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Thermophysical properties of the γ-TiAl alloy Ti-48Al-2Cr-2Ni in the liquid phase were investigated with a containerless electromagnetic processing device on board the International Space Station. Containerless processing is warranted by the high liquidus temperature T liq = 1 776 K and the high dissolution reactivity in the liquid phase. Thermophysical properties investigated include the surface tension and viscosity, density, specific heat capacity and the electrical resistivity. The experiments were supported by magnetohydrodynamic fluid flow calculations. The Ti-48Al-2Cr-2Ni alloy could be stably processed over extended times in the stable and undercooled liquid phase and exhibited an exceptional degree of undercooling before solidification. Experimental processes and thermophysical properties so obtained will be described. The experiments demonstrate the broad experimental capabilities of the electromagnetic processing facility on the International Space Station for thermophysical investigations in the liquid phase of metallic alloys not achievable by other methods.

During containerless processing, the oscillating drop method can be used to measure the surface tension and viscosity of a levitated melt. Through containerless processing, reactive melts that cannot be measured through conventional methods can be accurately measured; however, the accuracy of this method is dependent on the internal flow within the drop. While laminar flow does not redistribute the momentum of the oscillations, turbulent flow does redistribute the momentum of the flow and, as a result, dominates the damping. As a result, it is important to understand the internal flow behavior and the factors that affect the flow during these experiments. Models are used for the indirect quantification and characterization of the internal flow using the experimental parameters and material properties. In some cases, such as Cu50Zr50, the flow is laminar over the full range of the experiment. In other cases, including Al75Ni25, the sample is dominated by turbulent flow at high temperatures and applied electromagnetic fields, but upon cooling, transitions to laminar flow. Additionally, cases exist in which the flow is fully turbulent over the range of interest and valid measurements using the oscillating drop method are not possible. During the design phase of the experiment, the experimental parameters should be modeled to characterize the flow behavior and ensure a clean experiment.

Interfacial tension between molten high-temperature materials is to be measured using the electrostatic levitation furnace (ELF) and the electromagnetic levitator (ISS-EML) aboard the International Space Station. A levitated compound droplet of a concentric core–shell structure is excited either by an impulsive electromagnetic field or by a superimposed electrostatic field. The oscillation behavior of the compound droplet is analyzed to measure interfacial tension at the interface of the two phases. In support of the space experiments, a computational fluid dynamic model was developed to characterize the oscillation behavior of multiphase core–shell droplet. The developed model predicted the interfacial tension between molten copper-rich and cobalt-rich phases with a difference of 3.3 pct compared to the values reported in literature. The developed model is being utilized to investigate the influence of various test parameters on measured surface tension and also being extended for molten steel–molten slag systems.

Containerless processing of molten metals using electromagnetic levitation techniques is an important approach for material research, and the induced convection inside an electromagnetically levitated molten metal droplet is both difficult to measure experimentally and a key control parameter. This work proposed a surrogate model based on parametric numerical experiments of magnetohydrodynamic simulations of the ISS-EML facility, characterizing and quantifying the convective flow due to electromagnetic positioning power as a function of melt thermophysical properties. Feature mapping methods integrated with feature selection is used to construct an optimized formula in explicit form for the data fitting which maintains both of physical interpretability and mathematical tractability. The trained surrogate model is then applied for selected industrial important metals and alloys as examples that show the generalization ability of the model and how it can be utilized to quantify the induced convection and characterize internal flow patterns.

Pure, low-oxygen zirconium samples have been observed to nucleate a solid phase under conditions during which the sample was expected to remain liquid. This phenomenon was first seen during Spacelab Mission MSL-1R (materials science laboratory) experiments and has since also been observed in the International Space Station (ISS) electromagnetic levitation (EML) facility on a different sample. Current work has been able to replicate these anomalous solidification events under a range of conditions in the ISS MSL-EML facility. The solidification events are not well explained by classical homogeneous or heterogeneous nucleation. The current theory is that collapsing voids in the melt create a local region of high pressure that results in local material being deeply undercooled and a strong driving force for solidification.

Recrystallization and phase transformation reactions are driven by energy stored in the parent phase due to previous processing history. Grain boundaries, lattice strain, and dislocation networks may affect subsequent microstructural evolution, especially during metal powder consolidation processes. In this work, AlCrFe2Ni2 eutectic high-entropy alloy powders over a wide range of size distribution were used to investigate the correlation between dislocation density and microstructure as a function of particle size. Line profile analysis of the X-ray diffraction patterns of the as-received (quenched) samples shows that the dislocation density increases linearly with decreasing particle size. Based on microstructure analysis of the as-quenched and the annealed samples, it is found that the correlation is associated with the grain boundary length which increases with decreasing particle size, revealing that the key source of dislocation density is dislocations within the grain boundaries. The grain boundary energy acts as a driving force for the metastable to stable phase transformation of AlCrFe2Ni2, showing that the phase transformation kinetics is a function of particle size. This work shows a direct experimental observation and quantitative analysis that in metallic powder systems particle size is one of the key parameters which affects the dislocation density and the phase transformation kinetics most probably due to the different cooling rates achieved during powder production.

Electromagnetic levitation techniques are used in a microgravity environment to allow materials research under containerless conditions while limiting the influence of gravity. The induced advective flow inside a levitated molten alloy droplet is a key factor affecting solidification phenomena while potentially influencing the measurement of thermophysical properties of metallic alloy. It is thus important to predict the flow velocity under various operation conditions during melt processing. In this work, a magnetohydrodynamic model is applied over the range of conditions under which electromagnetically levitated droplets are processed to represent the maximum flow velocity and shear rate as a polynomial function of heating voltage, density, viscosity, and electrical conductivity of molten materials. An example is given for the ternary steel alloy Fe-19Cr-21Ni (at%) to demonstrate how internal advection under different heater settings becomes a strong function of alloy temperature and is a determining factor in the transition from laminar to turbulent flow conditions. The results are directly applicable to a range of other materials with properties in the range considered, including Ni-based superalloys, Ti-6Al-4V, and many other commercially-important alloys.

Recent experiments using electromagnetic levitation in reduced gravity have confirmed prior observations of anomalous nucleation of the solid in undercooled melts under specific conditions. All indications are that this effect is dynamical, not chemical: the same sample undercools over 300 °C before and after the anomalous event, but maintains the liquid state for only a few seconds when held at a more modest undercooling in the range of 0–50 °C. The new experimental results and related modeling will be examined in comparison to the hypothesis that the solidification is triggered by cavitation in the melt. This platform may provide data relevant to a better quantitative understanding of the effect of ultrasonic processing on grain refinement of terrestrial castings.

The density of a series of five different iron-cobalt alloys was measured using electrostatic levitator processing. Each sample was processed through multiple thermal cycles and the liquid density was measured, while the superheated sample was cooled down to its undercooled state. The volume of the sample was estimated by analyzing captured high-speed video data of the projected shape of the sample. The mass change during the melt cycle was also tracked using Langmuir’s equation of mass evaporation. The density was then calculated as a function of temperature based on these measurements of volume and mass. Density values obtained showed higher precision than existing data from the literature obtained using a variety of different techniques, although the accuracy was consistent.

A major reason to conduct solidification experiments in space is that the unique conditions accessible in reduced-gravity allow investigation of fundamental questions while limiting the influence of sedimentation or buoyancy-induced convection. When processing metallic alloys using containerless electromagnetic levitation, convection may be controlled over a wide range, spanning the laminar-turbulent transition, by proper selection of facility operating conditions. By measuring key thermophysical properties such as density, viscosity, and electrical resistivity on-orbit, the specific sample being processed may be characterized and the results used to update pre-mission magnetohydrodynamic model predictions of induced stirring within the droplet. Thus, convection becomes a controlled experimental parameter that can be applied to an investigation of how stirring influences the metastable-to-stable transformation during rapid solidification of FeCrNi alloys. For these alloys, the incubation or delay time is observed to be a weak function of undercooling and a strong function of applied convection.

Zirconium samples with different oxygen concentrations were tested using a ground-based electrostatic levitator at NASA Marshall Space Flight Center. The surface tension of liquid zirconium samples was measured in both undercooled and superheated conditions. The effect of oxygen on surface tension was determined: oxygen in zirconium samples decreases the surface tension of liquid samples, with only a small change in the temperature dependence.

Surface tension is an important property for processing of molten metals. Therefore, it is important to measure the surface tension accurately. Conventional contact methods for surface tension measurements compromise on accuracy due to impurities and oxidation at the molten metal and atmosphere interface. A contactless technique is being utilized for a more accurate measurement of surface tension of high temperature molten metals. A molten zirconium sample is electrostatically levitated using an electrostatic levitator at NASA Marshall Space Flight Center (MSFC) and oscillated at its natural frequency. The surface tension of the sample is related to the frequency of oscillation by Rayleigh’s formula. The oscillation of a molten metal droplet was simulated using a CFD package Fluent. Simulated oscillation was analyzed using Matlab code and fast Fourier transform was performed to extract oscillation frequency. Results from the simulations were in good agreement with the experimental data.

From December 2014, a series of space experiments will be performed to investigate the influence of the convection on the multiphase solidification phenomena of metallic alloys. For the success of the mission, it is of critical importance to predict the convection in molten samples under given test parameters. In this research, the convection induced in the molten Fe50Co50 alloy was predicted numerically. The magnetohydrodynamic model for the ground-based electromagnetic levitator developed in the previous research was extended to the space application. The same modeling strategies were applied to the electromagnetic levitator in space. Using the numerical model, the convection under various test conditions was predicted: The flow pattern was characterized as a function of the heating current. The maximum convection velocity at various temperatures was estimated with the increasing heating current. Finally, the range of accessible convection velocity was predicted as a function of the critical undercooling of the sample, the minimum positioner control voltage, and the undercooling of the sample. The results are expected to provide critical information for the design of the space experiments and the interpretation of the results.

This study investigates the bulk radiative properties of absorbing and scattering fibers. This type of problem can be applied to a group of geometrically similar applications including thermal radiators, insulation, and tube-and-shell heat exchangers. The specific application studied here is an ordered array of carbon fibers acting as a radiating fin for a space-based heat rejection system. High total emissivity is beneficial for this application, so this study focuses on how geometric factors affect the effective emissivity of an array of fibers. Photon scattering among fibers in an array can result in an effective emissivity greater than the emissivity of the fiber surfaces themselves.

Dynamic models are used to investigate the behavior of liquid droplets suspended in alternating current and direct current magnetic fields in microgravity and in various configurations providing conditions similar to microgravity. The realistic magnetic fields of solenoidal coils are used for the modeling experiments with electrically conducting (liquid silicon or metal) droplets. At high values of magnetic field, some oscillation modes are damped quickly, while others are modified with a considerable shift of the oscillating droplet frequencies and the damping constants from the nonmagnetic case. On a larger scale, the models are used to investigate the melting and heating process of reactive materials. It is demonstrated how 1 kg of liquid titanium in a traditional “cold” crucible-type furnace can be fully levitated without contact to wall to achieve high superheat of the melt.

The intimate links among structure, nucleation, and thermophysical properties are demonstrated from a few select experiments on high-melting-temperature metallic alloy liquids, made possible by containerless processing techniques. To complement the ground-based studies, future experiments on the International Space Station using a new electromagnetic levitation facility developed by the European Space Agency are discussed. The knowledge obtained from these studies is expected to deepen the understanding of supercooled liquid properties, elucidate their role in nucleation and glass formation, and test and develop more advanced theories of nucleation. All of these are expected to improve liquid phase processing techniques for crystalline and amorphous materials so that better functional properties may be achieved.

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (K p), heights (K h) and full-widths at half maximum (K FWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ∼0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr1− x Cu x (0.1 ≤ x ≤ 0.9), K p, K h and K FWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu–Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu–Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominate...

A need for higher service temperatures is driving development of new, higher temperature materials that are resistant to creep. However, conventional methods of measuring creep become increasingly difficult over about 1700 °C. A non-contact method with the capability of measurements at much higher temperature has been demonstrated on niobium using centrifugal loading of a spherical sample. Recent efforts have been made to extend this method to lower temperatures and higher stresses. Using material properties from the literature, we performed finite element simulations to determine the range of experimental parameters over which non-contact measurements of creep can be readily finished within a reasonable time duration for ZrB 2 and ZrB 2 + 25 vol.% SiC. Results from finite element analysis (FEA) model shows that the experiments are feasible at an angular velocity of 32,000 rps and a temperature 1900 °C for ZrB 2 + 25 vol.% SiC, but not for the pure ZrB 2 under the range of experimental conditions available.

Surface tension is an essential thermophysical property of liquids, and the oscillating droplet method is particularly effective for investigations involving reactive molten alloys. The Rayleigh equation is commonly used to evaluate surface tension from measurements of the damping frequency response of an oscillating droplet with small deformation, but non-linear effects are expected to arise for larger deformation. This work describes an improved method for interpreting frequency analysis and validates previous numerical simulation and theoretical analyses which predict a decrease in observed frequency at moderate deformation amplitude. Experimental results from microgravity tests are used to determine a correction of the Rayleigh equation to eliminate the influence of finite deformation.

Continuing pressures for higher performance and efficiency in energy conversion and propulsion systems are driving ever more demanding needs for new materials which can survive high stresses at the elevated temperatures. In such severe environments, the characterization of creep properties becomes indispensable. Conventional techniques for the measurement of creep are limited to about 1700 °C. A new method which can be applied at temperatures higher than 2000 °C is strongly demanded. This paper presents a non-contact method for creep measurements of ultra-high-temperature materials at 2300 °C. Using the electrostatic levitation facility at NASA MSFC, a spherical sample was rotated quickly enough to cause creep deformation due to centripetal acceleration. The deformation of the sample was captured with a digital camera, and the images were analyzed to measure creep deformation. To validate experimental results, numerical and analytical analyses on creep deformation of a rotating sphere have been conducted. The experimental, numerical, and analytical results showed good agreement with one another.

Glasses of Li2O.2SiO2 (LS2), and LS2 doped with 0.001wt% platinum (LS2 + 0.001 wt% Pt) compositions were melted, cooled and reheated at controlled rates while levitated (containerless) inside an Electrostatic Levitator (ESL) furnace at the NASA Marshall Space Flight Center, Huntsville, AL. The experiments were conducted in vacuum (approximately 10(exp -5) Pa) using spherical, 2.5 to 3 millimeter diameter, glass samples. The measured critical cooling rate for glass formation, R(sub c), for the LS2 and LS2+0.001 wt% Pt glasses processed at ESL were 14 plus or minus 2 Celsius per minute and 130 plus or minus 5 Celsius per minute, respectively. The values of R(sub c), for the same LS2 and LS2 + 0.001 wt% Pt glasses processed in a container were 62 plus or minus 3 Celsius per minute and 162 plus or minus 5 Celsius per minute, respectively. The effective activation energy for crystallization, E, for this LS2 glass processed without a container at ESL, 392 plus or minus 15 kiloJoules per mole, was higher than that, 270 plus or minus 15 kiloJoules per mole, for an identical glass processed in a container. These results suggest that the glass formation tendency for a containerless LS2 melt is significantly increased compared to an identical melt in contact with a container. The absence of heterogeneous nucleation sites that are inherently present in all melts held in containers, and or a change in the surface composition due to evaporation of Li2O during processing at ESL are likely reasons for the increased glass forming tendency of this containerless LS2 melt.

High-temperature measurement and study of reactive materials can be difficult with conventional processing methods because contamination from the measuring apparatus and container walls can adversely affect measurements. Containerless processing techniques can be employed to isolate samples from their environment, reducing contamination. Benefits ofcontainerless processing include reduction in heterogeneous nucleation sites, which in turn delays the onset of solidification and allows the study of meta-stable undercooled phases. However, property measurements must use noncontact methods as well. Fortunately, several optical-based methods have been developed and successfully employed to measure thermophysical properties, including surface tension, viscosity, density, and thermal expansion. Combining these techniques with the electrostatic levitator (ESL) located at the NASA Marshall Space Flight Center (MSFC) has resulted in an excellent facility to perform containerless material studies which support microgravity flight projects. Currently, studies of the thermophysical properties of liquid quasi-crystal forming and related alloys ranging from superheated to deeply undercooled states are being done with this facility in support of the NASA-funded flight project Quasi-crystalline Undercooled Alloys for Space Investigation (QUASI). While the primary purpose of these measurements is to support planned flight experiments, they are also a desirable resource for future manufacturing considerations and for fundamental insight in the physics of icosahedral ordering in liquids and solids. Presented here is an overview of the contactless measuring methods for surface tension, viscosity, density, and thermal expansion applied to Zr 54 Ti 8 Cu 20 Al 10 Ni 8 , for the superheated and meta-stable undercooled liquid phases, in support of QUASI.

A new hypothesis has been developed to explain the effect of internal fluid flow on the lifetime of a metastable phase in solidifying Fe-Cr-Ni alloys. The hypothesis shows excellent agreement with available experimental results, but microgravity experiments are required for complete validation. Certain Fe-Cr-Ni stainless steel alloys solidify from an undercooled melt by a two-step process in which the metastable ferrite phase forms first followed by the stable austenite phase. Recent experiments using containerless processing techniques have shown that the lifetime of the metastable phase is strongly influenced by flow within the molten sample. Simulations using a commercial computational fluid dynamics (CFD) package, FIDAP, were performed to determine the time required for collision of dendrites and compared to experimental delay time. If the convective velocities are strong enough to bend the primary arms, then the secondary arms of adjacent dendrites can touch. The points of collision form low-angle boundaries and result in high-energy sites that can serve as nuclei for the transformation to the stable phase. It has been determined that the convective velocities in electrostatic levitation (ESL) are not strong enough to cause collision. However, in ground-based electromagnetic levitation (EML), the convective velocities are strong enough to cause the dendrites to deflect so that the secondary arms of adjacent dendrites collide. There is quantitative agreement between the numerically determined time to collision and the experimentally observed delay time in EML. The strong internal velocity due to convection within the EML samples is the reason for the observed difference in delay times between ESL and EML. Microgravity testing is essential because the significant change in nucleation behavior occurs between the ranges accessible by ground-based ESL and EML. Testing in microgravity using EML will permit a large range of internal convective velocities including those that are inaccessible in 1 g.

In two-step solidification of alloys that form a metastable phase, the first stage of microstructural evolution following solidification of the primary (metastable) phase involves growth of the stable phase into the mushy-zone. The growth rate of the stable phase can be predicted using a simple dendrite growth model if the heat balance is modified to include isothermal melting of the pre-existing solid. This adiabatic remelting model successfully predicts the growth rates for the stable phase as measured experimentally in the Fe–Cr–Ni alloy system. The growth rate of the stable phase depends strongly on composition, but this dependence is countered by a reduction in the heat absorbed through melting of the metastable solid. Due to the small variation in thermophysical properties over a wide range of compositions, the net result of these two competing effects is a heat flux, which is proportional to the thermodynamic driving force.

Differences have been observed between the lifetimes of the metastable phases of undercooled samples of Fe–12 wt% Cr–16 wt% Ni alloy which had been subjected to electromagnetic levitation (EML) and electrostatic levitation (ESL). Internal flow is induced within the samples by positioning forces in EML and much weaker Marangoni forces in ESL. The hypothesis being tested is that the flow within EML samples is strong enough to cause the growing metastable dendrites to deflect so that the secondary arms of adjacent dendrites collide, resulting in early nucleation of the stable phase. Simulations using a commercial computational fluid dynamics software package were performed to determine the time required for collision of the secondary arms to occur. There is quantitative agreement between the numerical time to collision and the experimental lifetime of the metastable phase. It has been determined that the induced convective flow in EML samples is strong enough to cause collision and is the most likely cause o...

Using electrostatic-levitator (ESL)-based containerless processing combined with non-contact measuring techniques, surface tension and viscosity have been measured for Ti39.5Zr39.5Ni21, using the o...

The natural oscillation frequency of freely suspended liquid droplets can be related to the surface tension of the material, and the decay of oscillations to the liquid viscosity. However, the fluid flow inside the droplet must be laminar to measure viscosity with existing correlations; otherwise the damping of the oscillations is dominated by turbulent dissipation. Because no experimental method has yet been developed to visualize flow in electromagnetically levitated oscillating metal droplets, mathematical modeling can assist in predicting whether or not turbulence occurs, and under what processing conditions. In this paper, three mathematical models of the flow: (1) assuming laminar conditions, (2) using the k−ɛ turbulence model, and (3) using the RNG turbulence model, respectively, are compared and contrasted to determine the physical characteristics of the flow. It is concluded that the RNG model is the best suited for describing this problem when the interior flow is turbulent. The goal of the presented work was to characterize internal flow in an oscillating droplet of liquid metal, and to verify the accuracy of the characterization by comparing calculated surface tension and viscosity values to available experimental results.

We describe the spinning behavior of a suspended molten droplet subjected to electromagnetic heating. Our observations are derived from video images of droplets of palladium-silicon alloy in experiments on the MSL-1 (First Microgravity Science Laboratory) mission of the Space Shuttle (STS-83 and STS-94, April and July 1997). We inferred the resultant magnetohydrodynamic (MHD) flow inside the drop from motion of impurities on the surface. Digital particle tracking of the impurities is used to quantify the axial rotation of the levitated droplet. The analysis suggests that the levitated drop attains a constant rotational speed during the melting phase and formation of the co-rotating axisymmetric laminar toroidal structures. With continued electromagnetic heating, the sample's viscosity drops and the MHD flow accelerates, giving rise to instabilities of the internal flow. The rate of axial rotation increases significantly during this flow transition. The new data suggests a surprising interaction between the flow inside the levitated molten drop and the driving coils in the experiments. We explore the mechanisms that may be responsible for this spinning behavior.

Levitation techniques have been applied to a staggering range of materials, from liquid helium to aqueous solutions of proteins, to metals, ceramics, glasses and semiconductors. These experiments have encompassed temperatures from cryogenic to greater than 2500 °C, and samples from micrograms to tens of kilograms. It should come as no surprise that a wide variety of levitation principles have been employed for processing these samples, including electromagnetic (EML), electrostatic (ESL), aerodynamic and gas film, acoustic and dia- or paramagnetic levitation, as well as combinations of these and others. All of these containerless techniques share one key feature: internal flow in liquid samples. The flow may be driven directly by the positioning force, as in EML and aerodynamic levitation, or by temperature gradients through Marangoni convection and/or natural buoyancy, as in ESL. It is possible to reduce the positioning-driven and buoyancy flows by performing the experiments in microgravity; however, often even the reduced levels are important. For some experiments, such as viscosity measurements, only whether the flow is laminar or turbulent must be established. For other experiments, however, quantitative assessments of velocity, shear stress or shear strain rate are required. In most cases, it is difficult or impossible to measure the internal flow in levitated droplets directly. The samples are usually small, and often opaque, reactive, high-temperature, metastable, or all of the above. Furthermore, recirculating flow limits the utility of tracking surface particles, since they tend to collect in stagnation points rather than following the flow. Most research groups have chosen mathematical modelling to assess the internal flow in levitated droplets. Several different classes of experiments are examined in terms of the effect of fluid flow and the impact of flow modelling. This paper focuses on EML and ESL, although the techniques and many of the results are applicable to other levitation methods.

Different containerless processing techniques have different strengths and weaknesses. Applying more than one technique allows various parts of a problem to be solved separately. For two research projects, one on phase selection in steels and the other on nucleation and growth of quasicrystals, a combination of experiments using electrostatic levitation (ESL) and electromagnetic levitation (EML) is appropriate. In both experiments, convection is an important variable. The convective conditions achievable with each method are compared for two very different materials: a low-viscosity, high-temperature stainless steel, and a high-viscosity, low-temperature quasicrystal-forming alloy. It is clear that the techniques are complementary when convection is a parameter to be explored in the experiments. For a number of reasons, including the sample size, temperature, and reactivity, direct measurement of the convective velocity is not feasible. Therefore, we must rely on computation techniques to estimate convection in these experiments. These models are an essential part of almost any microgravity investigation. The methods employed and results obtained for the projects levitation observation of dendrite evolution in steel ternary alloy rapid solidification (LODESTARS) and quasicrystalline undercooled alloys for space investigation (QUASI) are explained.

The delay between conversion of metastable ferrite to stable austenite during ternary Fe-Cr-Ni alloy double recalescence is seen to differ by over an order of magnitude for tests conducted using electrostatic and electromagnetic levitation. Several possible reasons for this deviation are proposed. Thermodynamic calculations on evaporation rates indicate that potential composition shifts during testing are minimized by limiting test time and thermal history. Simulation indicates that deviation would be limited to a factor of 1.5 under worst-case conditions. Possible effects due to differences in sample size are also eliminated since the metastable array, where stable phase nucleation must occur, is significantly smaller than the sample. Differences in internal convection are seen to be the most probable reason for the observed deviation.

Recent X-ray scattering measurements show that icosahedral short-range order in Ti–Zr–Ni alloys is responsible for a change in phase selection from the stable C14 Laves phase to the quasicrystalline icosahedral phase, and that icosahedral short-range order increases at deeper undercoolings. This change in short-range order should be reflected in changes in the thermophysical properties of the melt. The surface tension and viscosity of quasicrystal-forming Ti–Zr–Ni alloys were measured over a range of temperature, including both stable and undercooled liquids, by an electrostatic levitation (ESL) technique. ESL is a containerless technique which allows processing of samples without contact, greatly reducing contamination and increasing access to the metastable undercooled liquid. The measured viscosity is typical of glass-forming alloys of similar composition to the quasicrystal-forming alloys studied here; however, the surface tension shows an anomaly at deep undercoolings.

During experiments on the MSL-1 (first microgravity science laboratory) mission of the space shuttle (STS-83 and STS-94, April and July 1997), a droplet of palladium-silicon alloy was electromagnetically levitated for viscosity measurements. For the nondeforming droplet, the resultant magnetohydrodynamic (MHD) flow inside the drop can be inferred from motion of impurity particulates on the surface. In the experiments, subsequent to melting, Joule heating produces a continuous reduction of viscosity of the fluid resulting in an acceleration of the flow with time. These observations indicate formation of a pair of co-rotating toroidal flow structures inside the spheroidal drop that undergo flow instabilities. As the fluid temperature rises, the amplitude of the secondary flow increases, and beyond a point, the tracers exhibit noncoherent chaotic motion signifying emergence of turbulence inside the drop. Assuming that the observed laminar-turbulent transition is shear-layer type, the internal structure of the toroidal loops is used to develop a semiempirical correlation for the onset of turbulence. Our calculations indicate that the suggested correlation is in modest agreement with the experimental data, with the transition occurring at a Reynolds number of 600.

The first undercooling measurements on electrostatic-levitated droplets of TiZrNi alloys that form the icosahedral quasicrystal phase are presented. The reduced undercooling for crystallization decreases with an increasing polytetrahedral order of the primary solidifying phase, suggesting the development of icosahedral short-range order in the undercooled liquid. X-ray diffraction measurements made at the advanced photon source on liquid droplets of these alloys, aerodynamically levitated and heated to near their liquidus temperature, however, show only weak evidence for increased icosahedral order. This suggests that significant ordering occurs below the melting temperature.

A magnetohydrodynamic model of internal convection of a molten Co-Cu droplet processed by the ground-based electromagnetic levitation (EML) was developed. For the calculation of the electromagnetic field generated by the copper coils, the simplified Maxwell’s equations were solved. The calculated Lorentz force per volume was used as a momentum source in the Navier–Stokes equations, which were solved by using a commercial computational fluid dynamics package. The RNG k-e model was adopted for the prediction of turbulent flow. For the validation of the developed model, a Co16Cu84 sample was tested using the EML facility in the German Aerospace Center, Cologne, Germany. The sample was subjected to a full melt cycle, during which the surface of the sample was captured by a high-speed camera. With a sufficient undercooling, the liquid phase separation occurred and the Co-rich liquid phase particles could be observed as they were floating on the surface along streamlines. The convection velocity was estimated by the combination of the displacement of the Co-rich particles and the temporal resolution of the high-speed camera. Both the numerical and experimental results showed an excellent agreement in the convection velocity on the surface.

Electromagnetic levitation is one area of the electromagnetic processing of materials that has uses for both fundamental research and practical applications. This technique was successfully used on the Space Shuttle Columbia during the Spacelab IML-2 mission in July 1994 as a platform for accurately measuring the surface tensions of liquid metals and alloys. In this article, we discuss the key transport phenomena associated with electromagnetic levitation, the fundamental relationships associated with thermophysical property measurement that can be made using this technique, reasons for working in microgravity, and some of the results obtained from the microgravity experiments.

The criticisms of Harvey and Gheribi (HG) are directed towards supporting evidence for ordering in supercooled Cu${}_{46}$Zr${}_{54}$ liquid from specific heat measurements and molecular dynamics simulations, not on the direct evidence that came from x-ray diffraction studies. In this reply, we demonstrate that the unique features observed in the specific heat [Cp(T)] are not artifacts of any specific assumptions, as suggested by HG. We have furnished additional details of the MD simulations and clarified related issues raised by HG. The basic conclusions, however, remain unchanged.

External triggering of nucleation in an undercooled liquid metal has been performed for the first time in an electrostatically levitated sample of undercooled eutectic Ni60Nb40. With the Electrostatic Levitator (ESL) facility at NASA/MSFC, NiNb alloys were externally nucleated at various undercooling increments. Observation of the resulting microstructure indicates that only one nucleation site is initiated with a trigger if the undercooling is sufficiently high. Well-defined transition zones exist in all the samples. An "anomalous" fine structured eutectic is initially grown from the nucleation point into the undercooled liquid followed by an unexpected variable width transition zone leading to a coarse structured region of dendrites. Our study tends to corroborate the findings of other studies on highly undercooled eutectics regarding a coupled growth zone within which the lamellar structure is formed and outside of which the anomalous is found.

Evidence for rapid ordering in a supercooled Cu${}_{46}$Zr${}_{54}$ liquid, obtained from high-energy x-ray diffraction in a containerless processing environment, is presented. Relatively sudden changes were observed in the topological and chemical short-range order near 850 \ifmmode^\circ\else\textdegree\fi{}C, a temperature that is 75 \ifmmode^\circ\else\textdegree\fi{}C below the liquidus temperature and 465 \ifmmode^\circ\else\textdegree\fi{}C above the glass transition temperature. A peak in the specific heat was observed with supercooling, with an onset near 850 \ifmmode^\circ\else\textdegree\fi{}C (the same temperature as the onset of ordering) and a maximum near 700 \ifmmode^\circ\else\textdegree\fi{}C, consistent with the prediction of a molecular-dynamics calculation using embedded atom potentials. The chemical and topological ordering measured here are in agreement with predictions of a rapid development of chemically ordered icosahedral clusters in the supercooled liquid.

Utilizing the containerless electrostatic levitation facility at NASA/MSFC, we were able to undercool the Ni59Nb41 (atomic) alloy by 210 K which was 160 degrees farther than the results of previous flight experiments. Undercoolings were clustered around 200 K during the repeated melting-freezing cycles on a single sample. Prior to this work, a metastable liquid separation had been presumed to limit the undercooling of this alloy. However, microstructural observations have revealed that undercooling was limited by crystal nucleation.