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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The influence of processing parameters on microstructural development of low-weight-percent primary, pro-eutectic, dendritic alloys during directional solidification

Alloy compositions centering about Sn- 5 to 15 wt pct Al and Pb- 4 to 20 wt pct Cu have been directionally solidified as a function of growth rate and orientation with respect to earth's gravity vector. The effect of these processing variables on macrosegregation is presented, and the consequence of a structural transition from primary columnar to equiaxed dendritic growth is examined. Effects detrimental to microstructure and attributed to density variations are shown to be minimized by a novel processing technique. In view of the ground-based results, some considerations follow regarding what might be gained by solidifying these alloys in a microgravity environment.

Grugel, R. N.↗
NASA NTRS1992

Macrosegregation During Steady-State Arrayed Growth of Dendrites in Directionally Solidified Pb-Sn Alloys

The paper examines the growth parameter dependence of the longitudinal macrosegregation due to interdendritic thermosolutal convection during directional solidification of Pb-Sn alloys with a steady-state dendritic arrayed morphology. The experimentally observed longitudinal macrosegregation can be described by an empirical parameter, effective partition coefficient (kE), obtained from Cs = kEC0(1 - fs) super KI-1, where the original tin content is C0 and that corresponding to the fraction distance solidified (Fs) is Cs. Channel formation is strongly associated with the onset of thermosolutal convection in the interdendritic melt. The critical effective Rayleigh number for the onset of channel formation is about 0.07.

Tewari, S. N.↗
NASA NTRS1993

Secondary and tertiary dendrite arm spacing relationships in directionally solidified Al-Si alloys

Secondary, lambda sub 2, and tertiary, lambda sub 3, dendrite arm spacings have been measured from Al-Si alloys which were directionally solidified as functions of growth velocity, V, temperature gradient, G, and composition, Co. Both lambda sub 2 and lambda sub 3 decreased as the imposed growth velocity and silicon concentrations were increased, and for each function a systematic variance in the rate was seen. Complications with measuring secondary arm spacings are shown and it was found that the tertiary arm data agree much better with coarsening theory, the implication being that lambda sub 3, when measurable, is a more representative and reliable measure of the solidification history than lambda sub 2.

Grugel, R. N.↗
NASA NTRS1993

Measured and predicted effects of gravity level on directional dendritic solidification of NH4Cl-H2O

Dendritic growth front rates during vertical directional solidification are predicted for gravity levels of 10 exp 0 g sub e (where e is earth gravity), 10 exp -1 g sub e, 10 exp -2 g sub e, 10 exp -3 g sub e, 10 exp -4 g sub e, and 10 exp -5 g sub e (microgravity) for the physical conditions used for a recent ammonium chloride-water solidification experiment on the International Microgravity Laboratory I (IMLI). The growth front rates at 10 exp 0 g sub e and 10 exp -5 g sub e are validated using ground based laboratory and IMLI experimental data. As the gravity decreases, the growth rates increase until they approach a maximum at approximately 10 exp -4 g sub e. The 10 exp -4 and 10 exp -5 levels are equivalent. Liquid concentration and volume fraction, temperature profiles and fluid flow velocities are also calculated. Kinetic energy calculations for each of the six gravity levels indicate that the threshold for fluid flow to affect the growth front rate is in the range of 10 exp -8 ergs.

Mccay, T. D.↗
NASA NTRS1992

Thermosolutal convection during dendritic solidification of a binary alloy

The FEM model of Heinrich (1988) is presently extended to the case of dendritic vertical solidification under steady-state conditions. An analysis is conducted of the nonlinear governing equations for linear stability, and FEM calculations are performed in cases where instability has been predicted for Pb-Sn melt systems; the results obtained show that the model is more stable than the plane front liquid-solid model, and shows similar, finger-like covection when unstable. It is found that convection in the 'mush' zone is confined to the top of that region; convective motion is driven by convection in the bulk fluid, in the absence of natural convection.

Heinrich, Juan C.↗
NASA NTRS1996

Suppressing The Growth Of Dendrites In Secondary Li Cells

Proposed technique for suppressing growth of lithium dendrites in rechargeable lithium electrochemical power cells involves periodic interruption of steady charging current with short, high-current discharge pulses. Technique applicable to lithium cells of several different types, including Li/TiS(2), Li/NbSe(3), Li/CoO(2), Li/MoS(2), Li/Vo(x), and Li/MnO(2). Cells candidates for use in spacecraft, military, communications, automotive, and other applications in which high-energy-density rechargeable batteries needed.

Davies, Evan D.↗
NASA NTRS1997

Role of Dendritic Cells in Immune Dysfunction

Specific aims include: (1) Application of the bioreactor to enhance cytokine-regulated proliferation and maturation of dendritic cells (DC); (2) Based on clues from spaceflight: compare the frequency and function of DC in normal donors and immunocompromised cancer patients; and (3) Initiate studies on the efficiency of cytokine therapy and DC-assisted immunotherapy (using bioreactor-expanded DC) in animal models of experimental fungal infections.

Savary, Cherylyn A.↗
NASA NTRS1998

Role of Dendritic Cells in Immune Dysfunction

The specific aims of the project were: (1) Application of the NASA bioreactor to enhance cytokine-regulated proliferation and maturation of dendritic cells (DC). (2) Compare the frequency and function of DC in normal donors and immunocompromised cancer patients. (3) Analyze the effectiveness of cytokine therapy and DC-assisted immunotherapy (using bioreactor-expanded DC) in a murine model of experimental fungal disease. Our investigations have provided new insight into DC immunobiology and have led to the development of methodology to evaluate DC in blood of normal donors and patients. Information gained from these studies has broadened our understanding of possible mechanisms involved in the immune dysfunction of space travelers and earth-bound cancer patients, and could contribute to the design of novel therapies to restore/preserve immunity in these individuals. Several new avenues of investigation were also revealed. The results of studies completed during Round 2 are summarized.

Savary, Cherylyn A.↗
NASA NTRS1999

Two Dimensional Dendritic Crystal Growth for Weak Undercooling

We discuss the framework and issues brought forth in the recent work of Kunka, Foster & Tanveer, which incorporates small but nonzero surface energy effects in the nonlinear dynamics of a conformal mapping function z(zeta,t) that maps the upper-half zeta plane into the exterior of a dendrite. In this paper, surface energy effects on the singularities of z(zeta,t) in the lower-half plane were examined, as they move toward the real axis from below. In particular, the dynamics of complex singularities manifests itself in predictions on nature and growth rate of disturbances, as well as of coarsening.

Tanveer, S.↗
NASA NTRS2002

Minimizing Segregation During the Controlled Directional Solidification of Dendritic Alloys Publication: Metallurgical and Materials Transactions

Gravity-driven thermosolutal convection that arises during controlled directional solidification (DS) of dendritic alloys promotes detrimental macro-segregation (e.g. freckles and steepling) in products such as turbine blades. Considerable time and effort has been spent to experimentally and theoretically investigate this phenomena; although our knowledge has advanced to the point where convection can be modeled and accurately compared to experimental results, little has been done to minimize its onset and deleterious effects. The experimental work demonstrates that segregation can be. minimized and microstructural uniformity promoted when a slow axial rotation is applied to the sample crucible during controlled directional solidification processing. Numerical modeling utilizing continuation and bifurcation methods have been employed to develop accurate physical and mathematical models with the intent of identifying and optimizing processing parameters.

Grugel, R. N.↗
NASA NTRS2003

Flight Planning for the International Space Station - Levitation Observation of Dendrite Evolution in Steel Ternary Alloy Rapid Solidification (LODESTARS)

During rapid solidification, a molten sample is cooled below its equilibrium solidification temperature to form a metastable liquid. Once nucleation is initiated, growth of the solid phase proceeds and can be seen as a sudden rise in temperature. The heat of fusion is rejected ahead of the growing dendrites into the undercooled liquid in a process known as recalescence. Fe-Cr-Ni alloys may form several equilibrium phases and the hypoeutectic alloys, with compositions near the commercially important 316 stainless steel alloy, are observed to solidify by way of a two-step process known as double recalescence. During double recalescence, the first temperature rise is associated with formation of the metastable ferritic solid phase with subsequent conversion to the stable austenitic phase during the second temperature rise. Selection of which phase grows into the undercooled melt during primary solidification may be accomplished by choice of the appropriate nucleation trigger material or by control of the processing parameters during rapid solidification. Due to the highly reactive nature of the molten sample material and in order to avoid contamination of the undercooled melt, a containerless electromagnetic levitation (EML) processing technique is used. In ground-based EML, the same forces that support the weight of the sample against gravity also drive convection in the liquid sample. However, in microgravity, the force required to position the sample is greatly reduced, so convection may be controlled over a wide range of internal flows. Space Shuttle experiments have shown that the double recalescence behavior of Fe-Cr-Ni alloys changes between ground and space EML experiments. This program is aimed at understanding how melt convection influences phase selection and the evolution of rapid solidification microstructures.

Flemings, Merton C.↗
NASA NTRS2000

Modern freshwater microbialite analogues for ancient dendritic reef structures

Microbialites are organosedimentary structures that can be constructed by a variety of metabolically distinct taxa. Consequently, microbialite structures abound in the fossil record, although the exact nature of the biogeochemical processes that produced them is often unknown. One such class of ancient calcareous structures, Epiphyton and Girvanella, appear in great abundance during the Early Cambrian. Together with Archeocyathids, stromatolites and thrombolites, they formed major Cambrian reef belts. To a large extent, Middle to Late Cambrian reefs are similar to Precambrian reefs, with the exception that the latter, including terminal Proterozoic reefs, do not contain Epiphyton or Girvanella. Here we report the discovery in Pavilion Lake, British Columbia, Canada, of a distinctive assemblage of freshwater calcite microbialites, some of which display microstructures similar to the fabrics displayed by Epiphyton and Girvanella. The morphologies of the modern microbialites vary with depth, and dendritic microstructures of the deep water (> 30 m) mounds indicate that they may be modern analogues for the ancient calcareous structures. These microbialites thus provide an opportunity to study the biogeochemical interactions that produce fabrics similar to those of some enigmatic Early Cambrian reef structures.

Non-NASA Center↗
NASA NTRS2012

Detachment of Tertiary Dendrite Arms during Controlled Directional Solidification in Aluminum - 7 wt Percent Silicon Alloys: Observations from Ground-based and Microgravity Processed Samples

Electron Back Scattered Diffraction results from cross-sections of directionally solidified aluminum 7wt% silicon alloys unexpectedly revealed tertiary dendrite arms that were detached and mis-oriented from their parent arm. More surprisingly, the same phenomenon was observed in a sample similarly processed in the quiescent microgravity environment aboard the International Space Station (ISS) in support of the joint US-European MICAST investigation. The work presented here includes a brief introduction to MICAST and the directional solidification facilities, and their capabilities, available aboard the ISS. Results from the ground-based and microgravity processed samples are compared and possible mechanisms for the observed tertiary arm detachment are suggested.

Grugel, Richard N.↗
NASA NTRS2012

Real Time In Situ Observations of Equiaxed Dendrite Coherency in Al-Cu Alloys Using High-Brilliance, 3rd Generation Synchrotron Sources

In the last decade synchrotron X-ray sources have fast become the tool of choice for performing in-situ high resolution imaging during alloy solidification. This paper presents the results of an experimental campaign carried out at the European Synchrotron Radiation Facility, using a Bridgman furnace, to monitor phenomena during solidification of Al-Cu alloys - specifically the onset of equiaxed dendrite coherency. Conventional experimental methods for determining coherency involve measuring the change in viscosity or measuring the change in thermal conductivity across the solidifying melt Conflicts arise when comparing the results of these experimental techniques to find a relationship between cooling rate and coherency fraction. It has been shown that the ratio of average velocity to the average grain diameter has an inversely proportional relationship to coherency fraction. In-situ observation therefore makes it possible to measure these values directly from acquired images sequences and make comparisons with published results.

Murphy, Andrew G.↗
NASA NTRS2015

Diagenetic Crystal Clusters and Dendrites, Lower Mount Sharp, Gale Crater

Since approximately Sol 753 (to sol 840+) the Mars Science Laboratory Curiosity rover has been investigating the Pahrump locality. Mapping of HiRise images suggests that the Pahrup locality represents the first occurrence of strata associated with basal Mount Sharp. Considerable efforts have been made to document the Pahrump locality in detail, in order to constrain both depositional and diagenetic facies. The Pahrump succession consists of approximately 13 meters of recessive-weathering mudstone interbedded with thin (decimeter-scale) intervals of more erosionally resistant mudstone, and crossbedded sandstone in the upper stratigraphic levels. Mudstone textures vary from massive, to poorly laminated, to well-laminated. Here we investigate the distribution and structure of unusual diagenetic features that occur in the lowermost portion of the Pahrump section. These diagenetic features consist of three dimensional crystal clusters and dendrites that are erosionally resistant with respect to the host rock.

Kah, L. C.↗
NASA NTRS2015

Evaluation of Primary Dendrite Arm Spacings from Aluminum-7wt% Silicon alloys Directionally Solidified aboard the International Space Station - Comparison with Theory

Aluminum – 7wt% silicon alloys were directionally solidified in the microgravity environment aboard the International Space Station as part of the “MIcrostructure Formation in CASTing of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions” (MICAST) European led program. Cross-sections of the sample during periods of steady-state growth were metallographically prepared from which the primary dendrite arm spacing (lambda 1) was measured. These spacings were found to be in reasonable agreement with the Hunt-Lu model which assumes a diffusion-controlled, convectionless, environment during controlled solidification. Deviation from the model was found and is attributed to gravity-independent thermocapillary convection where, over short distances, the liquid appears to have separated from the crucible wall.

Angart, Samuel↗
DOE OSTI2023

Metal-ion deposition regulator for suppressing dendrite formation in electrochemical cells

A metal-ion deposition regulator to regulate the flux and deposition of metal ions in an electrochemical cell. The regulator containing two membranes made of a polymer and a plurality of two-dimensional porous nanosheets sandwiched between the two membranes. The regulator is capable of distributing flux of metal ions passing through the metal-ion deposition regulator and regulating the deposition of the metal ions onto the cathode or anode thereby suppressing dendrite growth in the electrochemical cell. An electrochemical cell containing an anode, a cathode, a liquid electrolyte, and a metal-ion deposition regulator. A method of making a metal-ion deposition regulator. The method includes fabricating two-dimensional porous nanosheets utilizing graphene oxide as an expendable template, and sandwiching the porous nanosheets between two membranes made of a polymer such that the nanosheets are in contact with the two membranes.

Pol, Vilas Ganpat↗