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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Kinetics of Hydrogen Diffusion in LaNi(sub 5-x)Sn(sub x) Alloys

Solid-state diffusion of hydrogen in metal hydride (MH) alloys is recognized as the rate determining step in the discharge of MH alloys in alkaline Ni-MH rechargeable cells. In our pursuit of new ternary solutes in LaNi(sub 5) for extended cycle lifetimes, we have observed noticeable improvement in the cycle life with small substitutions of Sn and Ge for Ni. Furthermore, these substituents also facilitate enhanced charge transfer kinetics for hydriding-dehydriding process. In this paper, we report our studies on the kinetics of hydrogen diffusion in LaNi(sub 5-x) Sn(sub x) alloys by electrochemical pulse techniques, chronoamperometry and chronocoulometry.

electrochemical cycling↗
NASA NTRS1993

An approximate formula for recalescence in binary eutectic alloys

In alloys, solidification takes place along various paths which may be ascertained via phase diagrams; while there would be no single formula applicable to all alloys, an approximate formula for a specific solidification path would be useful in estimating the fraction of the solid formed during recalescence. A formulation is here presented of recalescence in binary eutectic alloys. This formula is applied to Ag-Cu alloys which are of interest in containerless solidification, due to their formation of supersaturated solutions.

Ohsaka, K.↗
NASA NTRS1984

Effect of alloy chemistry and exposure conditions on the oxidation of titanium

Multiwall is a new thermal protection system concept for advanced space transportation vehicles. The system consists of discrete panels made up of multiple layers of foil gage metal. Titanium is the proposed candidate metal for multiwall panels in the reentry temperature range up to 675 C. Oxidation and embrittlement are the principal concerns related to the use of Ti in heat shield applications. The results of a broad study on the oxidation kinetics of several titanium alloys subjected to different exposure conditions are described. The alloys include commercially pure titanium, Ti-6Al-4V, and Ti-6Al-2Sn-4Zr-2Mo. Oxidation studies were performed on these alloys exposed at 704 C in 5-760 torr air pressure and 0 to 50% relative humidity. The resulting weight gains were correlated with oxide thickness and substrate contamination. The contamination depth and weight gains due to solid solutioning were obtained from microhardness depth profiles and hardness versus weight percent oxygen calibration data.

Unnam, J.↗
NASA NTRS1980

Analysis and calculation of macrosegregation in a casting ingot. MPS solidification model. Volume 1: Formulation and analysis

The physical and numerical formulation of a model for the horizontal solidification of a binary alloy is described. It can be applied in an ingot. The major purpose of the model is to calculate macrosegregation in a casting ingot which results from flow of interdendritic liquid during solidification. The flow, driven by solidification contractions and by gravity acting on density gradients in the interdendritic liquid, was modeled as flow through a porous medium. The symbols used are defined. The physical formulation of the problem leading to a set of equations which can be used to obtain: (1) the pressure field; (2) the velocity field: (3) mass flow and (4) solute flow in the solid plus liquid zone during solidification is presented. With these established, the model calculates macrosegregation after solidification is complete. The numerical techniques used to obtain solution on a computational grid are presented. Results, evaluation of the results, and recommendations for future development of the model are given. The macrosegregation and flow field predictions for tin-lead, aluminum-copper, and tin-bismuth alloys are included as well as comparisons of some of the predictions with published predictions or with empirical data.

Maples, A. L.↗
NASA NTRS2003

Kinetic Monte Carlo Simulations of Rod Eutectics and the Surface Roughening Transition in Binary Alloys

In this paper we report on two related topics: Kinetic Monte Carlo simulations of the steady state growth of rod eutectics from the melt, and a study of the surface roughness of binary alloys. We have implemented a three dimensional kinetic Monte Carlo (kMC) simulation with diffusion by pair exchange only in the liquid phase. Entropies of fusion are first chosen to fit the surface roughness of the pure materials, and the bond energies are derived from the equilibrium phase diagram, by treating the solid and liquid as regular and ideal solutions respectively. A simple cubic lattice oriented in the {100} direction is used. Growth of the rods is initiated from columns of pure B material embedded in an A matrix, arranged in a close packed array with semi-periodic boundary conditions. The simulation cells typically have dimensions of 50 by 87 by 200 unit cells. Steady state growth is compliant with the Jackson-Hunt model. In the kMC simulations, using the spin-one Ising model, growth of each phase is faceted or nonfaceted phases depending on the entropy of fusion. There have been many studies of the surface roughening transition in single component systems, but none for binary alloy systems. The location of the surface roughening transition for the phases of a eutectic alloy determines whether the eutectic morphology will be regular or irregular. We have conducted a study of surface roughness on the spin-one Ising Model with diffusion using kMC. The surface roughness was found to scale with the melting temperature of the alloy as given by the liquidus line on the equilibrium phase diagram. The density of missing lateral bonds at the surface was used as a measure of surface roughness.

Bentz, Daniel N.↗
NASA NTRS1990

Effect of thermosolutal convection on the solid-liquid interface in Pb-Au alloy

Liquid-solid interface distortion in the region of primary dendrite tips has been investigated in directionally solidified Pb-8 wt pct Au alloy. The distortions are caused by thermosolutal convection despite choice of growth conditions which should have been thermally and solutally stabilizing. The convection produces clustering of primary dendrites on a plane perpendicular to the growth direction. It produces a mushy zone, where the primary dendrites do not protrude with a uniform length, across the specimen cross section, resulting in large macrosegregation in the transverse direction. However, little macrosegregation is observed along the growth direction. The mushy zone, with uneven dendrite lengths, forms in the beginning of directional solidification. Its shape and dendrite distribution do not show much change during subsequent solidification. Tip morphologies of primary dendrites, within the dendrite clusters, appear to follow the morphological stability relationship.

Tewari, S. N.↗
NASA NTRS2003

A Comparison between Growth Morphology of "Eutectic" Cells/Dendrites and Single-Phase Cells/Dendrites

Directionally solidified (DS) intermetallic and ceramic-based eutectic alloys with an in-situ composite microstructure containing finely distributed, long aspect ratio, fiber, or plate reinforcements are being seriously examined for several advanced aero-propulsion applications. In designing these alloys, additional solutes need to be added to the base eutectic composition in order to improve heir high-temperature strength, and provide for adequate toughness and resistance to environmental degradation. Solute addition, however, promotes instability at the planar liquid-solid interface resulting in the formation of two-phase eutectic "colonies." Because morphology of eutectic colonies is very similar to the single-phase cells and dendrites, the stability analysis of Mullins and Sekerka has been extended to describe their formation. Onset of their formation shows a good agreement with this approach; however, unlike the single-phase cells and dendrites, there is limited examination of their growth speed dependence of spacing, morphology, and spatial distribution. The purpose of this study is to compare the growth speed dependence of the morphology, spacing, and spatial distribution of eutectic cells and dendrites with that for the single-phase cells and dendrites.

Tewari, S. N.↗
NASA NTRS1985

High temperature properties of equiatomic FeAl with ternary additions

The aluminide intermetallic compounds are considered potential structural materials for aerospace applications. The B2 binary aluminide FeAl has a melting point in excess of 1500 K, is of simple cubic structure, exists over a wide range of composition with solubility for third elements and is potentially self-protecting in extreme environments. The B2 FeAl compound has been alloyed with 1 to 5 at. pct ternary additions of Si, Ti, Zr, Hf, Cr, Ni, Co, Nb, Ta, Mo, W, and Re. The alloys were prepared by blending a third elemental powder with pre-alloyed binary FeAl powder. Consolidation was by hot extrusion at 1250 K. Annealing studies on the extruded rods showed that the third element addition can be classified into three categories based upon the amount of homogenization and the extent of solid solutioning. Constant strain rate compression tests were performed to determine the flow stress as a function of temperature and composition. The mechanical strength behavior was dependent upon the third element homogenization classification.

Titran, R. H.↗
NASA NTRS1975

Directionally solidified pseudo-binary eutectics of Ni-Cr-(Hf, Zr)

A pseudo-binary eutectic, in which the intermetallic Ni7Hf2 reinforces the Ni-Cr solid solution phase, was previously predicted in the Ni-Cr-Hf system by a computer analysis. The experimental determination of pseudo binary eutectic compositions and the directional solidification of the Ni-Cr-Hf, Zr, and Ni-Cr-Zr eutectic alloys are discussed. To determine unknown eutectics, chemical analyses were made of material bled from near eutectic ingots during incipient melting. Nominal compositions in weight percent of Ni-18.6Cr-24.0HF, Ni19.6Cr-12.8Zr-2.8Hf, and Ni-19.2Cr-14.8Zr formed aligned pseudo-binary eutectic structures. The melting points were about 1270 C. The reinforcing intermetallic phases were identified as noncubic (Ni,Cr)7Hf2 and (Ni,Cr)7(Hf,Zr)2, and face centered cubic (Ni,Cr)5Zr. The volume fraction of the reinforcing phases were about 0.5.

Kim, Y. G.↗
NASA NTRS2002

Development Of A Novel Discontinuously-Reinforced Aluminum For Space Applications

Discontinuously-reinforced aluminum (DRA) has been used in aerospace structures such as Ventral Fins and Fan Exit Guide Vanes owing to its superior specific stiffness, specific strength, wear resistance, and thermal resistance as compared to the unreinforced aluminum alloys. In order to reduce engine weight, DRA materials are now being considered for space applications. Higher specific strength at ambient and cryogenic temperatures is one of the main requirements in certain rocket applications. The commercial DRA materials use 6xxx and 2xxx precipitation hardened aluminum alloys as matrices which have limited strengths. Therefore, an aluminum alloy which can provide significantly higher ambient and cryogenic strengths is required. In this paper, a novel aluminum alloy based on Al-Sc-X composition with improved ambient and cryogenic temperature strengthening capability is proposed. In addition, this alloy showed promise for improved strength at elevated temperature. The monolithic alloy and the composite with 15 volume percent SiC and B4C particles were processed using a powder metallurgy approach. The influence of processing parameters on the microstructures and mechanical properties of the monolithic and composite materials is discussed. The alloy showed very high strength and moderate ductility. The influence of hydrogen on the properties of monolithic and composite materials is discussed. The thermal stability of these materials is also evaluated. The strength of the material is discussed in terms of solid solution strengthening, Orowan strengthening, and antiphase boundary strengthening models.

Pandey, A. B.↗
NASA NTRS1985

The Development and Prevention of Channel Segregation During Alloy Solidification

During alloy solidification, interdendritic microsegregation causes density differences between entrapped liquid and bulk liquid which provides a driving force for convection. One of the consequences of this effect is the formation of long, nearly vertical liquid channels, rich in solute, flowing through the solid-liquid mushy zone somewhat as a river and tributary system. In ingot castings where the heat flow is vertically downwards and the solute less dense than the solute the convective flow is antiparallel with the flow caused by contraction on freezing. Identification of the mechanisms involved in these two problems is discussed. It is known that certain types of bulk liquid movements can inhibit channel development and the mechanism by which they do so are examined.

Hellawell, A.↗
NASA NTRS1991

Superalloy for high-temperature hydrogen environmental applications

A nickel-based superalloy is provided which is resistant to deterioration in hydrogen at high operating temperatures and pressures, and which thus can be used in hydrogen-fueled spacecraft such as the Space Shuttle. The superalloy is characterized by a two-phase microstructure and consists of a gamma-prime precipitated phase in a gamma matrix. The gamma matrix phase is a primary solid solution and the gamma precipitated phase will be an intermetallic compound of the type A.sub.3 B, such as nickel aluminide or titanide. Both phases are coherent, ordered, and compatible, and thus will retain most of their strength at elevated temperatures. The alloy consists essentially of (by weight): Ni 50-60%, Cr 10-20%, Al 2-6%, Co 2-5%, Ti 3-8%, W 5-12%, Mo 5-10%, Nb 1-3%, wherein the ratio W/MO is approximately equal to 1, and Ti/Al ranges from about 1 to about 2.

McKannan, Eugene C.↗
NASA NTRS1986

Crystallization microstructure in transparent monotectic alloys

The surprising funguslike morphology which occurs at the liquid(1)/liquid(2)/solid triple junction in directionally solidifying miscibility gap systems at the monotectic temperature is described. The monotectic temperature in a binary mixture is the one at which two immiscible solutions of the same two components form phases in equilibrium with the solid phase of one of the components. The growth of this morphology is described, and a similarity between it and that of a known crystal growth morphology is pointed out.

Kaukler, W. F.↗
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 NTRS1993

Thermosolutal convection during cellular arrayed growth of Pb-Sn alloys

Thermosolutal convection caused by the solute build up ahead of the growing arrays of cells and dendrites results in macrosegregation along the length of the Pb-Sn alloy (10 to 58 wt pct Sn) specimens when they are directionally solidified in a positive thermal gradient (melt on top, solid below, and gravity pointing down). At a constant thermal gradient, the extent of macrosegregation increases with decreasing growth speed as the microstructure changes from dendritic, to cellular and to planar. An empirical parameter, effective partition coefficient, obtained from the dependence of the longitudinal macrosegregation on fraction distance solidified can be used to represent the extent of macrosegregation.

Tewari, S. N.↗
NASA NTRS1999

In-Situ X-Ray Microscopy of Phase and Composition Distributions in Metal Alloys During Solidification

This research applies a state of the art X-ray Transmission Microscope, to image the solidification of metallic or semiconductor alloys in real-time. By employing a hard x-ray source with sub-micron dimensions, resolutions of up to 3 gm can be obtained with magnifications of over 800 X. Specimen growth conditions were optimized and the best imaging technologies applied to maintain x-ray image resolution, contrast and sensitivity. In addition, a special furnace design is required to permit controlled growth conditions and still offer maximum resolution and image contrast. We have successfully imaged in real-time: interfacial morphologies, phase growth, coalescence, incorporation of phases into the growing interface, and the solute boundary layer in the liquid at the solid-liquid inter-face. We have also measured true local growth rates and can evaluate segregation structures in the solid; a form of in-situ metallography. Composition gradients within the specimen cause vafiations in absorption of the flux such that the final image represents a spatial integral of composition (or thickness). During this study, the growth of secondary phase fibers and lameilae from eutectic and monotectic alloys have been imaged during solidification, in real-time, for the first time in bulk metal alloys. Keywords: x-ray, microscope, solidification, microfocus, real-time, microstructure

Kaukler, William F.↗
NASA NTRS1988

Enthalpies of a binary alloy during solidification

The purpose of the paper is to present a method of calculating the enthalpy of a dendritic alloy during solidification. The enthalpies of the dendritic solid and interdendritic liquid of alloys of the Pb-Sn system are evaluated, but the method could be applied to other binaries, as well. The enthalpies are consistent with a recent evaluation of the thermodynamics of Pb-Sn alloys and with the redistribution of solute in the same during dendritic solidification. Because of the heat of mixing in Pb-Sn alloys, the interdendritic liquid of hypoeutectic alloys (Pb-rich) of less than 50 wt pct Sn has enthalpies that increase as temperature decreases during solidification.

Poirier, D. R.↗
NASA NTRS1984

Influence of composition on the microstructure and mechanical properties of a nickel-base superalloy single crystal

The effects of cobalt, tantalum, and tungsten contents on the microstructure and mechanical properties of single crystal Mar-M247 were investigated. Elevated temperature tensile and creep-rupture properties of 001 oriented single crystals were related to microstructural features of the alloys. Substitution of Ni for Co in the high refractory metal alloys increased the lattice mismatch, which was considered to be the cause of the increases in tensile and creep strength. Substitution of Ni for Ta caused large decreases in tensile strength and creep life, consistent with decreases in gamma prime volume fraction, lattice mismatch, and solid solution hardening. Substitution of W for Ta resulted in decreased life at high stresses, which was related to small decreases in mismatch and volume fraction. However, the W substitution resulted in improved life at low stresses, which was related to solid solution strengthening by W.

Nathal, M. V.↗