Dendritic Growth Patterns Research Articles

Novel Fe-rich hypo-eutectic high entropy alloy developed for laser powder bed fusion in the Al-Co-Cr-Fe-Ni-Zr system

A novel heat resistant Fe-rich hypo-eutectic high entropy alloy (HEA) was manufactured using laser powder bed fusion (LPBF). The prealloyed Al1/4Co2Cr2Fe6Ni3/2Zr powder was characterized and processed using different preheating temperatures (650°C, 700°C and 800°C). The as-built microstructure showed dissimilar morphologies throughout the melt pool, with eutectic grains of ultra fine lamella spacing (λ≈25 nm) inside the core regions, and coarser dendritic and globulitic growth patterns at the interlayer boundaries (ILBs). An annealing treatment at 1000°C for 6 h yielded a homogenous composite microstructure of a Fe-rich matrix with ∼40 % of fine dispersed Zr-rich intermetallic (IM) phases of submicron size. Synchrotron high energy x-ray diffraction (HEXRD) revealed M2Zr Laves (M = Co, Fe, Ni) as the main IM for the as-built condition and the additional growth of M23Zr6 upon annealing. Further HEXRD in-situ high temperature tests examined the lattice parameter evolution and coefficient of thermal expansion (CTE) up to 1100°C. The mechanical performance of the as-built and annealed condition was evaluated through hardness and compression testing at room temperature (RT). Additional compression testing from RT to 1000°C were conducted on the annealed condition to assess the high temperature strength and give comparison to other high temperature materials like Ni-based superalloys.

A comparative investigation on the surface and physicochemical properties of organobimetallic thiocyanate complexes of Cadmium, Zinc, Mercury and Manganese

Abstract The single crystals of bimetallic thiocyanate ligands, namely manganese cadmium thiocyanate (MCTC), zinc cadmium thiocyanate (ZCTC), manganese mercury thiocyanate dimethylsulphoxide (MMTD), and cadmium mercury thiocyanate dimethylsulphoxide (CMTD), are cultivated through the utilization of slow solvent evaporation and gradual cooling methodologies. Through the utilization of optical microscopic techniques such as field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and epifluorescence, a state-of-the-art methodology extensively employed in the realms of biochemical, medical, and chemical research, we delve into the examination of growth mechanisms and surface topographies. It is additionally employed in LED, fluorescent, and various other luminous sources. The FESEM analysis of MCTC elucidates the manifestation of an extended dendritic growth pattern, which arises from the oscillation of the Mn and Cd metal ligands when connected by thiocyanate (SCN) bridges. The presence of three notable mounds exhibiting cavities within the multi-component thin film coating (MCTC) has been duly ascertained through the utilization of atomic force microscopy (AFM) imagery. The analysis of the histogram unveiled that the average diameter exhibited an augmentation concomitant with the alteration in the breadth of the distribution throughout the process of growth.

Development and Numerical Testing of a Model of Equiaxed Alloy Solidification Using a Phase Field Formulation

A computational framework is developed to understand the transient behavior of isothermal and non-isothermal transformation between liquid and solid phases in a binary alloy using a phase-field method. The non-isothermal condition was achieved by applying a thermal gradient along the computational domain. The bulk solid and liquid phases were treated as regular solutions, along with introducing an order parameter (phase field) as a function of space and time to describe the interfacial region between the two phases. An antitrapping flux term was integrated into the present phase-field model to mitigate the amount of solute trapping, which is characterized by the non-equilibrium partitioning of the solute. The governing equations for the phase field and the solute composition were solved by the cell-centered finite volume method using the open-source computational tool OpenFOAM. Simulations were carried out for the evolution of equiaxed dendrites inside an undercooled melt of a binary alloy, considering the effect of various computational parameters such as interface thickness, strength of crystal anisotropy, stochastic noise amplitude, and initial orientation. The simulated results show that the solidification morphology is sensitive to the magnitude of anisotropy as well as the amplitude of noise. A strong influence of interface thickness on the growth morphology and solute redistribution during solidification was observed. Incorporating antitrapping flux resulted in the solute partitioning close to the equilibrium value. Simulations show that the grain shape is unaffected by changes to crystallographic orientation with respect to the Cartesian computational grid. Thermal gradients exerted discernible effects on the solute distribution and the dendritic growth pattern. Starting with multiple nucleation events the model predicted realistic polycrystalline solidification and as-solidified microstructure.

Dendritic Growth Patterns in Rocks: Inverting the Driving and Triggering Mechanisms

AbstractMineral precipitation can form complex patterns under non‐equilibrium conditions, in which two representative patterns are rhythmic Liesegang stripes and fractal dendrites. Interestingly, both patterns occur in the same rock formations, including various dendritic morphologies found in different rocks, such as limestone and sandstone. However, the underlying mechanism for selecting the vastly different mineral precipitation patterns remains unclear. We use a phase‐field model to reveal the mechanisms driving pattern selection in mineral precipitation. Simulations allow us to explore the effects of diffusion parameters on determining the dendritic morphologies. We also propose a general criterion to distinguish the resulting dendrites in simulations and field observations based on a qualitative visual distinction into three categories and a quantitative fractal dimension (FD) phase diagram. Using this model, we reproduce the classified dendrites in the field and invert for the key parameters that reflect the intrinsic material properties and geological environments. This study provides a quantitative tool for identifying the morphology selection mechanism with potential applications to geological field studies, exploration for resource evaluation, and other potential industrial applications.

Melt flow-induced mechanical deformation of dendrites in alloy solidification: A coupled thermal fluid - solid mechanics approach

Melt flow during alloy solidification promotes more dendrite fragments or equiaxed grains due to dendrite fragmentation, while the complex interactions between melt flow and dendritic growth causing flow-induced dendrite deformation and dendrite fragmentations remain obscure. In this work, cellular automaton-finite volume approach and the displacement-based finite element method have been combined to simulate dendrite growth, fluid flow and flow-induced mechanical deformation in Al-4.5 wt%Cu alloy, and to reveal the stress evolution during dendritic growth under melt convection. It is found that dendrites can undergo visible mechanical bending under fluid flow. The stress increases with the enhancement of fluid flow. The primary dendritic trunk is the location of the dominant deformation under a parallel fluid flow. Though the secondary dendrite does not suffer the large stress, it significantly affects the stress concentration of the primary dendritic trunk. As flow velocity increases, the secondary dendrite arm spacing gradually decreases and bridging occurs due to the contact of tertiary dendrites. The bridging of secondary dendrites impedes the development of stress concentrations. Especially, as inflow velocity exceeds 0.05 m/s, the stress does not get larger than that under the velocity of 0.01 m/s under the complication interactions between the dendritic growth and flow patterns. Unlike previous understanding, the maximum stresses induced by melt flow are mainly located at the position where primary dendritic trunk intersects with unconnected secondary dendrites or the thin dendrite trunk.

Unveiling the influence of dendrite characteristics on the slip/twinning activity and the strain hardening capacity of Mg-Sn-Li-Zn cast alloys

This work explores the correlation between the characteristics of the cast structure (dendrite growth pattern, dendrite morphology and macro-texture) and strain hardening capacity during high temperature deformation of Mg-5Sn-0.3Li-0 and 3Zn multi-component alloys. The three dimensional (3D) morphology of the dendrite structure demonstrates the transition of the growth directions from<11 2 ¯ 3>,<11 2 ¯ 0> and <11 2 ¯ 2> to< 11 2 ¯ 3> and <11 2 ¯ 0> due to the addition of Zn. The simultaneous effects of growing tendency and the decrement of dendrite coarsening rate at the solidification interval lead to dendrite morphology transition from the globular-like to the hyper-branch structure. This morphology transition results in the variation of the solidification macro-texture, which has effectively influenced the dominant deformation mechanisms (slip/twin activity). The higher activity of the slip systems increases the tendency of the dendrite arms for bending along the deformation direction and fragmentation. Apart from this, the dendrite holding hyper-branch structure with an average thickness below 20 µm are more favorable for fragmentation. The dendrite fragmentation leads to considerable softening fractions, and as an effective strain compensation mechanism increases the workability of dendritic structure.

Dynamics of Dendritic Ice Freezing in Confinement.

We use high-speed photography to observe the dendritic freezing of ice between two closely spaced parallel plates. Measuring the propagation speeds of dendrites, we investigate whether there is a confinement-induced thermal influence upon the speed beyond that provided by a single surface. Plates of thermally insulating plastic and moderately thermally conductive glass are used alone and in combination, at temperatures between −10.6 and −4.8 °C, with separations between 17 and 135 μm wide. No effect of confinement was detected for propagation on glass surfaces, but a possible slowing of propagation speed was seen between insulating plates. The pattern of dendritic growth was also studied, with a change from curving to straight dendrites being strongly associated with a switch from a glass to a plastic substrate.

Tuning charge transport in organic semiconductors with nanoparticles and hexamethyldisilazane

In this study, we showcase the use of silicon dioxide (SiO2) nanoparticles and hexamethyldisilazane (HMDS) treatment to regulate the thin film morphology and manipulate charge transport of solution-processed, small-molecular, organic semiconductors. 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was blended with SiO2 nanoparticles as an exemplary organic semiconductor. A higher concentration of SiO2 nanoparticles were observed near the edge of TIPS pentacene crystals and enlarged the width of crystal edges. This greatly reduced the dendritic crystal growth pattern of TIPS pentacene and contributed to its enhanced orientation uniformity. In addition, the HMDS treatment effectively passivated the silanol groups on the hydrophilic gate dielectrics. As a result, bottom-gate, top-contact TIPS pentacene/SiO2 nanoparticle-based organic thin-film transistors boosted a performance consistency enhancement (defined by the ratio of average mobility to the standard deviation of mobility). The combinational approach to employ SiO2 nanoparticles and HMDS treatment as demonstrated in this work sheds light on important applications in high-performance organic electronics devices on flexible substrate.

Throughput study of diffusion along the twin boundaries in Mg-5Sn-0.3Li as-cast alloy and its effect on the homogenization during hot deformation

This study offers the effect of dendrite orientation selection on the defamation mechanisms and their subsequent effects on the homogenization of the dendritic structure during hot deformation processes. The results indicated that the solidification macro-texture has controlled by the dendrite growth pattern, which has effectively influenced the dominant deformation mechanisms (slip/twin activity). Interestingly, the boundary of the exhausted twins has been preferred for solute diffusion and has effectively contributed to the homogenization of the dendritic structure during hot deformation processes.

Effect of Ru on macro-/micro-structure evolution within platform of Ni-based superalloy single crystal blades

The effect of Ru-addition on the macro-/micro-structure evolution within platform of Ni-based superalloy single crystal turbine blades was investigated in this study. CM247 LC was used as the baseline alloy, and two modified alloy contained 3 wt% and 5 wt% Ru, respectively, were prepared at the expense of Ni in the baseline alloy. The results showed that the dendrite patterns within platform regions were non-uniform under the blade cluster condition. Different dendrite growth patterns were observed within platform overhangs on heater side and shadow side for these alloys. When the shadow side was free from stray grains, the secondary dendrites laterally spread into the overhang region and blocked by higher order dendrite branches, due to a high local undercooling. On the heater side, secondary dendrite arms could extend into the overhang region and reached extremities with few disruptions of higher order dendrite. With the Ru-addition increasing, stray grain occurrence increased in the platform regions of the blades, particularly on the shadow side. The Ru-addition also led to changes in solidification characteristics. In the solidified samples containing 3 wt% Ru, cracks along stray grain boundary were found, which may be caused by deficient of intergranular solid bridges and liquid feeding.

Determination of Secondary Dendrite Arm Spacing for In-738LC Gas-Tungsten-Arc-Welds

Microstructure and defect development in the gas tungsten arc weld process is influenced by the solidification and melt-pool dynamics. Melt-pool geometrical parameters which depend mainly on heat input have profound influence on the dendrite growth velocity and growth pattern in the melt pool. Temperature magnitude and history during the process directly determine the molten pool dimensions and surface integrity. However, due to the transient nature and small size of the molten pool, the temperature gradient and the molten pool size are very challenging to measure and control. The proposed research aims to establish a methodology for characterizing direct energy deposited metals by linking processing variables to the resulting microstructure and subsequent material properties. Secondary Dendrite Arm Spacing (SDAS) optical metallographic measurements of equiaxed solidified IN-738LC gas tungsten arc welds were conducted to find a new expression that links the cooling rate that is imposed on the welding during solidification, and the resultant scale of the grain substructure.

Simulation of three-dimensional dendritic growth using H-adaptive finite element method

Based on the AFEPack adaptive software package, the H-adaptive finite element method was used to solve the three-dimensional phase field model on the Linux platform. The growth of the three-dimensional dendrites in the undercooling melt was simulated numerically, and the effects of different undercooling on three-dimensional dendritic growth were studied. The real growth process of three-dimensional dendritic was reproduced. The dendritic growth pattern that we obtained was consistent with the crystallization theory. In order to verify the reliability of the simulation results, the three-dimensional dendrite tip growth rate and the tip radius were compared with the classical theory and the same conditions under the literature value. The results are compared with those obtained by Jeong et al. and obtained by finite difference method under the same conditions. It is found that the results obtained by H-adaptive finite element method are in agreement with the calculated results obtained by these two methods. It is proved that the H-adaptive finite element method is feasible to simulate the growth of three-dimensional dendrites. The method greatly reduces the computational cost of the phase field model solution and improves the efficiency of the solution.

Study on the microstructural evolution of different component alloys consisting of B2-NiSc intermetallics

Ni-50%Sc and Ni-51%Sc alloy were prepared with a vacuum arc smelting and water cooled copper mold suction-casting machine. The results showed that the two component alloys consisted of the primary phase B2-NiSc and lamellar (Ni2Sc+NiSc)eutectic due to the loss of Sc during melting. Two groups of alloys underwent (970 ℃, 72 h) homogenization heat treatment, and spherical or plate shape Ni2Sc particles were dispersed on the B2-NiSc matrix. With the increase of Sc content from 50% to 51%, the amount of the second phase in the alloy decreases, the microstructure becomes uniform, and the grain gradually changes from long bar to a spherical particle. According to the Jackson boundary theory, the Jackson factor α of B2-NiSc =0.5 &amp;lt; 2, so the interface is rough, which explains that the growth pattern of the B2-NiSc phase is a non-faceted growth. It is consistent with the dendritic growth pattern of the B2-NiSc phase, which is observed from the experiment. After a long heat treatment, the number of vacancies decreases and the microstructure became uniform. The loss rate of Sc in rapidly quenched solidification was higher than that after the heat treatment.

Formation of low-angle grain boundaries under different solidification conditions in the rejoined platforms of Ni-based single crystal superalloys

Abstract

A Phase-Field Lattice-Boltzmann Study on Dendritic Growth of Al-Cu Alloy Under Convection

Effects of convection (forced and natural) on dendritic evolution of the Al-Cu alloy were investigated using a phase-field lattice-Boltzmann approach. The non-linear coupled equations were solved by applying a parallel and adaptive mesh refinement algorithm. Important physical aspects including dendritic fragmentation, splitting, and formation of solute plumes were simulated. Results showed that the dendritic growth patterns under convection exhibited remarkable difference from those without convection. The presence of flow led to variation of solute diffusion and upstream–downstream dendritic growth difference, which further influenced the development of dendritic arms and multi-dendritic competitive growth. When the convection intensity was magnified, the convection-induced anisotropy became dominated, and the growth patterns changed accordingly to accommodate the local thermodynamic variation.

Cellular automaton modeling for dendritic growth during laser beam welding solidification process

A modified non-equilibrium cellular automaton (CA) model is proposed to simulate the dendrite growth along the fusion boundary under transient conditions during the solidification process of laser beam welding (LBW) pool. The main non-equilibrium solidification effects, including solute trapping, deviation of the liquidus line slope, and kinetic undercooling, are taken into consideration in this modified CA model. The evolution of dendrite morphology, concentration field, and velocity field was investigated. Four growth periods, i.e., the initial stable period, the instability period, the competitive growth period, and the short-term stable period, were observed during the solidification process of LBW pool. The dendrite growth patterns were different in these periods due to the transient solidification conditions. The solute distribution predicted by this modified CA model showed similar trends to that predicted by the previous phase field model. The dendrite tip velocity was significantly influenced by the thermal undercooling and constitutional undercooling in the LBW pool, and the tip velocity field can be divided into three stages, i.e., the planar growth stage, competitive stage, and short-term steady growth stage. In addition, further comparison between the simulated dendrite morphology and experimental dendrite morphology was performed. Results showed that the simulated morphology of dendrites agreed well with the experimental results. In particular, the primary dendrite arm spacing predicted by this model was in good agreement with that obtained from experiments.

Nanocrystalline TON-type zeolites synthesized under static conditions

Pure-phase zeolites of the TON-type were synthesized under static hydrothermal conditions, which previously have, typically, required stirring during synthesis. Heterocyclic structure directing agents (SDAs) were observed to play important roles in both the selection of the polymorphs as well as in affecting the morphology of the particles formed. When an imidazole-based SDA was used snowflake-shaped particles formed, which indicated a dendritic growth pattern of the zeolite. These zeolite particles possessed intercrystalline mesopores. To the best of our knowledge, it is for the first time that snowflake-shaped particles have been observed for TON-type zeolites. Other synthesis parameters were optimized to obtain crystals with short c-axes. The c-axis shortened with an increased solid concentration used during synthesis. This shortening was attributed to both the degree of supersaturation, and a change of the crystal growth mechanism. Short c-axes could increase the concentration of pore mouths in TON-type zeolites. Altogether, synthesis of nanocrystalline zeolites of the TON type under static condition could, potentially, be advantageous to large-scale production.

Three-dimensional α-Mg dendritic morphology and branching structure transition in Mg-Zn alloys

Three-dimensional dendritic morphology and branching structure of hexagonal close-packed α-Mg in magnesium-zinc alloys possessing different zinc concentrations were studied using synchrotron X-ray microtomography. Interestingly, dendrite growth patterns were observed to undergo a morphologic transition with increasing zinc addition. Dendrite arms in the basal plane of hexagonal close-packed α-Mg tend to split and deviate from the basal plane continuously and symmetrically as a function of zinc concentrations. A hyperbranched dendrite structure was also found with an interim zinc concentration. Several types of dendrite growth patterns with different morphologies and branching structure were proposed in order to reflect the evolution of α-Mg dendrites with various zinc contents.

Computational study of electro-convection effects on dendrite growth in batteries

Dendrite formation on the anode surface of a battery is closely related to the safety and capacity of high energy density batteries, thus suppressing dendrite growth will significantly improve the performance of batteries. Many experimental reports reveal that convection near the dendrite nucleation site can change the local mass transport, and ultimately affect dendrite growth. Investigation of the convection effect in batteries will guide the development of strategies to suppress dendrite growth in a convective electrolyte. Most of the existing electro-convection computational models for dendrite growth studies are based on Eulerian frameworks. These methods have difficulty modeling the moving boundaries associated with dendrite growth and are less computationally efficient in simulating convective fluid motion. In this paper we adopt a mesh-free particle based Lagrangian method to address the challenges of previous grid based Eulerian electro-convection models. The developed model is verified by comparison to analytical solutions, including verification of ion migration and the electric potential. Simulation results show that the predicted dendrite growth and electro-convective flow patterns compare well with experimental results during early dendrite growth stages. Parametric studies reveal that low viscosity electrolytes suppress the dendrite growth by increasing the mass transport of ions near the anode/electrolyte interface.

In situ observation of solidification patterns in diffusive conditions

We present a review of recent in situ experimentation studies of solidification front patterns and microstructures in alloys. Front-tracking diagnostics and real-time observation methods using high-resolution optical or X-ray imaging devices currently apply to model transparent systems as well as metallic alloys in thin and bulk samples. On a theoretical basis that spans the physics of nonequilibrium pattern formation and materials science, in combination with time-resolved numerical simulations, conclusive results of both fundamental- and applied-science interest have been obtained on major problems relative to multiscale microstructure selection, morphological transitions, and crystallographic effects during single- and multi-phase solidification. We will mainly focus on the dynamics of cellular, dendritic, and eutectic growth patterns in diffusive-growth conditions, that is, in the absence of convection in the liquid. This can be achieved in (semi-)thin samples, or, for bulk solidification, in the reduced-gravity environment of orbiting facilities. A selection of emerging work on, e.g., faceted growth and adaptive control of solidification patterns will furthermore be reported. We conclude by pointing out open questions and new perspectives for future research.