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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.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Coarsening in binary solid-liquid mixtures

A theory of Ostwald ripening has been developed for a solid-liquid mixture cosisting of a low volume fraction array of spherical solid particles in a liquid wherein the coarsening process proceeds via the transport of both heat and mass. It is found that the simultaneous transport of heat and mass during ripening does not alter the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large decrease in the rate constant can be expected with small additions of solute to a pure metal. Possible extensions of this theory to the analogous problem of ripening in isothermal ternary alloys are also discussed.

Voorhees, P. W.↗
NASA NTRS2003

Alloy Interface Interdiffusion Modeled

With renewed interest in developing nuclear-powered deep space probes, attention will return to improving the metallurgical processing of potential nuclear fuels so that they remain dimensionally stable over the years required for a successful mission. Previous work on fuel alloys at the NASA Glenn Research Center was primarily empirical, with virtually no continuing research. Even when empirical studies are exacting, they often fail to provide enough insight to guide future research efforts. In addition, from a fundamental theoretical standpoint, the actinide metals (which include materials used for nuclear fuels) pose a severe challenge to modern electronic-structure theory. Recent advances in quantum approximate atomistic modeling, coupled with first-principles derivation of needed input parameters, can help researchers develop new alloys for nuclear propulsion.

Bozzolo, Guillermo H.↗
NASA NTRS1987

A critical examination of the dendrite growth models Comparison of theory with experimental data

Three dendrite growth models for directionally solidified succinonitrile-acetone, succinonitrile-salol, aluminum-copper, and lead-paladium alloys are evaluated. The characteristics of the Burden and Hunt (1974) model, the Laxmanan (1985) model, and the Trivedi (1980) model are described. The dendrite tip temperature, tip radius, liquid composition, and primary arm spacing for the alloys are analyzed in terms of growth speed, alloy composition, and temperature gradient. It is observed that the Burden and Hunt model accurately predicts the proper behavior of the parameters, but does not provide good quantitative predictions. A good fit between the experimental data and the Trivedi and Laxmanan models is detected. The advantages of the Trivedi marginal stability analysis and the Laxmanan minimum dendrite tip undercooling approaches are discussed.

Tewari, S. N.↗
NASA NTRS2015

Self-Repairing Fatigue Damage in Metallic Structures for Aerospace Vehicles Using Shape Memory Alloy Self-healing (SMASH) Technology

This DAA is for the Phase II webinar presentation of the ARMD-funded SMASH technology. A self-repairing aluminum-based composite system has been developed using liquid-assisted healing theory in conjunction with the shape memory effect of wire reinforcements. The metal matrix composite was thermodynamically designed to have a matrix with a relatively even dispersion of low-melting phase, allowing for repair of cracks at a pre-determined temperature. Shape memory alloy wire reinforcements were used within the composite to provide crack closure. Investigators focused the research on fatigue cracks propagating through the matrix in order to optimize and computer model the SMASH technology for aeronautical applications.

fatigue↗
NASA NTRS1982

Electrical characterization of Hg(1-x)Cd(x)Te alloys

Theoretical models are described for calculations of charge-carrier concentrations, Fermi energy, and conduction electron mobility as functions of x, temperature, and ionized and neutral defect concentrations of Hg(1-x)Cd(x)Te alloys. Measurements are reported of electron concentration and electron mobility from 5-300 K for alloys with x values of between 0.17 and 0.30. The electrical data are in reasonable agreement with theory, and were analyzed to obtain estimates of donor and acceptor state concentrations. The electron mobilities are calculated in terms of a microscopic theory of electrical conduction derived from the solution of the Boltzmann equation for the perturbed steady-state electron distribution function, and they show that longitudinal optical-phonon and charged and neutral defect scattering are the dominant mobility-limiting mechanisms.

Lehoczky, S. L.↗
NASA NTRS2019

The Effect Stacking Fault Segregation and Phase Transformations have on Creep Strength in Ni-based Superalloys

In this study, two commercially similar polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760C and 552MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active gamma prime shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a gamma prime -to- gamma phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a gamma prime-to-D019 phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.

Superalloy↗
NASA NTRS1939

Local instability of centrally loaded columns of channel section and Z-section

Charts are presented for the coefficients in formulas for the critical compressive stress at which cross-sectional distortion begins in a thin-wall member with either a channel section or a Z-section with identical flanges. The energy method of Timoshenko was used in the theoretical calculations required for the construction of the charts. The deflection equations were carefully selected to give good accuracy. The calculation of the critical compressive stress at stresses beyond the elastic range is briefly discussed. In order to demonstrate the use of the formulas and the charts in engineering calculations, two illustrative problems are included.

ELASTICITY↗
NASA NTRS1990

Thermomechanical deformation testing and modeling in the presence of metallurgical instabilities

A number of viscoplastic constitutive models were developed to describe deformation behavior under complex combinations of thermal and mechanical loading. Questions remain, however, regarding the validity of procedures used to characterize these models for specific structural alloys. One area of concern is that the majority of experimental data available for this purpose are determined under isothermal conditions. This experimental study is aimed at determining whether viscoplastic constitutive theories characterized using an isothermal data base can adequately model material response under the complex thermomechanical loading conditions typical of power generation service. The approach adopted was to conduct a series of carefully controlled thermomechanical experiments on a nickel-based superalloy, Hastelloy Alloy X. Previous investigations had shown that this material experiences metallurgical instabilities leading to complex hardening behavior, termed dynamic strain aging. Investigating this phenomenon under full thermomechanical conditions leads to a number of challenging experimental difficulties which up to the present work were unresolved. To correct this situation, a number of advances were made in thermomechanical testing techniques. Advanced methods for dynamic temperature gradient control, phasing control and thermal strain compensation were developed and incorporated into real time test control software. These advances allowed the thermomechanical data to be analyzed with minimal experimental uncertainty. The thermomechanical results were evaluated on both a phenomenological and microstructural basis. Phenomenological results revealed that the thermomechanical hardening trends were not bounded by those displayed under isothermal conditions. For the case of Hastelloy Alloy X (and similar dynamic strain aging materials), this strongly suggests that some form of thermomechanical testing is necessary when characterizing a thermoviscoplastic deformation model. Transmission electron microscopy was used to study the microstructural physics, and analyze the unique phenomenological behavior.

Castelli, Michael G.↗
NASA NTRS2003

Heats of Segregation of BCC Metals Using Ab Initio and Quantum Approximate Methods

Many multicomponent alloys exhibit surface segregation, in which the composition at or near a surface may be substantially different from that of the bulk. A number of phenomenological explanations for this tendency have been suggested, involving, among other things, differences among the components' surface energies, molar volumes, and heats of solution. From a theoretical standpoint, the complexity of the problem has precluded a simple, unified explanation, thus preventing the development of computational tools that would enable the identification of the driving mechanisms for segregation. In that context, we investigate the problem of surface segregation in a variety of bcc metal alloys by computing dilute-limit heats of segregation using both the quantum-approximate energy method of Bozzolo, Ferrante and Smith (BFS), and all-electron density functional theory. In addition, the composition dependence of the heats of segregation is investigated using a BFS-based Monte Carlo procedure, and, for selected cases of interest, density functional calculations. Results are discussed in the context of a simple picture that describes segregation behavior as the result of a competition between size mismatch and alloying effects

Good, Brian↗
NASA NTRS1991

Calculation of Gallium-metal-Arsenic phase diagrams

Electrical contacts and metallization to GaAs solar cells must survive at high temperatures for several minutes under specific mission scenarios. The determination of which metallizations or alloy systems that are able to withstand extreme thermal excursions with minimum degradation to solar cell performance can be predicted by properly calculated temperature constitution phase diagrams. A method for calculating a ternary diagram and its three constituent binary phase diagrams is briefly outlined and ternary phase diagrams for three Ga-As-X alloy systems are presented. Free energy functions of the liquid and solid phase are approximated by the regular solution theory. Phase diagrams calculated using this method are presented for the Ga-As-Ge and Ga-As-Ag systems.

Scofield, J. D.↗
NASA NTRS1999

Undercooling Limits and Thermophysical Properties in Glass Forming Alloys

The primary objective of this program is to produce deeply undercooled metallic liquids and to identify factors that limit undercooling and glass formation. The main research objectives are: (1) Investigating undercooling limits in glass-forming alloys and identifying factors that affect undercooling; (2) Measuring thermophysical properties and investigating the validity of the classical nucleation theory and other existing theories in the extreme undercooled states; and (3) To investigate the limits of electrostatic levitation technology in the ground base and to identify thermophysical parameters that might require reduced-g environment.

Rhim, Won-Kyu↗
NASA NTRS2003

The Solidification Velocity of Undercooled Nickel and Titanium Alloys with Dilute Solute

The study of solidification velocity is important for two reasons. First, understanding the manner in which the degree of undercooling of the liquid and solidification velocity affect the microstructure of the solid is fundamental. Second, there is disagreement between theoretical predictions of the relationship between undercooling and solidification velocity and experimental results. Thus, the objective of this research is to accurately and systematically quantify the solidification velocity as a function of undercooling for dilute nickel-and titanium-based alloys. The alloys chosen for study cover a wide range of equilibrium partition coefficients, and the results are compared to current theory.

Algoso, Paul R.↗
NASA NTRS2006

Quantitative Assessment of Fatigue Damage Accumulation in Wavy Slip Metals from Acoustic Harmonic Generation

A comprehensive, analytical treatment is presented of the microelastic-plastic nonlinearities resulting from the interaction of a stress perturbation with dislocation substructures (veins and persistent slip bands) and cracks that evolve during high-cycle fatigue of wavy slip metals. The nonlinear interaction is quantified by a material (acoustic) nonlinearity parameter beta extracted from acoustic harmonic generation measurements. The contribution to beta from the substructures is obtained from the analysis of Cantrell [Cantrell, J. H., 2004, Proc. R. Soc. London A, 460, 757]. The contribution to beta from cracks is obtained by applying the Paris law for crack propagation to the Nazarov-Sutin crack nonlinearity equation [Nazarov, V. E., and Sutin, A. M., 1997, J. Acoust. Soc. Am. 102, 3349]. The nonlinearity parameter resulting from the two contributions is predicted to increase monotonically by hundreds of percent during fatigue from the virgin state to fracture. The increase in beta during the first 80-90 percent of fatigue life is dominated by the evolution of dislocation substructures, while the last 10-20 percent is dominated by crack growth. The model is applied to the fatigue of aluminium alloy 2024-T4 in stress-controlled loading at 276MPa for which experimental data are reported. The agreement between theory and experiment is excellent.

Cantrell, John H.↗
NASA NTRS1985

Theoretical Studies of the Surface Tension of Liquid Metal System

A major goal of this project is to understand the surface tension and other thermophysical properties of liquid metals and alloys from a fundamental viewpoint. The approach is to calculate these quantities by a first principles technique which combines the statistical-mechanical theory of the liquid state with an electronic pseudopotential theory of electrons in metals. The inhomogeneity of the surface is treated using an ionic-density-functional formalism developed with the support of NASA. Of particular interest are the variation of surface tension with temperature and impurity concentration: such variations strongly influence the types of convection which make take place in a low-gravity environment. Some progress has already been achieved in computing the reduction of surface tension due to the presence of low-surface-tension impurities, and the corresponding surface segregation of such impurities. In the coming year, it is planned to concentrate on the surface properties of materials of particular interest to the MSA program: Si, Ga and GaSn alloys. An additional goal is to gain some theoretical understanding of the high temperature thermophysical properties of liquid metals, particularly high melting point materials which have not been studied extensively from a theoretical viewpoint.

Stroud, D. G.↗
NASA NTRS2004

Measuring the Thermodynamics of the Alloy/Scale Interface

A method is proposed for the direct measurement of the thermodynamic properties of the alloy and oxide compound at the alloy/scale interface observed during steady-state oxidation. The thermodynamic properties of the alloy/scale interface define the driving force for solid-state transport in the alloy and oxide compound. Accurate knowledge of thermodynamic properties of the interface will advance our understanding of oxidation behavior. The method is based on the concept of local equilibrium and assumes that an alloy+scale equilibrium very closely approximates the alloy/scale interface observed during steady-state oxidation. The thermodynamics activities of this alloy+scale equilibrium are measured directly by Knudsen effusion-cell mass spectrometer (KEMS) using the vapor pressure technique. The theory and some practical considerations of this method are discussed in terms of beta-NiAl oxidation.

Copland, Evan↗
NASA NTRS1985

Molecular orbital studies in oxidation: Sulfate formation and metal-metal oxide adhesion

The chemical mechanisms for sulfate formation from sodium chloride and sulfur trioxide, which is a product of jet fuel combustion was determined. Molten sodium sulfate leads to hot corrosion of the protective oxide layers on turbine blades. How yttrium dopants in nidkel-aluminum alloys used in turbine blades reduce the spalling rate of protective alumina films and enhance their adhesion was also determined. Two other fulfate mechanisms were deduced and structure of carbon monoxide on a clean chronium and clean platinum-titanium alloys surfaces was determined. All studies were by use of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Seven studies were completed. Their titles and abstracts are given.

Anderson, A. B.↗
NASA NTRS1995

Three-dimensional CTOA and constraint effects during stable tearing in a thin-sheet material

A small strain theory, three-dimensional elastic-plastic finite element analysis was used to simulate fracture in thin sheet 2024-T3 aluminum alloy in the T-L orientation. Both straight and tunneled cracks were modeled. The tunneled crack front shapes as a function of applied stress were obtained from the fracture surface of tested specimens. The stable crack growth behavior was measured at the specimen surface as a function of applied stress. The fracture simulation modeled the crack tunneling and extension as a function of applied stress. The results indicated that the global constraint factor, alpha(sub g), initially dropped during stable crack growth. After peak applied stress was achieved, alpha(sub g) began to increase slightly. The effect of crack front shape on alpha(sub g) was small, but the crack front shape did greatly influence the local constraint and through-thickness crack-tip opening angle (CTOA) behavior. The surface values of CTOA for the tunneled crack front model agreed well with experimental measurements, showing the same initial decrease from high values during the initial 3mm of crack growth at the specimen's surface. At the same time, the interior CTOA values increased from low angles. After the initial stable tearing region, the CTOA was constant through the thickness. The three-dimensional analysis appears to confirm the potential of CTOA as a two-dimensional fracture criterion.

Dawicke, D. S.↗