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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Isothermal dendritic growth: A low gravity experiment

The Isothermal Dendritic Growth Experiment is an active crystal growth experiment designed to test dendritic growth theory at low undercoolings where convection prohibits such studies at 1 g. The experiment will be essentially autonomous, though limited in-flight interaction through a computer interface is planned. One of the key components of the apparatus will be a crystal growth chamber capable of achieving oriented single crystal dendritic growth. Recent work indicates that seeding the chamber with a crystal of the proper orientation will not, in and of itself, be sufficient to meet this requirement. Additional flight hardware and software required for the STS flight experiment are currently being developed at NASA Lewis Research Center and at Rensselaer Polytechnic Institute.

Glicksman, M. E.↗
NASA NTRS1988

Microsegregation in directionally solidified Pb-8.4 at. pct Au alloy

The dependence of microsegregation behavior on growth rate and thermal gradient has been examined in a Pb-8.4 at. pct Au alloy material partially directionally solidified and quenched. The composition of the quenched 'liquid' at the dendrite tip (Ct), that of the eutectic-like solid phase freezing from the interdendritic liquid at the base of dendrite (Cse), the volume fraction of this eutectic-like region (fe), and solute profiles in the interdendritic quenched liquid and ahead of the dendrite have been measured. Two dendritic growth models for solidification of a binary alloy melt in a positive thermal gradient at the liquid-solid interface, one for dendrites with 'minimum undercooled dendrite tip' and the other for an Ivantsov type of dendrite with 'marginally stable tip', have been examined for a quantitative comparison with measured values of Ct, Cse, and fe. Convection in the melt, possibly due to horizontal density gradients, is found to be a serious limitation for theoretical understanding of the observed experimental behavior and meaningful comparison of theories.

Tewari, S. N.↗
NASA NTRS1988

Separation processes in monotectic systems

Low-gravity work is reviewed and various factors influencing phase distribution are analyzed. The behavior of miscibility gap type materials in low gravity is studied. The nature of the attraction between the solution and the container or surface affects the volume of the minority phase available to form dispersed droplets as well as the extent of undercooling of the monotectic reaction.

Facemire, Barbara R.↗
NASA NTRS1989

Spectral emissivities and optical constants of electromagnetically levitated liquid metals as functions of temperature and wavelength

The development of a noncontact temperature measurement device utilizing rotating analyzer ellipsometry is described. The technique circumvents the necessity of spectral emissivity estimation by direct measurement concomittant with radiance brightness. Using this approach, the optical properties of electromagnetically levitated liquid metals Cu, Ag, Au, Ni, Pd, Pt, and Zr were measured in situ at four wavelengths and up to 600 K superheat in the liquid. The data suggest an increase in the emissivity of the liquid compared with the incandescent solid. The data also show moderate temperature dependence of the spectral emissivity. A few measurements of the optical properties of undercooled liquid metals were also conducted. The data for both solids and liquids show excellent agreement with available values in the literature for the spectral emissivities as well as the optical constants.

Krishnan, S.↗
NASA NTRS1990

Asymptotic theory of steady axisymmetrical needlelike crystal growth

The present paper is concerned with the stationary needle crystal growth with arbitrary undercooling. Two classes of asymptotic solutions are discussed: (1) the regular-tip solutions, and (2) the smooth-root solutions. When the surface tension is nonzero, the regular-tip solutions may not have smooth roots. Among the regular-tip solutions, however, one can identify a 'principal regular-tip solution', which has the best behavior in the far field and is physically acceptable.

Xu, Jian-Jun↗
NASA NTRS1990

Influence of convection on microstructure

In eutectic growth, as the solid phases grow they reject atoms to the liquid. This results in a variation of melt composition along the solid/liquid interface. In the past, mass transfer in eutectic solidification, in the absence of convection, was considered to be governed only by the diffusion induced by compositional gradients. However, mass transfer can also be generated by a temperature gradient. This is called thermotransport, thermomigration, thermal diffusion or the Soret effect. A theoretical model of the influence of the Soret effect on the growth of eutectic alloys is presented. A differential equation describing the compositional field near the interface during unidirectional solidification of a binary eutectic alloy was formulated by including the contributions of both compositional and thermal gradients in the liquid. A steady-state solution of the differential equation was obtained by applying appropriate boundary conditions and accounting for heat flow in the melt. Following that, the average interfacial composition was converted to a variation of undercooling at the interface, and consequently to microstructural parameters. The results obtained show that thermotransport can, under certain circumstances, be a parameter of paramount importance.

Wilcox, William R.↗
NASA NTRS1990

Containerless processing in the European microgravity programme

Reasonable acoustic levitation for undercooled melts in space was obtained, but some residual instabilities in times of high thermal transients need to be eliminated. Electrostatic levitation was developed with the acoustic levitator with similar applications in mind. The system tested used a tetrahedral electrode configuration with unchanged samples. ESA's involvement in electromagnetic levitation is concentrated on accommodation studies for the European Containerless Processing Lab for the Space Station Freedom. The gas film technique is based on the processing of samples confined by porous walls. Air flow through the walls create air cushions which inhibit wall contact. This technique is particularly promising for glasses.

Behrmann, P.↗
NASA NTRS1990

High temperature metal purification using a compact portable rf heating and levitation system on the wake shield

The Wake Shield Facility (WSF) can provide an ideal vacuum environment for the purification of high temperature metals in space. The Modular Electromagnetic Levitator (MEL), will provide the opportunity to study undercooling of metals in space and allow to determine material properties in space. The battery powered rf levitation and heating system developed for the MEL demonstrated efficiency of 36 percent. This system is being considered to purify metals at temperatures below 3000 C.

Hahs, C. A.↗
NASA NTRS1990

Cation distributions on rapidly solidified cobalt ferrite

The cation distributions in two rapidly solidified cobalt ferrites have been determined using Moessbauer spectroscopy at 4.2 K in an 8-T magnetic field. The samples were obtained by gas atomization of a Co0-Fe2O3-P2O5 melt. The degree of cation disorder in both cases was greater than is obtainable by cooling unmelted cobalt ferrite. The more rapidly cooled sample exhibited a smaller departure from the equilibrium cation distribution than did the more slowly cooled sample. This result is explained on the basis of two competing effects of rapid solidification: high cooling rate of the solid, and large undercooling.

De Guire, Mark R.↗
NASA NTRS1990

A numerical and analytical study of nonlinear bifurcations associated with the morphological stability of two-dimensional single crystals

The nonlinear stability of a two-dimensional single crystal of pure material in an undercooled melt is studied both analytically and numerically. The quasi-steady state approximation is used for the thermal fields, and the effects of different solid and liquid thermal conductivities, isotropic interfacial growth kinetics, and isotropic surface tension are included. The bifurcation analysis is performed by calculating the instantaneous value of the fundamental component of the local normal growth speed for an interface perturbed by a single Fourier shape component. Numerically, the fundamental component of the interfacial growth speed is found by Fourier analysis of the solution to an integrodifferential equation obeyed at the interface. Analytically, an expansion technique is used to derive a solvability condition defining each of these bifurcation points. The analytical and numerical results are in very close agreement. Almost all of the bifurcations are subcritical, and the results are presented by giving values of the Landau coefficient as a function of the different dimensionless parameters used in the model.

Brush, L. N.↗
NASA NTRS1991

Free energy change of off-eutectic binary alloys on solidification

A formula for the free energy difference between the undercooled liquid phase and the stable solid phase is derived for off-eutectic binary alloys in which the equilibrium solid/liquid transition takes place over a certain temperature range. The free energy change is then evaluated numerically for a Bi-25 at. pct Cd alloy modeled as a sub-subregular solution.

Ohsaka, K.↗
NASA NTRS1991

A conceptual approach for noncontact calorimetry in space

A concept is developed and described which allows to measure the heat capacity and the effective thermal conductivity of stable and undercooled liquid metals and alloys in an electromagnetic levitation apparatus. A proposal is made to use an ac pulse heating method which is used nowadays as a standard technique for precision measurement of low-temperature heat capacities. The ideal process parameters, including the drop diameter D, temperature T, and frequency of measurement omega, can be optimized when the following relations hold for the external and internal relaxation time constants tau-1 and tau-2, respectively: omega(tau-1) greater than 10 and omega(tau-2) less than 0.1. Then, heat capacity data can be obtained with an accuracy of better than 1 percent with D about 5 to 10 mm, T between 1200 and 1800 K, and omega between 0.1 and 1 Hz for typical metals and alloys.

Fecht, H. J.↗
NASA NTRS1991

Thermal and solutal conditions at the tips of a directional dendritic growth front

The line-of-sight averaged, time-dependent dendrite tip concentrations for the diffusion dominated vertical directional solidification of a metal model (ammonium chloride and water) were obtained by extrapolating exponentially fit diffusion layer profiles measured using a laser interferometer. The tip concentrations were shown to increase linearly with time throughout the diffusion dominated growth process for an initially stagnant dendritic array. The process was terminated for the cases chosen by convective breakdown suffered when the conditionally stable diffusion layer exceeded the critical Rayleigh criteria. The transient tip concentrations were determined to significantly exceed the values predicted for steady state, thus producing much larger constitutional undercoolings. This has ramifications for growth speeds, arm spacings and the dendritic structure itself.

Mccay, T. D.↗
NASA NTRS1991

Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition

A combination of bulk undercooling in an electromagnetic levitation apparatus and splat quenching between two copper plates is used to process Nb-Si alloys in order to maximize rapid solidification conditions and minimize the effects of recalescence, with emphasis on the solidification of characteristics of alloys in the 21 to 27 at. pct Si range of composition. SEM and TEM as well as X-ray diffraction are used to characterize the microstructures of the processed samples. In the range of compositions studied, the splat-quenched drops always formed the tetragonal Nb3Si phase directly from the liquid. Drops solidified in the coil were characterized by the presence of the primary intermetallic Nb5Si3 and the absence of both peritectic Nb3Si and the equilibrium eutectic. In these cases, a metastable alpha-Nb + beta-Nb5Si3 eutectic formed. The results are discussed in terms of possible metastable configurations of the Nb-Si phase diagram as well as concepts of nucleation and growth kinetics applied to the Nb3Si and Nb5Si3 intermetallics.

Hofmeister, W. H.↗
NASA NTRS1991

Applications of containerless processing in the studies of metals and alloys

The use of containerless processing in nucleation and microstructural studies of alloys is reviewed in terms of approaches and recent results. Drop-tube studies of niobium based on a statistical approach are discussed, and experiments are described which permit the study of the solidification front across the surface of a specimen. The various alloys studied are reviewed in terms of facilitated observations of metastability, chemical homogeneity, and microstructural refinement. Heterogeneous nucleation is reported for a preexponential factor of 10 exp 27 and a critical free energy of 46-56kT. The theoretical relationship between undercooling and solidification velocity correlates with containerless test observations at large solute concentrations. Containerless processing is effective for studying the properties of liquid metals or accessing the metastable thermodynamic regime.

Bayuzick, R. J.↗
NASA NTRS1992

Formation of a metastable ferromagnetic tau phase during containerless melt processing and rapid quenching in Mn-Al-C alloys

Solidification of selected Mn-Al-C alloys during containerless levitation and rapid quenching has yielded the first report for a ferromagnetic metastable tau phase formed directly from the melt. Complete solidification to tau phase was interrupted by the competitive evolution of an equilibrium epsilon phase during recalescence. The amount of undercooling required to produce the metastable ferromagnetic tau phase in a Mn(0.55)Al(0.433)C(0.017) alloy during solidification was estimated as approximately 470 K based on differential thermal analysis results. When the alloy carbon content was increased to 3.4 at. pct, transition in structure development occurred so that the samples exhibited gamma 2 phase formation as well as tau and epsilon phases.

Kim, Y. J.↗
NASA NTRS1992

Thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height

Satellite images of large volcanic explosions reveal that the tops of volcanic eruptions columns are much cooler than the surrounding atmosphere. It is proposed that this effect occurs whenever a mixture of hot volcanic ash and entrained air ascends sufficiently high into a stably stratified atmosphere. Although the mixture is initially very hot, it expands and cools as the ambient pressure decreases. It is shown that cloud-top undercoolings in excess of 20 C may develop in clouds that penetrate the stratosphere, and it is predicted that, for a given cloud-top temperature, variations in the initial temperature of 100-200 C may correspond to variations in the column height of 5-10 km. It is deduced that the present practice of converting satellite-based measurements of the temperature at the top of volcanic eruptions columns to estimates of the column height will produce rather inaccurate results and should therefore be discontinued.

Woods, Andrew W.↗