Solid-liquid Interface Morphology Research Articles

Role of dielectric force and solid extraction in electrohydrodynamic flow assisted melting

A numerical study of non-isothermal solid-liquid phase change process in a differentially heated square cavity aided with an electrohydrodynamic (EHD) flow is reported. Melting of a phase change material (PCM) subjected to an electric field with different charge injection strengths is studied. The study aims to numerically demonstrate the solid extraction phenomenon and its role in accelerating the melting process. The EHD flow modifies the flow structure and notably alters the solid–liquid interface morphology. The dielectric force extracts the semi-solid PCM from the melt interface into the liquid bulk with high electric field intensity. The dielectric force causes melting-rate enhancement during the initial stages of melting. While, the later stages of melting are influenced by the combined action of Coulomb and dielectric forces. The role of the Coulomb force is weaker in the weaker charge-injection regimes. Higher electric potential and stronger charge injection generally lead to increased melting rates. Up to 62.12 % decrease in total time taken for melting is achieved within the parameters considered herein.

Solidification furnace for in situ observation of bulk transparent systems and image analysis methods.

This paper aims to describe the experimental framework of the Directional Solidification Insert, installed onboard the International Space Station, dedicated to the in situ and real-time characterization of the dynamic selection of the solid-liquid interface morphology in bulk samples of transparent materials under diffusive growth conditions. The in situ observation of the solid-liquid interface is an invaluable tool for gaining knowledge on the time evolution of the interface pattern because the initial morphological instability evolves nonlinearly and undergoes a reorganization process. The result of each experiment, characterized by the sample concentration, a thermal gradient, and a pulling rate, is a large number of images. The interpretation of these images necessitates a robust identification of each cell/dendrite's position and size during the entire solidification. Several image analysis methods have been developed to reliably achieve this goal despite varying contrast and noise levels and are described in detail. Typical solidification experiments are presented, and the dynamics of the pattern formation are analyzed to illustrate the application of the image analysis methods.

A solid–liquid interface enhancement algorithm for X-ray in situ observation of space materials

China Manned Space Station has planned a high-temperature material science experimental rack designed to study the mechanism of the material preparation process under microgravity conditions. The X-ray real-time observation module observes the solid–liquid interface morphology and interface transport effect in situ during sample preparation, an important component of high temperature rack. Aiming at the sparsity of X-ray images of space high temperature materials, a solid–liquid interface enhancement algorithm for X-ray in situ observation of space materials is proposed to solve low contrast and blurry interface of X-ray images. Compared with the existing methods, the algorithm enhances the contrast and provides a new idea for solving the material interface dynamics from the perspective of image. The enhanced image can accurately calculate the moving speed of the solid–liquid interface of the materials, and the interface dynamics of material growth can be effectively studied by combining the temperature measured in the experiment. This research is of great significance for studying growth mechanisms of materials and guiding the preparation of foundation materials.

Effect of segregation and advection governed heterogeneous distribution of nanoparticles on NEPCM discharging behavior

A numerical study is performed to investigate the role of heterogeneous distribution of nanoparticles in discharging behavior of nanoparticle-enhanced phase change materials (NEPCM). The numerical model considers solidification, multiphase convection, nanoparticles segregation, sedimentation, drags on nanoparticles and Brownian and thermophoresis diffusion phenomena. The discharging behavior of NEPCM is analyzed for the solidification of n-octadecane (PCM)-Cu-nanoparticles in a rectangular cavity. The discharging stage of NEPCM is also being investigated experimentally. The experiments were equipped with Particle Image Velocimetry (PIV), high-resolution imaging and thermocouples to measure real-time flow field, solidified layer thickness and local temperature. The numerically predicted results are compared with these experimental results to validate the model. Using the numerical model, the transient evolution of solidification interface morphology, nanoparticles concentration, fluid flow, temperature field, and thermophysical properties during solidification of NEPCM is described. Subsequently, the effect of process parameters such as initial nanoparticles concentration and Stefan number on system's thermal performance is delineated. It is noticed that adding nanoparticles up to a certain limit increases the discharging rate. The discharging time for 0 wt%, 2.5 wt%, 4.5 wt% and 6.5 wt% Cu-nanoparticles concentration are found to be 1115, 895, 980 and 1075 min, respectively. Thus, NEPCM with 2.5 wt% Cu-nanoparticles discharges 19.7 % faster than pure PCM. Furthermore, it is found that the heterogeneous distribution of the rejected nanoparticles significantly affects the solid-liquid interface morphology, solidification rate and alters the thermophysical properties locally as well as globally. The effective density and thermal conductivity increase with an increase in the local concentration of nanoparticles, whereas the effective specific heat capacity decreases with the increase in the local concentration of nanoparticles.

Microstructure evolution and compressive property variation of AlxCoCrFeNi high entropy alloys produced by directional solidification

The AlxCoCrFeNi (molar radio, x = 0.3, 0.6, 0.9 and 1.2) high entropy alloys (HEAs) were directionally solidified at the withdrawal rate of 150 μm/s. The effects of Al content on microstructure evolution, element segregation as well as compressive property were investigated. It was found that the increase of Al content resulted in the formation of BCC crystal structure. Meanwhile, the microstructure of AlxCoCrFeNi HEAs transformed from dendrite to equiaxed dendrite. The Al0.3CoCrFeNi HEA formed a single FCC solid solution phase, the Al0.6CoCrFeNi HEA consisted of CoFeCr-enriched FCC solid solution phase + NiAl-enriched BCC solid solution phase, the Al0.9CoCrFeNi and Al1.2CoCrFeNi HEAs were composed of NiAl-enriched B2 solid solution phase + CoFeCr-enriched BCC solid solution phase. Morphology of solid-liquid interface was coarse dendrite or typical dendrite. As the Al content increased, the compressive strain of directionally solidified AlxCoCrFeNi HEAs decreased from 21.26% to 18.05%, and the ultimate compressive strength increased first and then decreased. The improvement of strength results from crystal structure transition and solution strengthening. As the Al contents increased from 0.6 to 0.9, the crystal structure of AlxCoCrFeNi HEAs transformed from CoFeCr-enriched FCC structure + NiAl-enriched BCC structure to CoFeCr-enriched BCCstructure + NiAl-enriched B2 structure. The strain of Al0.3CoCrFeNi HEA was higher than others, and Al0.9CoCrFeNi HEA exhibited the maximum ultimate compressive strength of 2114.57 MPa.

Numerical Simulation of Transport Phenomena in Directional Solidification Castings with Changeable Cross-Section and Solidification Interface Control

The roles of traveling magnetic fields (TMFs) within the transport phenomena during the directional solidification of nickel-based superalloys were simulated. The evolution of thermal field, flow field and solid-liquid interface morphology during the solidification process under both natural and forced convection conditions were also simulated and compared. The strength of TMFs window that suppresses the flow of the interfacial front in the melt was quantified. The association between flow velocity at the interface front and defect formation was discussed.

Effect of Vertical High Magnetic Field on the Morphology of Solid-Liquid Interface during the Directional Solidification of Zn-2wt.%Bi Immiscible Alloy

The morphology of the solid-liquid (S-L) interface is crucial for the directionally solidified microstructures of various alloys. This paper investigates the effect of vertical high magnetic field (VHMF) on the morphology evolution of the S-L interface and the solidified microstructure during the directional solidification of Zn-2wt.%Bi immiscible alloy. The results indicate that the morphology of the S-L interface is highly dependent on the VHMF, resulting in various solidified microstructures. When the growth rate was 1 μm/s, the aligned droplets were formed directly at the disturbed S-L interface under a 1 T VHMF. However, the stability of the S-L interface was improved to form a stable Bi-rich fiber under a 5 T VHMF. When the growth rate was 5 μm/s, the S-L interface was changed from cellular to dendritic to cellular again with increasing magnetic flux density. A theory regarding constitutional supercooling and efficient solute diffusion has been proposed to explain the S-L interface transition under the VHMF. The difference in the effective diffusion capacity of the solute originates from the thermoelectric magnetic effect and the magneto-hydrodynamic damping effect. The present work may initiate a new method to transform the solidified microstructures of immiscible alloys via an applied magnetic field during directional solidification.

Enrichment characteristic of carbon atoms in solid-liquid zone of high carbon steel under different directional solidification rates

To reveal the formation mechanism and main influencing factors of C-segregation in high carbon steel under different solidification rates (40, 80, 160, 200 and 320 µm·s−1), the enrichment characteristics of carbon atoms in the solid-liquid zone of Fe-0.61%C steel were studied using a zone melting liquid metal cooling apparatus and electron probe microanalysis. The relationships among micro-segregation of carbon atoms, solid-liquid interface morphology and solidification rate were fully discussed. The results show that large dendrite spacing and a slow-moving dendritic interface create less resistance and more time for the migration of interdendritic carbon atoms to liquid zone. This results in the continuous enrichment of carbon atoms in liquid zone, further expands the solid-liquid temperature range, prolongs the solidification time of molten steel, and causes the formation of carbon micro-segregation at the solidification end as the solidification rate is 40 µm·s−1. Conversely, abundant and elongated secondary dendrite arms with small spacing seriously impede the diffusion of interdendritic carbon-rich molten steel to liquid zone. As a result, there is only obvious dendrite segregation, but little difference in the carbon content along the solidification direction as solidification rate exceeds 200 µm·s−1.

Machine learning approach to automated analysis of atomic configuration of molecular dynamics simulation

Three-dimensional convolutional neural network (3D-CNN) is employed for automated analysis of atomic configuration of molecular dynamics (MD) simulation. Solid and liquid atoms in the solid-liquid biphasic system of various elements at high temperature are identified by a 3D-CNN architecture. Accuracy of 3D-CNN successfully achieves more than 90% independent of crystal structure, whereas accuracy of common neighbor analysis (CNA) is approximately 50% at most for the same system. 3D-CNN can extract the morphology of solid-liquid interface very clearly including roughness at atomistic scale. Moreover, 3D-CNN trained by the data set of a certain element (iron) can be applied for another element (tungsten) of same crystal structure without further training. It is significant in this study to shed light on a high potential of machine learning (ML)-based approach for automated analysis of atomistic configuration since it is not straightforward to develop an identifier of atomic configuration manually when we face a new problem out of existing methodologies.

Microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy

The microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy were investigated using OM, SEM, EDS, TEM and tensile tests. The microstructure of as-cast alloy, including the well-aligned lamellar structures, the radial emanating lamellar structure and mesh-like structure, consists of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. From the longitudinal section of directionally solidified alloy, when the withdrawal rate is 6 μm/s, the solid-liquid interface morphology is planar, and the well-aligned lamellar structure is observed. The three-dimensional morphologies of well-aligned lamellar structures are well characterized by deep etching. With increasing the withdrawal rate to 15 μm/s, the solid-liquid interface morphology change from planar to cellular, and the microstructure changes from planar eutectic to cellular eutectic, even the dendritic eutectic at local region occurs at 60 and 120 μm/s. From the transverse section, the well-aligned lamellar structures occur in the cell interior, and the interlamellar spacing decreases gradually with increasing the withdrawal rates. The TEM result indicates that NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase of [1¯11] NiAl-rich//[1¯22]CoCrFeNi-rich and (211)NiAl-rich//(022¯)CoCrFeNi-rich. The nano-precipitates occur in NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. The directionally solidified alloy possesses a good combination of strength and ductility at 60 μm/s. In particular, using the directional solidification technology is a good way to improve the ductility of the alloy (∼37%). The strengthening and plasticizing mechanisms are discussed by analyzing the fracture and side surfaces.

Sensitivity of Numerical Predictions to the Permeability Coefficient in Simulations of Melting and Solidification Using the Enthalpy-Porosity Method

The high degree of uncertainty and conflicting literature data on the value of the permeability coefficient (also known as the mushy zone constant), which aims to dampen fluid velocities in the mushy zone and suppress them in solid regions, is a critical drawback when using the fixed-grid enthalpy-porosity technique for modelling non-isothermal phase-change processes. In the present study, the sensitivity of numerical predictions to the value of this coefficient was scrutinised. Using finite-volume based numerical simulations of isothermal and non-isothermal melting and solidification problems, the causes of increased sensitivity were identified. It was found that depending on the mushy-zone thickness and the velocity field, the solid–liquid interface morphology and the rate of phase-change are sensitive to the permeability coefficient. It is demonstrated that numerical predictions of an isothermal phase-change problem are independent of the permeability coefficient for sufficiently fine meshes. It is also shown that sensitivity to the choice of permeability coefficient can be assessed by means of an appropriately defined Péclet number.

Effect of crystallizer′s three-dimension on the solid-liquid interface morphology of the large-scale Ti64 during EBCHM

Solid-liquid interface morphology of the titanium ingot has great influence on the microstructure during the solidification process. This paper focused on the effect of Crystallizer’s 3D dimension of the large-scale Ti64 (Ti–6Al–4V) slab ingot on the solid-liquid interface morphology. The results show that when the cross-section length of Ti64 slab ingot exceeds 450 mm, the cooling capacity of the crystallizer does not increase. When the inner length exceeds the effective distance, the depth of molten pool and the width of mushy zone will not change with the increase of the inner length in the crystallizer. Moreover, increasing the ratio of the crystallizer’s inner length to inner width is helpful to improve the production quality. The simulation results show that it is better to choose between 4:1 and 6:1. When the height of the crystallizer is higher than 300 mm, the depth of the molten pool and the width of mushy zone will not change. Therefore, the certain range theoretical basis was obtained to design the crystallizer’s three-dimension during EBCHM for the large-scale Ti64 slab ingot.

Transition of solid-liquid interface and tensile properties of CoCrFeNi high-entropy alloys during directional solidification

In order to understand effect of solidification velocity on microstructure evolution and mechanical properties of high entropy alloys (HEAs), CoCrFeNi HEA samples were directionally solidified at different solidification velocities (5–50 μm/s). The results show that all samples were directionally solidified, but the solid-liquid interface and columnar grains are different. Microstructure of them is only composed of FCC solid solution phase. Morphology of solid-liquid interface transforms from cellular, to cellular-dendritic, and then to dendritic with increasing of solidification rate. The criterion of solidification rate for cellular/dendrite transition is calculated as about 26.1 μm/s based on experimental results. The width of columnar grains decreases from 200 μm to 100 μm with solidification rate increasing from 5 μm/s to 50μm/s. Meanwhile, secondary dendrite spacing decreases from 200 μm to 30 μm for dendritic solidified samples (v = 10–50 μm/s). Tensile testing show that the yield strength improves from 233 MPa to 383 MPa, and the ultimate tensile strength improves from 431 MPa to 596 MPa with increasing of solidification rate. Meanwhile, the elongation of all samples is more than 34.3%, but rupture morphology analysis shows that the rupture changes from quasi-cleavage fracture to ductility fracture. The improvement of tensile properties is resulted from grain boundary strengthening of the columnar grain and second dendrite.

Mathematical Modeling of Solid-Liquid Interface Morphology in The Process of Polycrystalline Silicon Directional Solidification

Mathematical Modeling of Solid-Liquid Interface Morphology in The Process of Polycrystalline Silicon Directional Solidification

An Electromagnetic Compounding Technique for Counteracting the Thermoelectric Magnetic Effect During Directional Solidification Under a Transverse Static Magnetic Field

An external direct current (DC) was introduced to counteract the thermoelectric magnetic effect during the directional solidification of Sn-10 wt pct Bi alloy under a 0.5 T transverse static magnetic field. The results show that the single-side Bi segregation induced by the thermoelectric magnetic effect gradually becomes weakened, disappeared, and then reversed with the increasing current density of the external DC. Therefore, a new method for counteracting the thermoelectric magnetic effect during materials processing under the magnetic field was proposed. In order to verify this method, the absolute thermoelectric power of Sn-10 wt pct Bi alloy and the internal thermoelectric current under the experimental conditions were measured and calculated, respectively. A 2D numerical simulation was established to simulate the evolution of the solid–liquid interface morphology, flow field, and composition segregation with the increasing current density of the external DC during directional solidification under a transverse static magnetic field. The present study not only facilitates the understanding of the effects of the forced flows on the directionally solidified microstructures and composition segregation, but also provides a new way for eliminating segregation and obtaining the higher-quality solidification structure by using the electromagnetic compounding technique.

The Diffusion and Solid-Liquid Phase Transformation in Directional Solidification of Alloy: A Quantitative Phase Field Characterization and Real-Time Observation

Directional solidification is a paradigm process to gain the desired microstructure via certain applied solidification parameters. A thorough understanding of the diffusion-limited solid-liquid interface morphology evolution from initial transient to steady state is of uppermost importance to optimize the solidification processes. The rapid development of quantitative phase-field model provides a feasible computational tool to explore the underlying physics of the morphological transition at different stages. On basis of the diffusion-limited quantitative phase-field simulations using adaptive finite element method, the directional solidification of Al-4wt.%Cu alloy is characterized and both the solid interface propagation speed and solute profile are analyzed. The simulations are then compared with the in situ and real-time observation by means of synchrotron radiation x-ray radiography image. Good agreements are obtained between simulations and experimental data. Detailed mechanism that controls the morphological instability and transition are then addressed.

Macrosegregation of Impurities in a Metallurgical Silicon Ingot After Transient Directional Solidification

Metallurgical grade silicon was melted and directionally solidified in transient conditions by extracting heat from the base of a cylindrical ingot and inserting heat at the ingot top. A heat-transfer mathematical model was implemented to predict the solidification velocity and temperature gradient using cooling curves measured directly in the silicon melt. Nearly 70% of the resulting ingot displays a region of columnar grains aligned with the ingot axis. In this region, the concentration of metallic impurities is usually below the quantification limit of the analytical technique and intermetallic particles are absent, strongly indicating significant purification. The transition from the purified region of the ingot to the ingot top, where impurity concentrations increase and intermetallic particles are seen, is consistent with a change of the solid-liquid interface morphology from planar to cellular/dendritic, as similarly reported in the literature and as indicated by a preliminary analysis with the constitutional undercooling criterion.

Solid-liquid interface morphology of white carbide eutectic during directional solidification

AbstractIn this paper the analysis of solid-liquid interface morphology in white carbide eutectic was made. In a vacuum Bridgman-type furnace, under an argon atmosphere, directionally solidified sample of Fe - C alloy was produced. The pulling rate was v = 125 μm/s (450 mm/h) and constant temperature gradient G = 33.5 K/mm. The microstructure of the sample was frozen. The microstructure of the sample was examined on the longitudinal section using an light microscope and scanning electron microscope.

Microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified novel NiAl-Cr(Fe) alloy

The microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified NiAl-Cr(Fe) alloy were investigated using OM, SEM, EDS, DSC and three-point bending tests. From the as-cast microstructure and DSC result, NiAl-34Cr-4Fe (at.%) is a eutectic alloy which consists of eutectic cells in different sizes. The half-baked mesh-like structure is observed at the cell center, and the radial emanating thicker or longer Cr(Fe) phases embedded within NiAl matrix are observed near or at the cell boundary. In the directional solidification process, the solid-liquid interface morphology has an evolutionary process of planar to cellular, even dendritic interface with increasing the withdrawal rates, and the eutectic cell and the microstructure at the cell center refines gradually. From the transverse microstructure, the characteristic of eutectic cell is similar to that of eutectic cell in as-cast alloy. It can be seen from the longitudinal colony/cell center that the broken (short) Cr(Fe) rods are observed at 6 μm/s, and they evolve to granular Cr(Fe) phases when the withdrawal rate increases further. Moreover, regardless of vacuum induction melting (as-cast) and directional solidification, NiAl-34Cr-4Fe (at.%) eutectic alloy possesses a poor fracture toughness due to the inferior brittleness of both NiAl and Cr(Fe) phases. Meanwhile, the crack propagation and fracture surface are observed to better understand the fracture behavior.

Effect of Solid–Liquid Interface Morphology on Grain Boundary Segregation during Colloidal Polycrystallization

The effect of the solid–liquid interface morphology on grain boundary (GB) segregation during colloidal polycrystallization was investigated by in situ optical observations.The time evolution of the impurity distribution in the liquid in the vicinity of solid–liquid interface was directly observed during crystal growth. Impurities were distributed homogeneously along the direction perpendicular to the growth orientation during the initial stage of the growth and were then gathered in the groove that formed at the solid–liquid interface as crystallization proceeded. GBs were exposed to the liquid at the bottom of the groove, and a high impurity concentration at the GB (CGB) was obtained when the area of the groove was large. The area of the groove changes depending on the crystallographic orientation of neighboring grains across GBs, which results in a difference in the interfacial energies.