Electromagnetic Levitation Technique Research Articles

Microstructural Study of Containerless Solidification of Al–20wt%Ce Alloy

Containerless solidification of Al–20wt%Ce is investigated experimentally using the electromagnetic levitation (EML) and impulse atomization (IA) techniques. In the processed EML samples, small primary undercooling and minimal eutectic undercooling are shown. The microstructure consists of large primary Al11Ce3 dendrites surrounded by an α‐Al–Al11Ce3 eutectic. Phase fractions determined by neutron diffraction are similar to the values obtained from a Scheil–Gulliver solidification simulation. Larger IA powders (between 425 and 1000 μm) show a microstructure qualitatively similar to that of EML samples, implying that they follow a similar solidification path. Quantitatively, microstructural features are finer due to the higher cooling rates involved in the solidification process. Atomized particles with a size lower than 425 μm show a strikingly different microstructure, with a very fine eutectic giving way to large intermetallic plates surround by a regular eutectic. Microhardness testing of the structures shows a significant increase in hardness as the sample size decreases, going from 45.8 ± 3.6 HV for the EML sample (lowest cooling rate) to 142.2 ± 12.0 HV for particles in the 106–150 μm range (highest cooling rate).

Liquid state properties and amorphous solidification kinetics of multicomponent Fe_{50-x}Co_{x}Cr_{14}Mo_{14}C_{9}B_{8}Tm_{5} alloys investigated under containerless processing conditions.

The liquid state thermophysical properties and amorphous solidification kinetics of Fe_{50-x}Co_{x}Cr_{14}Mo_{14}C_{9}B_{8}Tm_{5} (x=10, 15, 20, and 25) alloys were explored by electromagnetic and electrostatic levitation techniques. It was found that the surface tension of liquid alloys with Fe contents below 30 at.% had a strong temperature dependence. The high surface tension led to a sharp increase in the interfacial free-energy penalty. A high nucleation barrier was formed inside the melt, which greatly inhibited the nucleation rate. The liquid viscosity revealed the strong liquid feature of this alloy series, which became the dominant kinetic factor determining their solidification mechanisms. The high viscosity hindered atomic diffusion and delayed nucleation. The long crystallizing incubation time of Fe_{30}Co_{20}Cr_{14}Mo_{14}C_{9}B_{8}Tm_{5} ensured a strong glass-forming ability and a low critical cooling rate of 2.11×10^{3}Ks^{-1}. As a result, a bulk metallic glass rod with an 8-mm diameter was successfully prepared by a convenient casting procedure. This rod could remain in a glassy state at a higher temperature and over a wider temperature range due to its high glass-transition temperature and large undercooled liquid region. The apparent activation energy for nucleation was derived as 463.8 kJmol^{-1} according to the nonisothermal crystallization kinetics, indicating that the bulk metallic glass had to absorb a large amount of energy to overcome the potential barriers before nucleation and thus exhibited excellent thermal stability.

Surface wave phenomenon and microstructure evolution mechanism of electromagnetically levitated ternary Ti-Al-Fe alloy

Electromagnetic levitation technique was performed to investigate the surface wave phenomenon and microstructure evolution mechanism of undercooled ternary Ti50Al30Fe20 alloy. Under the condition of high undercooling, the formation process of surface waves on the levitated alloy surface were directly detected by high-speed camera. The cooling gas provided the external perturbation that promoted surface nucleation, providing the suitable site for surface wave generation. On the rotating semisolid alloy surface, the competition between centrifugal force and surface tension caused the surface wrinkling, leading to the formation of surface waves. The residual liquid phase was solidified in the vortex circulation flow, and the vortex-shape pattern was preserved during rapid solidification. With the increases of undercooling, the microstructure evolved from dendrites and interdendritic lamellar eutectics into uniform anomalous eutectics because of the independent nucleation and competitive growth of (βTi) and TiFe phase. The surface wave was formed only at high undercoolings, indicating this phenomenon was related to the fluidity in eutectic growth. In this case, the uniform microstructure was closer to satisfying the continuum medium hypothesis, and the homogeneous medium reduced the energy damage during wave propagation. In addition, the micromechanical properties were remarkably increased as undercooling rose owing to the microstructural morphology and solute trapping effect.

TEMPUS-A microgravity electromagnetic levitation facility for parabolic flights.

During the ∼22s lasting free fall phase in an aircraft flying a parabola, the aboard installed electromagnetic levitation facility "TEMPUS" is used to investigate contactless and undisturbed of gravity induced convection thermophysical properties and microstructure formations of hot and highly reactive metal or semiconductor melts. The completely contactless handling and measurement of a liquid by the levitation technique keeps the melt free of contamination and enables the extension of the accessible sample temperature range far into the undercooled liquid state below the melting point. Additionally, the state of reduced weight during parabolic flights allows us to considerably decrease the strongly disturbing electromagnetic levitation forces acting in ground-based facilities on the suspended liquids. The present paper explains in detail the basic principle and the technical realization of the TEMPUS levitation facility and its attached measurement devices. Furthermore, it presents some typical experiments performed in TEMPUS, which also show the advantages resulting from the combination of reduced weight, electromagnetic levitation, and contactless measurement techniques. The control and data recording, as well as the planning, preparation, and operation of the TEMPUS experiments within the parabolic flight campaign, are another aspect outlined in the following.

Crystalline Microstructure, Microsegregations, and Mechanical Properties of Inconel 718 Alloy Samples Processed in Electromagnetic Levitation Facility

The solidification of Inconel 718 alloy (IN718) from undercooled liquid is studied. The solidification kinetics is evaluated in melted and undercooled droplets processed using the electromagnetic levitation (EML) technique by the temperature–time profiles and solid/liquid (S/L) interface movement during recalescence. The kinetics is monitored in real time by special pyrometrical measurements and high-speed digital camera. It is shown that the growth velocity of γ-phase (the primary phase in IN718), the final crystalline microstructure (dendritic and grained), and the mechanical properties (microhardness) are strongly dependent on the initial undercooling ΔT at which the samples started to solidify with the originating γ-phase. Particularly, with the increase in undercooling, the secondary dendrite arm spacing decreases from 28 μm to 5 μm. At small and intermediate ranges of undercooling, the solidified droplets have a dendritic crystalline microstructure. At higher undercooling values reached in the experiment, ΔT>160 K (namely, for samples solidified with ΔT=170 K and ΔT=263 K), fine crystalline grains are observed instead of the dendritic structure of solidified drops. Such change in the crystalline morphology is qualitatively consistent with the behavior of crystal growth kinetics which exhibits the change from the power law to linear law at ΔT≈160 K in the velocity–undercooling relationship (measured by the advancement of the recalescence front in solidifying droplets). Study of the local mechanical properties shows that the microhardness increases with the increase in the γ″-phase within interdendritic spacing. The obtained data are the basis for testing the theoretical and computational of multicomponent alloy samples.

Metastable phase separation and crystalline orientation feature of electromagnetic levitation processed CoCrCuFeNi high entropy alloy

The metastable liquid phase separation and rapid solidification kinetics of CoCrCuFeNi high entropy alloy (HEA) has been investigated by electromagnetic levitation (EML) technique. The maximum liquid undercooling attained 452 K (0.27TL), and the critical undercooling to initiate phase separation was determined as 172 K. By modulating the degree of alloy undercooling, the overall macrosegragation pattern achieved a transition from homogeneous dendrites into dispersed structures and finally into core-shell structures. Both microstructure and electron backscatter diffraction (EBSD) characterizations revealed that liquid undercooling played a crucial role in the modulation of dendritic anisotropy correlated with crystalline orientations. Coarse high entropy face-centered cubic (HEF) dendrites were produced with 〈100〉 preferred orientation at small undercoolings. Once undercooling exceeded the threshold of 172 K, the previously uniform liquid alloy was separated into Cu-depleted and Cu-rich zones. Numerous equiaxed HEF grains of relatively random orientation together with some twin crystals were displayed in Cu-depleted zone, whereas in Cu-rich zone HEF phase solidified into many slender dendrites. Theoretical analyses indicated that equiaxed grains were produced in the regime of thermal diffusion-controlled dendritic growth with actual solidifying velocity up to 42.7 m s−1. Furthermore, physical properties examinations demonstrated that large liquid undercoolings enhanced the alloy microhardness and low temperature saturated magnetization but reduced its electrical resistivities.

Microstructure dependence of electrochemical corrosion resistance for rapidly solidified Ti50Al48Mo2 alloy

The rapid solidification of undercooled liquid Ti50Al48Mo2 alloy was achieved by the electromagnetic levitation (EML) technique. At small and medium undercoolings, primary (βTi) dendrite reacted with surrounding liquid to drive a peritectic transformation into the (αTi) phase. The solutal Mo and Al segregations were located within the dendrite center and the grain boundary during peritectic transformation, consequently B2 phase in the dendrite center and γ phase at the grain boundary formed. Once undercooling exceeded 253 K, the peritectic transformation was completely inhibited, and the formation of the B2 phase and γ phase was completely suppressed. The ultrafine eutectoid structure was formed and a complete solute trapping effect was realized. Homogeneous solute distribution facilitated the formation of thicker passivation film with lower defect density and higher film resistance on the alloy surface. Moreover, this weakened micro-galvanic effect reduced the susceptibility to pitting corrosion, and consequently the corrosion resistance of the alloy was improved.

Experimental observation of the vertical displacement between heating and levitation regions in an electromagnetic levitation coil

Scientific progress in the relevant fields of science and technology requires the production of crystals with quality beyond the current state of the art. Electro-magnetic levitation (EML) is a prospective method for the growth of high-purity crystals, allowing for avoidance of any contact between the crystal-melt and the crucible. Contactless crystal growth reduces the number of crystal defects commonly abundant in conventional crystal growth methods. The EML method also allows crystal growth of materials with very high melting points. In this article, we report detailed measurements of the EML method. The induction coil used in this study has three turns and one counterturn. We subject different metal material (Al, Cu, Sn, and Ni) samples to the induction coil’s electromagnetic field. For each sample, we measure the induced lift force, Joule heating, and components of magnetic induction as a function of position inside the coil. The results show that the maximum heating in an EML coil is emitted in the area below the levitation zone, a discrepancy not reported earlier. Our findings suggest that this shift should be considered in coil design to avoid instability of the levitated material. We hope this study will serve as a stepping stone for developing EML techniques. The experimental results we provide will be used to evaluate the accuracy of current and future theoretical models of EML coils. This, in turn, will facilitate progress in the application of EML to the growth of larger crystals of higher quality.

Density and surface tension measurements of molten Al–Si based alloys

AbstractThis study is part of a series of studies aimed at measuring the thermophysical properties of molten phase change material-type metallic thermal energy storage materials near 873 K (600°C). The target material is Al–Si based alloys. First, as a feasibility study, density measurements of the molten state of three Al–Si binary alloys (Al–12.2Si, Al–50Si and Al–90Si in atomic%) were performed. A highly accurate non-contact density measurement method based on the static magnetic field superposition electromagnetic levitation (EML) method was employed as the density measurement method. The validity of this experimental method was confirmed, and density of molten Al–Si base alloys (ADC12 and Al–5.9mass%Si–1.6mass%Fe) were measured as a function of temperature with an expanded uncertainty of 1.2%. In addition, the surface tension of the alloys was measured by the droplet oscillation method using the EML technique. The surface tension was successfully obtained as a function of temperature with expanded uncertainty of 2.3%.

Effect of Undercooling on the Microstructure and Mechanical Properties of Hyper-eutectic Ni–Sn Alloy

In this study, container-less solidification of hyper-eutectic Ni–Sn alloy has been performed by using the electromagnetic levitation technique. The effect of undercooling on the formed microstructure and on the mechanical properties have been investigated. Growth velocities were determined by high-speed video-imaging of the solidification process. A step change in the growth velocities that are increasing with increasing undercooling is observed. This aligns with an observed first change in the microstructure between low and intermediate undercoolings. At the lower undercoolings, a pro-eutectic Ni3Sn phase along with lamellar eutectic structure in the inter-dendritic region is found. At intermediate undercooling of 100–150 K, a divorced eutectic microstructure is observed whereas at undercoolings above 165 K α-Ni precipitates are observed within β-Ni3Sn dendrites. Microhardness testing revealed higher strength for the lamellar phase as compared to the non-lamellar phase. Nano-indentation has been performed to determine the hardness and strength of individual phases in the microstructure.

Complete solute trapping of rapidly growing nickel dendrites within liquid Ni72Mo28 hypoeutectic alloy

The rapid dendrite growth in highly undercooled liquid Ni72Mo28 hypoeutectic alloy was accomplished by containerless processing via electromagnetic levitation and drop tube techniques. The (Ni) dendrites achieved a high growth velocity of 26 m/s at the maximum undercooling of 226 K (0.14 TL) under levitated state. Remarkable dendritic structure refinement and Mo solubility extension were observed with the increase in undercooling. For freely falling alloy droplets, the largest undercooling was enhanced to 246 K (0.15 TL), which resulted in thorough solute trapping and almost segregationless solidification. A microstructure transition from columnar dendrite to equiaxed dendrite took place once alloy undercooling exceeded a threshold about 74–79 K. In addition, the Vickers hardness of primary (Ni) dendrite was significantly improved, which was caused by the extension of Mo solubility and microstructure refinement.

Methane pyrolysis rate measurement using electromagnetic levitation techniques for turquoise hydrogen production: Liquid In, Ga, Bi, Sn, and Cu as catalysts

Turquoise hydrogen production is a promising technology because hydrocarbons are pyrolyzed into hydrogen and carbon without the generation of carbon dioxide. A high-temperature bubble column reactor with molten catalytic metal is suitable for this technology. However, the production efficiency of a reactor depends on various factors. A research methodology to assess the catalytic performance of molten metal catalysts is presented by employing an electromagnetic levitation technique. The catalytic reaction rate was selectively measured, which is independent of the motion of the bubbles in the bubble column reactor. Therefore, the proposed methodology is suitable for selecting an optimum molten-metal catalyst based on its catalytic properties. The hydrogen production rate per unit area of the molten metal catalyst (rH2) was measured for Bi, Cu, Ga, In, and Sn. The highest reaction rate above 1100 °C was obtained for Bi, while the highest reaction rate below 1000 °C was obtained for Ga. The reaction rate was inversely proportional to the first ionization energy of the metals. This study provides quantitative data on methane pyrolysis via in the presence of molten pure-metal catalysts using a reliable experimental technique.

Microstructural study of containerless solidification of Al-5wt%Ce alloy

Containerless solidification of Al-5wt%Ce is investigated experimentally using the electromagnetic levitation technique. The processed samples show small primary undercooling and minimal eutectic undercooling. Phase fractions determined by neutron diffraction are similar to the values obtained from a Scheil-Gulliver solidification simulation. Significant solidification shrinkage at the top of the samples indicate that solidification started at the bottom and the front propagated upwards. X-ray microscopy results show multiple nucleation events throughout the samples.

Microscopic structure evolution and amorphous solidification mechanism of liquid quinary Zr<sub>57</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>5</sub> alloy

The substantial undercooling and rapid solidification of liquid quinary Zr<sub>57</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>5</sub> alloy are achieved by electromagnetic levitation (EML) technique. The amorphous solidification mechanism is revealed with molecular dynamics (MD) simulation. It is observed in EML experiment that the containerlessly solidified alloy is characterized by a core-shell structure, with mainly amorphous phase becoming the core and crystalline ZrCu, Zr<sub>2</sub>Cu and Zr<sub>8</sub>Cu<sub>5</sub> phases forming the shell. The volume fraction of amorphous core structure increases with undercooling and attains a value up to 81.3% at the maximum experimental undercooling of 300 K, which indicates that the critical undercooling required for complete amorphous solidification is 334 K. TEM analyses show that the alloy microstructure is mainly composed of Zr<sub>8</sub>Cu<sub>5</sub> phase, whereas the ZrCu phase and Zr<sub>2</sub>Cu phase are suppressed when liquid undercooling approaches this threshold. Once the critical undercooling is reached, amorphous solidification prevails over the crystallization of Zr<sub>8</sub>Cu<sub>5</sub> phase. In addition, a small quantity of amorphous phases are found in the crystalline shell and a little trace of Zr<sub>8</sub>Cu<sub>5</sub> nano-cluster is detected among the amorphous core. It is further verified by MD simulation that the formation of amorphous phase in the shell is caused by the microsegregation-induced solutal undercooling when liquid alloy attains the critical undercooling, while the nano-clusters within the core is mainly ascribed to the micro-thermal fluctuation effect inside highly undercooled liquid phase.

Observation of Pattern Formation during Electromagnetic Levitation Using High-Speed Thermography

Electromagnetic levitation (EML) was employed for studying the velocity and morphology of the solidification front as a function of undercooling of metallic materials. The limitation of the EML technique with respect to low melting alloys that emit outside the visible light spectrum was overcome by employing state-of-the-art high-speed mid-wavelength infrared cameras (MWIR cameras) with a photon detector. Due to the additional thermography contrast provided by the emission contrast of the solid and liquid phases, conductor, and semi-conductor, the pattern formation of Al-based alloys was studied in detail, revealing information on the nucleation, phase selection during solidification, and the influence of convection.

Temperature dependence of crystal growth behavior of AlN on Ni–Al using electromagnetic levitation and computer vision technique

Bulk AlN single crystal has a great demand as a substrate material for AlGaN-based optical and electronic devices such as deep ultraviolet light-emitting diodes and high-power transistors. We have previously proposed a novel solution-growth method using Ni–Al solution with an in-situ observation system for solution growth of AlN crystal using electromagnetic levitation. In this paper, to investigate AlN formation behavior on 40 mol%Al–Ni droplets, two precisely synchronized high-speed cameras from the horizontal and vertical directions were installed. AlN formation behavior was evaluated quantitatively using computer vision image processing techniques. Based on the results, we demonstrated the growth of thick AlN film at 2030 K for 1 h. A 3.8-μm-thick c-axis oriented AlN film successfully formed on the droplet, and the film was also oriented in-plane. The + c-direction AlN film grew towards the droplet center from the surface by reacting dissolved nitrogen and Al atoms.

Surface tension of liquid Fe-Ni, Fe-Cr, and Ni-Cr with physics-informed statistical modeling on the influence of oxygen content

• Accurate measurements of the surface tension of liquid Fe-Ni, Fe-Cr, and Ni-Cr using electromagnetic levitation. • A generalized parametric model was developed to predict the surface tension of binary alloys under the influence of oxygen. • The developed statistical model was successfully applied to predict the surface tension of liquid Fe-Ni-O. Surface tensions of liquid Fe-Ni, Fe-Cr, and Ni-Cr were measured by oscillating droplet method using the electromagnetic levitation technique. The measurements were performed in gas atmosphere of Ar-5% H 2 or Ar, where the resulting oxygen contents of the samples are less than 0.003 at.% and 0.03–0.10 at.%, respectively. The influence of oxygen on the surface tension is demonstrated to be significant, and the decrease in surface tension strongly depends on the oxygen contents and temperature. A Langmuir-based oxygen adsorption model for binary alloys was developed from a physics-informed statistical approach considering the distribution of oxygen allocation in different elements, which statistically predicts and physically interprets the oxygen-induced surface tension shift in binary alloy melts. This generalized parametric model was successfully applied to liquid Fe-Ni-O and showed a good agreement with the experimental data and the Butler model.

Eutectic growth kinetics and microscopic mechanical properties of rapidly solidified CoCrFeNiMo0.8 high entropy alloy

Liquid CoCrFeNiMo0.8 high entropy alloy (HEA) was undercooled by up to 205 K and thus rapidly solidified with electromagnetic levitation technique. A kinetics transition from slow lamellar eutectic growth to rapid anomalous eutectic growth was observed, which showed a significant effect on crystal orientation and mechanical properties. At small undercoolings below 89 K only (σ-CrFeMo + FCC) lamellar eutectic growth proceeded. Once the undercooling exceeded this threshold, anomalous eutectic would grow directly and lead to an extra recalescence peak. The maximum growth velocity of lamellar eutectic was measured as 0.012 m s−1, whereas that of anomalous eutectic attained 2.547 m s−1 at the largest undercooling of 205 K. It was revealed that the eutectic growth mechanism transition resulted from the independent nucleation and cooperative branched growth of the two eutectic phases. An orientation relationship (OR) of {110}σ // {111}FCC and <111>σ // <110>FCC was determined between σ-CrFeMo and FCC phases by electron backscatter diffraction (EBSD) orientation analysis. Furthermore, mechanical measurements indicated that the hardness, compression strength and ductility of this HEA showed an increasing trend with the rise of liquid undercooling, which may be attributed to structural refinement and the volume fraction increase of anomalous eutectic.

Fluid flow and vortex-shaped structure formation mechanisms ofAl-Ag-Ge alloy

<p indent="0mm">Fluid flow and solidification mechanisms of Al₇₀Ag₂₀Ge₁₀ alloy were systematically investigated using electromagnetic levitation, induction, arc, and resistance melting techniques coupled with finite element simulation method. A unique type of vortex-shaped microstructure was formed in these four experiments. This was different from the result under slow solidification conditions. The formation of the vortex-shaped structure was affected by fluid flow and cooling rate of alloy melt. The Lorentz force caused turbulent flow in alloy melt, while the turbulent flow during arc melting was driven by arc force under electromagnetic levitation or induction melting conditions. The surface tension and buoyancy force caused laminar flow in alloy melt during the resistance melting process. Due to rapid flow, the Ag₂Al phase nucleated ahead of the Al phase, primarily under near-equilibrium conditions. The metastable Ag₂Al dendrites grew as vortex-shaped morphology and preserved the melt flow pattern during solidification. The increased cooling rate caused a rapid growth of metastable Ag₂Al dendrites with increased vortex density. The curvature radius of metastable Ag₂Al dendrites decreased with enhanced melt flow; meanwhile, the more drastic nucleation and growing competition among phases occurred. Finally, irregular Al, Ag₂Al, and Ge ternary eutectics fully formed inside the vortex-shaped structure.

Molar heat capacity of liquid Ti, Al20Ti80 and Al50Ti50 measured in electromagnetic levitation

Temperature dependent isobaric molar heat capacity cP was measured in a containerless way for liquid Ti and two AlTi binary liquid alloys. The technique of electromagnetic levitation was used in combination with laser modulation calorimetry. In all cases, linear temperature dependencies were found: At the corresponding liquidus temperatures, cP equals 49.75(±2.0) J∙K-1mol-1, 57.43(±2.9) J∙K-1mol-1, and 42.60(±2.2) J∙K-1mol-1, for Ti, Al20Ti80 and Al50Ti50, respectively. The respective temperature coefficients amount to -1.67∙10-2J∙K-2mol-1, -2.73∙10-2J∙K-2mol-1, and +7.83∙10-2J∙K-2mol-1. For liquid Ti, there is a good agreement with existing literature data. The results are discussed in relation to the Neumann-Kopp rule.