its performance on electrochemical energy storage. Although the ZnO matrix is removed by alkali etching, it provides a basis for the growth of Ni-Co oxide. Using ZnO as template is simple to operate and can directly determine the shape and size of the hollow structure. Meanwhile, the ZnO template core can be well separated from the hollow structure, and the template can be used without surface functionalization, which is more simple and efficient. The obtained material can be directly used as electrode material without further processing, offering the benefit of unique structural advantages and low electron transmission resistance, as there is no need to use a binder. The as-obtained NiCo2O4 nanothorn electrode exhibits excellent electrochemical performance. It shows ultrahigh specific capacitance of 1534 C g−1 at 1 A g−1 current density, good rate discharge performance (75.8% retention from 0.5 A g−1 to 5 A g−1), values superior to those of nanothorn NiO, Co3O4 and bare Ni-Co 1-2 nanosheet structures and excellent cycling ability (83% retention after 5000 cycles). The as-assembled Ni-Co 1-2//AC hybrid supercapacitor device also exhibits high energy density of 64.1 Wh kg−1 at the power density of 450 W kg−1.

Microstructure evolution and fracture toughness of Nb–Si–Ti based alloy with Cr, Mo and W elements addition

This paper deals with microstructural evolutions, phase constitutions and fracture toughness of Nb–16Si–22Ti-(6Mo/8Cr/2W) alloys using the vacuum non-consumable arc-melting. The results show that the addition of 6 at.% Mo or 8 at.% Cr promotes the eutectic reaction of (Nb, X)3Si phase → Nbss/(Nb, X)5Si3 eutectic. The common addition of 8 at.% Cr and 6 at.% Mo leads to the microstructure transform from hypoeutectic microstructure to near-eutectic microstructure. The Nb–16Si–22Ti–8Cr alloy with coupled structure of Nbss/(Nb, X)5Si3 eutectic has the maximum fracture toughness of 13.4 MPa m1/2. The 6 at.% Mo or 2 at.% W addition has little influence on the room-temperature fracture toughness KQ value, but the common addition of 8 at.% Cr, 6 at.% Mo and 2 at.% W can improve room-temperature fracture toughness KQ value. The high content of Nbss phase and Nbss dendritic crystal in the matrix can contribute to fracture toughness of NbSi alloy. The coupled structure of Nbss/(Nb, X)5Si3 eutectic can prevent the crack propagation and improve the fracture toughness.

Microstructural evolution and mechanical properties in Zr–Cu–Al–Nb bulk metallic glass composites prepared by laser metal deposition

In this work, laser metal deposition (LMD), a direct energy deposition (DED) additive manufacturing technology, is used to prepare Zr59.3Cu28.8Al10.4Nb1.5 bulk metallic glass composites (BMGCs). Phase constitutions and thermal behavior of LMD-processed Zr–Cu–Al–Nb samples are examined using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The microstructure evolution parallel to the build direction is characterized in detail utilizing scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microscopy analysis (EPMA) and electron backscatter diffraction (EBSD). Young's modulus and hardness in typical microstructures are evaluated employing nanoindentation. Within each deposited layer, a periodic microstructure, exhibiting crystalline features of three distinctly different morphologies, is observed, depending on the location within that layer. Along the build direction, the dominating microstructure of the melt pool evolves from a nearly featureless amorphous state over fine nanocrystals to coarse nanocrystals while the morphology of most micro-scale dendritic crystals changes from flower-shaped to columnar, simultaneously a coarse microstructure appearing in upper layers. The fine Al-rich phases gradually coarsen directionally parallel to the build direction. Furthermore, the correlation between microstructure and mechanical properties is investigated. This work enhances our knowledge about microstructural characteristics and mechanical properties of BMGCs in-situ fabricated by DED additive manufacturing.

INFLUENCE OF SUPERPLASTICIZERS МС POWER FLOW 3100 ON CALCIUM SULPHATE DIHYDRATE TECHNICAL PARAMETERS

In recent years, there has been a decrease in gypsum products demand. Theimproving of gypsum binders properties can help to expand the scope of gypsum binders and, as aresult, increase gypsum products demand. The structure of the gypsum binder is the carrier of itsproperties. Studying the relationship between structure and properties is the way to directedelimination of disadvantages. One of the ways to eliminate the disadvantages is the modification ofsurfactants and nanoparticles, which affect and allow to control the hydration process. Therefore,the aim of the research is to study the effect of superplasticizer MC Powerflow 3100 on themicrostructure. physical and technical properties of calcium dihydrate formed during the hardeningof gypsum binder. The increase in the strength of gypsum binder at the compression of 77 percentand the bending strength of 17.6 percent was received during the research. Alsow the water-gypsumratio was reduced by 10 percent and the curing time was reduced: beginning from 5 to 2.5 minutes,end from 12 to 8 minutes. It was also found that the presence of the superplasticizer MC Plaflow 3100in the curing system of gypsum binder changes the process of hardening and crystal formation. Itaffects on the morphology of gypsum dihydrate crystals. To study the effect of the superplasticizer onthe morphology of gypsum dihydrate crystals, the crystals were grown in the presence ofsuperplasticizer MC Powerflow 3100 by the method of counter growth. When superplasticizer MCPowerflow 3100 is added to the solution, the thickening of crystals occurs and alsow their doublingwith the formation of dendritic crystals form. The performed researches can be the basis for thedirected management of material structure for the purpose of receiving the new improved indicatorsof gypsum binders properties that in turn will allow to expand scope of application of this material.

Structural Phase States and Surface Properties of Steel 45 after Electroexplosive Borocoppering and Electron-Beam Treatment

The paper concerns improving the microhardness and wear resistance of steel 45 by the combined treatment of electroexplosive borocoppering with the subsequent electron-beam treatment. It is found that surface roughness at the area of the electroexplosive treatment increases along with the absorbed power density and the mass of boron powder. The electron-beam treatment leads to a decrease of roughness and appearance of craters instead of radial melt flow traces. The depth structure of the electroexplosive alloying area with a thickness of 25 µm includes a coating layer, near-surface, intermediate, and boundary layers. The surface microhardness and the depth of the hardening zone after the electroexlosive alloying increase along with the absorbed power density and boron concentration and reach the values of 1400 HV The electron-beam treatment causes merging of the coating and the surface layers and increases the hardening zone depth up to 80 µm. A cellular or dendritic crystallization structure is formed near the surface, and a grain structure is formed in the depth. The inhomogeneous distribution of alloying elements over the volume of the alloying area and its adjustment during the electron-beam treatment are established. The inter-dendritic distances and grain diameters increase as the absorbed power density becomes higher with the increase of the electron-beam treatment exposure time. Also, the size of martensite needles increases in the depth. The combined treatment produces the sub microcrystalline strengthening phases-borides FeB, Fe2B, FeB2, carboboride Fe23 (C, B)6 , and carbide B4C. The microhardness level is reduced to 800 HV, and the wear resistance increases up to five times when compared to the wear resistance of the base.

Microstructure and Mechanical Properties of Additively Manufactured Ni-Al Bronze Parts Using Cold Metal Transfer Process

In this paper, the microstructure and mechanical properties of the SG-CuAl8Ni6 Ni-Al bronze straight wall were studied, which was fabricated by the cold metal transfer (CMT) arc additive manufacturing technology. This Ni-Al bronze cladding layer of SG-CuAl8Ni6 is composed mainly of α-Cu, residual β phase, rich Pb phase and κ phase. The microstructure of this multilayer single-channel Ni-Al bronze straight wall circulating presents the overall periodic law, which changes from fine cellular crystals, columnar crystals to dendritic crystals with the increase of the distance from the substrate. The Vickers hardness value of the Ni-Al bronze straight wall decreases with the distance of substrate are between 155 and 185 HV0.5. The microhardness and elastic modulus of the Ni-Al bronze specimen are 1.57 times and 1.99 times higher than these of the brass matrix, respectively. The ultimate tensile strength (UTS) of the straight wall in the welding direction and 45° downward-sloping is greater than that of about 550 MPa in the stacking direction, and the elongation value in the welding direction is the highest. With the increase in interlayer temperature, the grain size increased gradually, and the tensile strength decreases slightly.

Lacustrine carbonate towers of Lake Abhe, Djibouti: Interplay of hydrologic and microbial processes

Lake Abhe, located at the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rift trends, is a hypersaline and alkaline closed lake that is known for its exposures of massive carbonate chimneys on the flat-lying sediments of the lake's eastern margin. This study describes the morphological, textural, and petrographic characteristics of these chimneys at the basin- to microscopic-scale in order to constrain the carbonate depositional system in the context of basin hydroclimate history. Chimneys occur in three main fields and are assessed according to 1) large-scale (>5 m) morphological variations, 2) meso-scale (cm to <5 m) textures, and 3) micro-scale (<1 cm) fabrics. The dominant chimney fabric is a porous crystalline framework of trigonal prismatic and dendritic calcite crystals that locally contain spherulites, sickle-cell calcite fabrics, and entombed microbial cocci. In addition to the crystalline chimneys, other carbonate sediments include stromatolitic crusts composed of microdigitate laminated columns dominated by crystalline fan microfabrics, as well as carbonate-rich mudstones and relatively rare, localized, carbonate-rich diatomites. Chimneys are interpreted to form primarily as products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals, while stromatolitic crusts are interpreted to form during lowstand lake levels. Our interpretation of mixing processes is supported by the δ18O composition of chimney calcite, which is representative of lake water and area hot spring endmembers. The observation of stromatolitic crusts with more positive values of δ13C and δ18O than crystalline chimneys indicates that crusts formed during periods of high evaporation and low lake level. Crusts also contain Mg-silicate minerals, which are not present in crystalline chimneys, and further support the interpretation of lowstand depositional conditions. Although the age of chimney formation is not well constrained, evidence from seismic reflection data suggests a pattern of chimney formation during lake level rise and highstand times, followed by lake level fall, subaerial exposure and weathering that occurred at several times throughout the Late Pleistocene and Holocene.

Morphology of Barite Synthesized by In-Situ Mixing of Na2SO4 and BaCl2 Solutions at 200 °C

Barite is an abundant sulfate mineral in nature. Especially, the variety of morphologies of barite is often driven by the mixing of Ba-bearing hydrothermal fluid and sulfate-bearing seawater around hydrothermal chimneys. In order to better understand the factors affecting the morphology and precipitation mechanism(s) of barite in seafloor hydrothermal systems, we synthesized barite by a new method of in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C while varying Ba concentrations and ratios of Ba2+/SO42−, and at room temperature for comparison. The results show that barite synthesized by in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C forms a variety of morphologies, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped morphologies, while room temperature barites display relatively simple, granular, or leaf-like morphologies. Thus, temperature affects barite morphology. Moreover, dendritic barite crystals only occurred at conditions where Ba2+ is in excess of SO42− at the experimental concentrations. The dendritic morphology of barite may be an important typomorphic feature of barite formed in high-temperature fluids directly mixing with excess Ba2+ relative to SO42−.

MoS&lt;sub&gt;2&lt;/sub&gt;/Graphene Heterostructure Anode for Li-Ion Battery Application: A First-Principles Study

The development of next generation Li ion battery has attracted many attentions of researchers due to the rapidly increasing demands to portable energy storage devices. General Li metal/alloy anodes are confronted with challenges of dendritic crystal formation and slow charge/discharge rate. Recently, the prosperity of two-dimensional materials opens a new window for the design of battery anode. In the present study, MoS2/graphene heterostructure is investigate for the anode application of Li ion battery using first-principles calculations. The Li binding energy, open-circuit voltage, and electronic band structures are acquired for various Li concentrations. We found the open-circuit voltage decreases from ~2.28 to ~0.4 V for concentration from 0 to 1. Density of states show the electrical conductivity of the intercalated heterostructures can be significantly enhanced. The charge density differences are used to explain the variations of voltage and density of states. Last, ~0.43 eV diffusion energy barrier of Li implies the possible fast charge/discharge rate. Our study indicate MoS2/graphene heterostructure is promising material as Li ion battery anode.

The shape of dendritic tips: a test of theory with computations and experiments.

This article is devoted to the study of the tip shape of dendritic crystals grown from a supercooled liquid. The recently developed theory (Alexandrov & Galenko 2020 Phil. Trans. R. Soc. A 378, 20190243. (doi:10.1098/rsta.2019.0243)), which defines the shape function of dendrites, was tested against computational simulations and experimental data. For a detailed comparison, we performed calculations using two computational methods (phase-field and enthalpy-based methods), and also made a comparison with experimental data from various research groups. As a result, it is shown that the recently found shape function describes the tip region of dendritic crystals (at the crystal vertex and some distance from it) well. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

A review on the theory of stable dendritic growth.

This review article summarizes the main outcomes following from recently developed theories of stable dendritic growth in undercooled one-component and binary melts. The nonlinear heat and mass transfer mechanisms that control the crystal growth process are connected with hydrodynamic flows (forced and natural convection), as well as with the non-local diffusion transport of dissolved impurities in the undercooled liquid phase. The main conclusions following from stability analysis, solvability and selection theories are presented. The sharp interface model and stability criteria for various crystallization conditions and crystalline symmetries met in actual practice are formulated and discussed. The review is also focused on the determination of the main process parameters-the tip velocity and diameter of dendritic crystals as functions of the melt undercooling, which define the structural states and transitions in materials science (e.g. monocrystalline-polycrystalline structures). Selection criteria of stable dendritic growth mode for conductive and convective heat and mass fluxes at the crystal surface are stitched together into a single criterion valid for an arbitrary undercooling. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Dendritic growth of ice crystals: a test of theory with experiments

Motivated by an important application of dendritic crystals in the form of an elliptical paraboloid, which widely spread in nature (ice crystals), we develop here the selection theory of their stable growth mode. This theory enables us to separately define the tip velocity of dendrites and their tip diameter as functions of the melt undercooling. This, in turn, makes it possible to judge the microstructure of the material obtained as a result of the crystallization process. So, in the first instance, the steady-state analytical solution that describes the growth of such dendrites in undercooled one-component liquids is found. Then a system of equations consisting of the selection criterion and the undercooling balance that describes a stable growth mode of elliptical dendrites is formulated and analyzed. Three parametric solutions of this system are deduced in an explicit form. Our calculations based on these solutions demonstrate that the theoretical predictions are in good agreement with experimental data for ice dendrites growing at small undercoolings in pure water.

Effect of laser heat input on the properties of cladding layer

By means of optical microscope, SEM and EDS, the microstructure of laser cladding layer under 1200W laser heat input was analyzed. Through the research, the relationship between precipitate characteristics and properties of the cladding layer was studied, and failure reason was found. The results show that Co, Cr and W elements have the tendency of short-range diffusion in the surface region in Stellite 6 alloy with a jagged shape. In this area, Co-Cr-W-Fe precipitated and hardened layer formed with high hardness and abrasive resistance but a lower ductility. Meanwhile, intergranular phase M23C6 precipitated. Some parts the big temperature gradient and overheat accelerated the precipitation process, led to the incomplete fusion of dendritic crystal, crack initiation and propagation on account of the integrated factors. Some solutions were put forward to reduce the failure risk.

Porous SiC using polycarbosilane/camphene solutions: Roles of freeze casting parameters

In order to control the pore characteristics and macroscopical performance of porous ceramics, roles of the freeze casting parameters are the key points. Herein, aligned dendritic porous SiC was fabricated by freeze casting of PCS-camphene solutions with different solid loading, freeze front velocity, temperature gradient, and freezing temperature. Influence of these parameters on the microstructure and compressive strength of porous SiC was investigated. With increasing the PCS content, freeze temperature, freeze front velocity or temperature gradient, degree of undercooling of the camphene was increased, resulting in the formation of smaller pore size, decreased porosity and increased compressive strength. Compared to variables of freeze temperature and temperature gradient, increased freeze front velocity was more efficiency in improving the compressive strength of porous SiC, owing to the formation of smaller pore size and longer secondary dendritic crystals. Promising micron-sized porous SiC with high porosity (79.93 vol%) and satisfactory strength (15.84 MPa) was achieved for 10% PCS-camphene solution under optimized freezing conditions.