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At least 1,567 records · Page 87

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NASA NTRS1992

The effect of compositionally-generated elastic stresses on morphological instability during directional solidification

The effect of compositionally-generated elastic stresses on the conditions for morphological instability during directional solidification of a dilute, ideal binary alloy is investigated using the Gibbs-Thomson equations for an elastically-stressed solid with zero surface stress in equilibrium with a liquid. It is found that these stresses lead to a small stabilization of the Mullins and Sekerka cellular mode of instability. The steady mode is stabilized by a stress-induced modification to the interfacial concentration of the solid, which in turn alters the amount of solute rejected to inhibit the growth of perturbations. The presence of elastic stresses could generate a new oscillatory instability which is most likely to be found in experiments near absolute stability for materials with segregation coefficients near unity and large solute expansion coefficients.

Spencer, B. J.↗

NASA NTRS1991

Attentional issues in superimposed flight symbology

The mechanisms of the failure to simultaneously process HUD superimposed symbology and the out-the-window view may lie in the many cognitive and perceptual cues that inform the pilot that the HUD, although collimated at infinity, is an instrument on the aircraft. Thus, in relation to the world scene, the HUD forms a perceptually segregated object that has a different frame of reference. Research is being explored to determine the mechanisms in this problem, and to develop advanced designs that maximize the cognitive and perceptual compatibility of the superimposed symbology by integrating the important information into the world scene.

Foyle, David C.↗

NASA NTRS1992

Starbursts in the nuclei of shell galaxies

Simulations of shell-forming galaxy mergers are reported in which gas and stars are tracked separately to study the implications of observations that indicate fine structures in elliptical galaxies. The hybrid N-body/hydrodynamics code by Hernquist and Katz (1989) is employed neglecting the self-gravity of galaxies in tidal disruptions by massive primaries of small dwarf galaxies. The evolution of the galactic debris is examined showing that the gas and stars are effectively segregated with the stars forming sharp-edged features and the gas forming disks in the nucleus of the primary. Star formation is theorized to occur in the nuclei of shell galaxies due to shocks in the gas, and the added presence of a black hole can lead to an active galactic nucleus. The simulation is of interest to the interpretation of such observations as those of NGC1275 in which dormant elliptical galaxies are activated by mergers between large galaxies and gas-rich satellites.

Hernquist, Lars↗

NASA NTRS1992

Microstructural development during directional solidification of peritectic alloys

Despite the widespread commercial use of peritectic alloys (e.g., steels, brass, bronze, intermetallic compounds, Co based superalloys and A3B type superconductors), the characterization of the microstructural development during directional solidification of peritectics has historically lagged behind similar efforts directed towards other types of binary invariant reactions such as eutectic or monotectic. A wide variety of possible microstructures has been shown to form in peritectics depending upon the imposed temperature gradient, G, the solidification velocity, V, as well as the presence or absence of convection in the melt. This has important technological implications since many commercially important alloys exhibit peritectics and processing methods such as casting and welding often involve widely changing conditions. It has been the aim of this project to examine, in a systematic fashion, both experimentally and theoretically, the influence of gravitationally driven convection on segregation and microstructural development during solidification in peritectic systems under terrestrial conditions. The scientific results of the project will be used to establish ground based data in support of a meaningful microgravity flight experiment.

Lograsso, Thomas A.↗

NASA NTRS1993

Development of dispersion-strengthened XD (trademark) Cu alloys for high heat-flux applications

In a previous effort sponsored by NASA LeRC, the XD(trademark) process was used to produce ZrB2 particulate reinforcements in Cu and the resulting extruded material was microstructurally characterized and evaluated in tension over a range of temperatures. A problem that was encountered in that study was microstructural inhomogeneity resulting from the frequent presence of 'ZrB2 agglomerates' that were several microns in size. The presence of these agglomerates was attributed to improper mixing of powders in the green compact used in the XD process for elemental boron powder segregation. In this program, several milling parameters were examined in an effort to optimize this processing step; two levels of ZrB2 reinforcements were considered (7 vol. percent and 15 vol. percent). Microstructures of the reacted powder mass were examined to verify the absence of these agglomerates. Larger bathes of powder were then mixed, reacted, machined to size, canned, and extruded. The microstructure and tensile properties of these extrusions were examined, and the measured properties were correlated with the observed microstructure. Large unreacted or partially reacted Zr particles were present. These particles affected the mechanical properties deleteriously, and their presence is attributed to insufficient heat of reaction during XD synthesis. Alternate processing routes are recommended.

Kumar, K. Sharvan↗

NASA NTRS1991

Space transfer vehicle concepts and requirements study. Volume 3, book 1: Program cost estimates

The Space Transfer Vehicle (STV) Concepts and Requirements Study cost estimate and program planning analysis is presented. The cost estimating technique used to support STV system, subsystem, and component cost analysis is a mixture of parametric cost estimating and selective cost analogy approaches. The parametric cost analysis is aimed at developing cost-effective aerobrake, crew module, tank module, and lander designs with the parametric cost estimates data. This is accomplished using cost as a design parameter in an iterative process with conceptual design input information. The parametric estimating approach segregates costs by major program life cycle phase (development, production, integration, and launch support). These phases are further broken out into major hardware subsystems, software functions, and tasks according to the STV preliminary program work breakdown structure (WBS). The WBS is defined to a low enough level of detail by the study team to highlight STV system cost drivers. This level of cost visibility provided the basis for cost sensitivity analysis against various design approaches aimed at achieving a cost-effective design. The cost approach, methodology, and rationale are described. A chronological record of the interim review material relating to cost analysis is included along with a brief summary of the study contract tasks accomplished during that period of review and the key conclusions or observations identified that relate to STV program cost estimates. The STV life cycle costs are estimated on the proprietary parametric cost model (PCM) with inputs organized by a project WBS. Preliminary life cycle schedules are also included.

Peffley, Al F.↗

NASA NTRS1993

INEL Spray-forming Research

Spray forming is a near-net-shape fabrication technology in which a spray of finely atomized liquid droplets is deposited onto a suitably shaped substrate or mold to produce a coherent solid. The technology offers unique opportunities for simplifying materials processing without sacrificing, and oftentimes substantially improving, product quality. Spray forming can be performed with a wide range of metals and nonmetals, and offers property improvements resulting from rapid solidification (e.g., refined microstructures, extended solid solubilities and reduced segregation). Economic benefits result from process simplification and the elimination of unit operations. Researchers at the Idaho National Engineering Laboratory (INEL) are developing spray-forming technology for producing near-net-shape solids and coatings of a variety of metals, polymers, and composite materials. Results from several spray forming programs are presented to illustrate the range of capabilities of the technique as well as the accompanying technical and economic benefits. Low-carbon steel strip greater than 0.75 mm thick and polymer membranes for gas/gas and liquid/liquid separations that were spray formed are discussed; recent advances in spray forming molds, dies, and other tooling using low-melting-point metals are described.

Mchugh, Kevin M.↗

NASA NTRS1993

Do galactic potential wells depend on their environment?

Using galaxies in complete samples as tracers of the galaxy density field and about 1000 galaxies with measured circular velocities as targets, we examine the cross-correlation functions between the targets and tracers as a function of galaxy circular velocities. The correlation strength does not vary with the circular velocities except for elliptical galaxies with the highest velocity dispersions, where the effect may well be due to morphological segregations in clusters of galaxies. This is contrasted with the strong dependence of the correlation functions of dark halos on their circular velocities in some models of galaxy formation.

Mo, H. J.↗

NASA NTRS1993

Can cluster environment modify the dynamical evolution of spiral galaxies?

Over the past decade many effects of the cluster environment on member galaxies have been established. These effects are manifest in the amount and distribution of gas in cluster spirals, the luminosity and light distributions within galaxies, and the segregation of morphological types. All these effects could indicate a specific dynamical evolution for galaxies in clusters. Nevertheless, a more direct evidence, such as a different mass distribution for spiral galaxies in clusters and in the field, is not yet clearly established. Indeed, Rubin, Whitmore, and Ford (1988) and Whitmore, Forbes, and Rubin (1988) (referred to as RWF) presented evidence that inner cluster spirals have falling rotation curves, unlike those of outer cluster spirals or the great majority of field spirals. If falling rotation curves exist in centers of clusters, as argued by RWF, it would suggest that dark matter halos were absent from cluster spirals, either because the halos had become stripped by interactions with other galaxies or with an intracluster medium, or because the halos had never formed in the first place. Even if they didn't disagree with RWF, other researchers pointed out that the behaviour of the slope of the rotation curves of spiral galaxies (in Virgo) is not so clear. Amram, using a different sample of spiral galaxies in clusters, found only 10% of declining rotation curves (2 declining vs 17 flat or rising) in opposition to RWF who find about 40% of declining rotation curves in their sample (6 declining vs 10 flat or rising), we will hereafter briefly discuss the Amram data paper and compare it to the results of RWF. We have measured the rotation curves for a sample of 21 spiral galaxies in 5 nearby clusters. These rotation curves have been constructed from detailed two-dimensional maps of each galaxy's velocity field as traced by emission from the Ha line. This complete mapping, combined with the sensitivity of our CFHT 3.60 m. + Perot-Fabry + CCD observations, allows the construction of high-quality rotation curves. Details concerning the acquisition and reduction procedures of the data are given in Amram. We present and discuss our preliminary analysis and compare them with RWF's results.

Amram, P.↗

NASA NTRS1992

Thermal and dynamical regimes of single- and two-phase magmatic flow in dikes

The coupling between thermal and dynamical regimes of single- and two-phase magmatic flow in dikes, due to temperature-dependent viscosity and dissipation, was investigated using finite element calculations of magma flow in dikelike channels with length-to-width ratios of 1000:1 or more. Solutions of the steady state equations governing magma flow are obtained for a variety of conditions ranging from idealized plane-parallel models to cases involving nonparallel geometry and two-phase flows. The implications of the numerical simulations for the dynamics of flow in a dike-reservoir system and the consequences of dike entrance conditions on magmatic storage are discussed. Consideration is also given to an unmixing/self-lubrication mechanism which may be important for the lubrication of silicic magmas rising to the earth's surface in mixed magma ascent scenarios, which naturally segregates magma mixtures of two components with differing viscosities to minimize the driving pressure gradient.

Carrigan, Charles R.↗

NASA NTRS1992

Core formation by giant impacts

The present model for the timing and mechanisms of planetary core formation argues that once a planet reaches a certain minimum mass, the large impacts that are typical of late accretion can trigger core formation. This model circumvents the difficulties posed by the large-scale segregation of molten iron into diapirs, and the displacement of the cold, elastic interior of the planet by the iron. The analytical melting model used is based on the Hugoniot equations, the empirical relationship for the decline of particle velocity with distance, and the linear shock-particle velocity relationship.

Tonks, W. B.↗

NASA NTRS1992

Advances in materials and current collecting networks for AMTEC electrodes

Electrode materials for the Alkali Metal Thermal to Electric Converter (AMTEC) play a significant role in the efficiency of the device. RhW and PtW alloys have been studied to determine the best performing material. While RhW electrodes typically have power densities somewhat lower than PtW electrodes, PtW performance is strongly influenced by the Pt/W ratio. The best performing Pt/W ratio is about 3.4. RhW electrodes sinter more slowly than PtW and are predicted to have operating lifetimes up to 40 years; PtW electrodes are predicted to have lifetimes up to 7 years. Interaction with the current collection network can significantly decrease lifetime by inducing metal migration and segregation and by accelerating the sintering rate.

Ryan, M. A.↗

NASA NTRS1991

Interface demarcation in Bridgman-Stockbarger crystal growth of II-VI compounds

In preparation for anticipated microgravity space missions carrying the Advanced Automated Directional Solidification Furnace, experiments were conducted to determine growth of interface in growing crystals of cadmium-doped germanium (test case), cadmium telluride, and mercury zinc telluride, using three different techniques. It was found that compositional variations detectable by microprobe and EDS techniques were most successful for detecting interfaces in materials with extensive transient regions, large segregation coefficients, and high alloy content.

Gillies, Donald C.↗

NASA NTRS1991

Steady state modeling of large diameter crystal growth using baffles

Buoyancy driven flow in the crystal melt is one of the leading causes of segregation. Natural convection arises from the presence of thermal and/or solutal gradients in the melt and it is not possible to completely eliminate the convection even in the low gravity environment of space. This paper reports the results of computational modeling research that is being done in preparation for space-based experiments. The commercial finite element code FIDAP was used to simulate the steady convection of a gallium-doped germanium alloy in a Bridgman-Stockbarger furnace. In particular, the study examines the convection-suppressing benefits of inserting cylindrical baffles in the molten region to act as viscous dampers. These thin baffles are assumed to be inert and noncontaminating. The results from this study show the manner in which the streamlines, velocities, and temperature fields at various gravity levels are affected by the presence of baffles. The effects of changing both the number and position of the baffles are examined and the advantages and disadvantages of using baffles are considered.

Sahai, Vivek↗

NASA NTRS1992

Effect of high temperature annealing on the microstructure of SCS-6 SiC fibers

The effect of annealing the SCS-6 SiC fiber for one hour at 2000 C in an argon atmosphere is reported. The SiC grains in the fiber coarsen appreciably and the intergranular carbon films segregate to the grain junctions. It would appear that grain growth in the outer part of the fiber is primarily responsible for the loss in fiber strength and improvement in fiber creep resistance.

Ning, X. J.↗

NASA NTRS1992

Recycling and source reduction for long duration space habitation

A direct mathematical approach has been established for characterizing the performance of closed-loop life support systems. The understanding that this approach gives clearly illustrates the options available for increasing the performance of a life support system by changing various parameters. New terms are defined and utilized, such as Segregation Factor, Resource Recovery Efficiency, Overall Reclamation Efficiency, Resupply Reduction Factor, and Life Support Extension Factor. The effects of increases in expendable system supplies required due to increases in life support system complexity are shown. Minimizing resupply through increased recycling and source reduction is illustrated. The effects of recycling upon resupply launch cost is also shown. Finally, material balance analyses have been performed based on quantity and composition data for both supplies and wastes, to illustrate the use of this approach by comparing ten different closed-loop life support system cases.

Hightower, T. M.↗

NASA NTRS1993

Thermal effects in rapid directional solidification - Linear theory

We study the morphological instability of the planar solid/liquid interface for a unidirectionally-solidified dilute binary mixture. We use a model developed by Boettinger et al. (1985, 1986), Aziz (1982), and Jackson et al. (1980), which allows for nonequilibrium effects on the interface through velocity-dependent segregation and attachment kinetics. Two types of instabilities are found in the linear stability analysis: (1) a cellular instability, and (2) an oscillatory instability driven by disequilibrium effects. Merchant and Davis (1990) characterized these instabilities subject to the frozen-temperature approximation (FTA). The present work relaxes the FTA by including the effects of latent heat and the full temperature distribution. Thermal effects slightly postpone the onset of the cellular instability but dramatically postpone the onset of the oscillatory instability; however, the absolute-stability conditions, at which at high speed the cellular and oscillatory instabilities are suppressed, remain unchanged from the FTA.

Huntley, D. A.↗

NASA NTRS1990

Collaboration using roles

Segregation of roles into alternative accounts is a model which provides not only the ability to collaborate but also enables accurate accounting of resources consumed by collaborative projects, protects the resources and objects of such a project, and does not introduce new security vulnerabilities. The implementation presented here does not require users to remember additional passwords and provides a very simple consistent interface.

Bishop, Matt↗