Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “undercooled”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

The formation kinetics of lunar glasses

The kinetic treatment of crystallization and glass formation, involving the construction of time-temperature-transformation curves (TTT) corresponding to a given degree of crystallinity, is extended to permit the description of crystallization of a body initially cooled to a glassy state. The key assumption is that if at any time and temperature a crystallite is smaller than the critical size corresponding to that temperature, it will melt completely and can be ignored in any further calculations of the crystal distribution. This approach is used to predict the temperature of maximum crystallization rate for the matrix composition of lunar breccia 67975; results are shown to be in excellent agreement with experiment. Theoretical results obtained for anorthite indicate a barrier to nucleation in the range of 75 kT when the ratio of the undercooling to the liquidus temperature is 0.2. Measured nucleation barriers for the 67975 matrix composition are in the range of 42 to 45 kT.

Uhlmann, D. R.↗
NASA NTRS1979

A simplified model for glass formation

A simplified model of glass formation based on the formal theory of transformation kinetics is presented, which describes the critical cooling rates implied by the occurrence of glassy or partly crystalline bodies. In addition, an approach based on the nose of the time-temperature-transformation (TTT) curve as an extremum in temperature and time has provided a relatively simple relation between the activation energy for viscous flow in the undercooled region and the temperature of the nose of the TTT curve. Using this relation together with the simplified model, it now seems possible to predict cooling rates using only the liquidus temperature, glass transition temperature, and heat of fusion.

Uhlmann, D. R.↗
NASA NTRS1980

SPAR VI Technical Report for Experiment 76-22: Directional Solidification of Magnetic Composites

Samples of eutectic Bi/MnBi were directionally solidified during a low-g interval aboard the SPAR 6 flight and in a l-g environment under identical furnace velocity and thermal conditions. The Bi/MnBi eutectic is characterized by a regular rod eutectic whose morphology may be sensitive to thermo-solutal convection and by its components, MnBi, which is ferromagnetic. Morphological analyses on samples show statistically smaller interrod spacings and rod diameters with respect to samples grown under identical solidification furnace conditions in l-g. An adjustment between the interrod spacing, growth velocity, and total undercooling at the solidification interface is proposed. Morphological analyses on samples grown in l-g indicate little difference between results for different growth orientations with respect to the gravity vector. The magnetic properties are significantly affected, however, by the presence of a nonequilibrium magnetic phase and the nonequilibrium phase transforms to the equilibrium ferromagnetic phase during isothermal heat treatment.

Pirich, R. G.↗
NASA NTRS1980

Si-rich Fe-Ni grains in highly unequilibrated chondrites

Consideration is given to the Si contents of Fe-Ni grains in highly unequilibrated chondrites, which have undergone little metamorphosis and thus best preserve the record of processes in the solar nebula. Electron microprobe determinations of silicon content in grains of the Bishunpur chondrite are presented for the six Si-bearing Fe-Ni grains for which data could be obtained, five of which were found to be embedded in olivine chondrules. In addition, all grains are found to be Cr-rich, with Cr increased in concentration towards the grain edge, and to be encased in FeS shells which evidently preserved the Si that entered the FeNi at higher temperatures. A mechanism for the production of Si-bearing metal during the condensation of the cooling solar nebula is proposed which considers the metal to have condensed heterogeneously while the mafic silicates condensed homogeneously with amounts of required undercooling in the low-pressure regions where ordinary and carbonaceous chondrites formed, resulting in Si mole fractions of 0.003 at nebular pressures less than 0.000001 atm.

Rambaldi, E. R.↗
NASA NTRS1982

Heat loss and drag of spherical drop tube samples

Analysis techniques for three aspects of the performance of the NASA/MSFC 32 meter drop tube are considered. Heat loss through the support wire in a pendant drop sample, temperature history of a drop falling through the drop tube when the tube is filled with helium gas at various pressures, and drag and resulting g-levels experienced by a drop falling through the tube when the tube is filled with helium gas at various pressures are addressed. The developed methods apply to systems with sufficiently small Knudsen numbers for which continuum theory may be applied. Sample results are presented, using niobium drops, to indicate the magnitudes of the effects. Helium gas at one atmosphere pressure can approximately double the amount of possible undercooling but it results in an apparent gravity levels of up to 0.1 g.

Wallace, D. B.↗
NASA NTRS1982

Rapid solidification processing

Rapid solidification in materials was studied. Crystalline and noncrystalline metals, polymers, ceramics, solidification mechanism and resulting structure, innovative processing techniques, properties and applications are investigated. The following programs are outlined: alloys which offer high strength at elevated temperatures, and processing methods necessary to achieve these goals; structure and structure property relations in ferrous alloys; Corrosion related aspects and high temperature oxidation resistance of fine grained alloys; relation of solidification theory to structures produced, on innovative processes for rapid solidification, and on solidification at high undercoolings. Studies on rapid solidification of polymeric materials are demonstrated.

Source record↗
NASA NTRS1982

Crystal nucleation in glass-forming alloy and pure metal melts under containerless and vibrationless conditions

The undercooling behavior of large spheroids of Pd40Ni40P40 was investigated. By surface etching, supporting the specimens on a fused silica substrate, and successive heating and cooling, crystallization can be eliminated, presumable due to the removal of surface heterogeneities. By this method samples up to 3.2g with a 0.53 mm minor diameter, were made entirely glassy, except for some superficial crystals comprising less than 0.5% of the volume. These experiments show that a cooling rate of approximately 1 K/sec is adequate to avoid copious homogeneous nucleation in the alloy, and that by eliminating or reducing the effectiveness of heterogeneous nucleation sites, it is possible to form bulk samples of this metallic glass with virtually unlimited dimensions.

Spaepen, F.↗
NASA NTRS1982

Theory and applications of electromagnetic levitation

A simple treatment of the electromagnetic levitation problem is presented, with emphasis placed on approximate formulas useful in planning and interpreting laboratory measurements. Consideration is also given to numerical solutions for fields, eddy currents, and Lorentz forces for rapidly varying applied fields, with particular reference made to traveling wave levitation experiments. Applications of levitation processing are briefly reviewed, including thermophysical property measurements, undercooling studies, containerless crystal growth, and continuous casting of cylinders.

Frost, R. T.↗
NASA NTRS1982

Crystal nucleation in Pd-Si alloys

A study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd(84.5)Si(15.5) eutectic composition is reported. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. It is found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with a previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.

Drehman, A. J.↗
NASA NTRS1982

Solidification studies of Nb-Ge alloys at large degrees of supercooling

A 32 meter evacuated drop tube has been used to investigate the solidification of Nb-Ge alloys after deep undercooling. Samples have been supercooled as much as 500 K below the liquidus by using free-fall conditions to eliminate crucible induced nucleation. Final microstructures are dependent on the quenching rates at the bottom of the drop tube with a striking extension of the beta phase solubility limit at the higher quenching rates.

Lacy, L. L.↗
NASA NTRS1982

The effect of solute on the homogeneous crystal nucleation frequency in metallic melts

A complete calculation that extends the classical theory for crystal nucleation in pure melts to binary alloys has been made. Using a regular solution model, approximate expressions have been developed for the free energy change upon crystallization as a function of solute concentration. They are used, together with model-based estimates of the interfacial tension, to calculate the nucleation frequency. The predictions of the theory for the maximum attainable undercooling are compared with existing experimental results for non-glass forming alloys. The theory is also applied to several easy glass-forming alloys (Pd-Si, Au-Si, Fe-B) for qualitative comparison with the present experimental experience on the ease of glass formation, and for assessment of the potential for formation of the glass in bulk.

Thompson, C. V.↗
NASA NTRS1984

Bulk formation of metallic glasses and amorphous silicon from the melt

Procedures and compositions for producing metallic glasses in bulk at slow cooling rates were investigated. An attempt was made to form the amorphous phase of the tetrahedrally coordinated elements (Si or Ge) by undercooling the melt. The crystal nucleation behavior of pure liquids and glass formers were examined.

Spaepen, F.↗
NASA NTRS1984

Models of Rapid Solidification

Laser annealing studies provide much information on various consequences of rapid solidification, including the trapping of impurities in the crystal, the generation of vacancies and twins, and on the fundamental limits to the speed of the crystal-melt interface. Some results obtained by molecular dynamics methods of the solidification of a Lennard-Jones liquid are reviewed. An indication of the relationship of interface speed to undercooling for certain materials can be derived from this model. Ising model simulations of impurity trapping in silicon are compared with some of the laser annealing results. The consequences of interface segregation and atomic strain are discussed.

Gilmer, G. H.↗
NASA NTRS1983

Homogeneous crystal nucleation in binary metallic melts

A method for calculating the homogeneous crystal nucleation frequency in binary metallic melts is developed. The free energy of crystallization is derived from regular solution models for the liquid and solid and is used, together with model-based estimates of the interfacial tension, to calculate the nucleation frequency from the classical theory. The method can account for the composition dependence of the maximum undercooling observed in a number of experiments on small droplet dispersions. It can also be used to calculate the driving force for crystal growth and to obtain more precise estimates of the homogeneous crystal nucleation frequency in glass-forming alloys. This method, although approximate, is simple to apply, and requires only knowledge of the phase diagram and a few readily available thermodynamic quantities as input data.

Thompson, C. V.↗
NASA NTRS1984

Formation of bulk metallic glass by fluxing

Bulk specimens (0.4-4 g mass) of the alloy Pd40Ni40P20 have been undercooled consistently to the glass state, with no detectable superficial crystallinity, in a molten flux of dehydrated boron oxide. The minimum dimension of the most massive glass specimen, so formed, was 1.0 cm. The absence of crystallinity in the specimens was confirmed by X-ray diffraction, scanning electron microscopy, and calorimetry.

Kui, H. W.↗
NASA NTRS1985

Hybrid Contactless Heating and Levitation

Acoustic and electromagnetic fields applied. In contactless processing apparatus, acoustic and electromagnetic levitating fields employed alternately or simultaneously with amplitude of each controlled to produce various combinations of heating, cooling, and levitation. Apparatus provides rapid heating and cooling or slow heating and cooling for such processes as nucleation, crystallization, incubation, deep undercooling, and heterogeneity control.

Lee, M. C.↗
NASA NTRS1984

Microgravity containerless processing in long drop tubes

Extensive experience in utilizing long drop tubes for studying effects of microgravity on the solidification of alloys was obtained. While some modifications are necessary to improve versatility, the facility proved to be most useful. Both an electron beam furnace and an electromagnetic levitation furnace can be used. The electron beam furnace is used with vacuum environments (0.00001 torr), whereas the levitation furnace is presently used only in inert gas environments (above 100 torr). Experiments are best applied to refractory alloys because of the sensitivity of the detectors now being used to observe solidification. Processing of lower melting point metals and alloys simply cannot be recorded. On the other hand, expected improvements in detector sensitivity will allow experimentation with relatively low melting alloys. In such cases, solidification will occur in flight only if higher inert gas pressure is used (100 to 760 torr) to increase heat loss by convection. Under these conditions microgravity conditions no longer apply. However, as shown by results to date, it is not microgravity as such that is important in drop tube solidification. Instead it is the containerless nature of the process that is significant, leading to large degrees of undercooling before solidification and therefore to unique alloys.

Bayuzick, R. J.↗