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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Heat storage in alloy transformations

The theory of eutectic transformation was examined to find guidelines to the best material combinations to examine. The heats of transformation were measured calorimetrically, and the volume changes of expanding solid mixtures and homogeneous liquid solutions, especially during the transformation between the two states at fixed temperature, were measured by changes in X-ray absorption. Heat flow models appropriate to storage in phase change materials were developed along with efficient calculating procedures so that the relative importance of the problems associated with energy storage density, heat conduction, and similar properties could be assessed.

Birchenall, C. E.↗
NASA NTRS1984

Dendritic solidification in a binary alloy melt: Steady-state versus morphological stability theories

A model for dendritic growth in a binary alloy melt, in the presence of a positive temperature gradient in the liquid, is presented. The model describes satisfactorily the transition from a dendritic interface to a planar interface at very low and very large growth rates. A dendrite tip stability parameter is derived, strictly from steady state considerations, without resorting to a perturbation analysis. The estimates of the parameter agree with those obtained by models based on perturbation analyses.

Laymanan, V.↗
NASA NTRS2005

New Theoretical Technique for Alloy Design

During the last 2 years, there has been a breakthrough in alloy design at the NASA Lewis Research Center. A new semi-empirical theoretical technique for alloys, the BFS Theory (Bozzolo, Ferrante, and Smith), has been used to design alloys on a computer. BFS was used, along with Monte Carlo techniques, to predict the phases of ternary alloys of NiAl with Ti or Cr additions. High concentrations of each additive were used to demonstrate the resulting structures.

Ferrante, John↗
NASA NTRS1974

Stress corrosion cracking of titanium alloys

The effect of hydrogen on the properties of metals, including titanium and its alloys, was investigated. The basic theories of stress corrosion of titanium alloys are reviewed along with the literature concerned with the effect of absorbed hydrogen on the mechanical properties of metals. Finally, the basic modes of metal fracture and their importance to this study is considered. The experimental work was designed to determine the effects of hydrogen concentration on the critical strain at which plastic instability along pure shear directions occurs. The materials used were titanium alloys Ti-8Al-lMo-lV and Ti-5Al-2.5Sn.

Statler, G. R.↗
NASA NTRS1999

Effect of a Nonplanar Melt-Solid Interface On Lateral Compositional Distribution During Unidirectional Solidification of a Binary Alloy with a Constant Growth Velocity V: Theory - Pt. 1

Infrared detected materials, such as Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te have energy gaps almost linearly proportional to their composition. Due to the wide separation of liquidus and solidus curves of their phase diagram, there are compositional segregation in both of the axial and radial directions of these crystals grown in the Bridgman system unidirectionally with constant growth rate. It is important to understand the mechanisms, which affect lateral segregation such that large radially uniform composition crystal can be produced. Following Coriel, etc's treatment, we have developed a theory to study the effect of a curved melt-solid interface shape on lateral composition distribution. The model is considered to be a cylindrical system with azimuthal symmetry and a curved melt-solid interface shape which can be expressed as a linear combination of a series of Bessell's functions. The results show that melt-solid interface shape has a dominant effect on the lateral composition distribution of these systems. For small values of beta, the solute concentration at the melt-solid interface scales linearly with interface shape with a proportional constant of the produce of beta and (1 -k), where beta = VR/D, with V as growth velocity, R as the sample radius, D as the diffusion constant and k as the distribution constant. A detailed theory will be presented. A computer code has been developed and simulations have been performed and compared with experimental results. These will be published in another paper.

Wang, Jai-Ching↗
NASA NTRS1999

Effect of a Nonplanar Melt-Solid Interface on Lateral Compositional Distribution during Unidirectional Solidification of a Binary Alloy with a Constant Growth Velocity V: Theory - Part 1

Infrared detector materials, such as Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te have energy gaps almost linearly proportional to its composition. Due to the wide separation of liquidus and solidus curves of their phase diagram, there are compositional segregations in both of axial and radial directions of these crystals grown in the Bridgman system unidirectionally with constant growth rate. It is important to understand the mechanisms which affect lateral segregation such that large uniform radial composition crystal is possible. Following Coriell, etc's treatment, we have developed a theory to study the effect of a curved melt-solid interface shape on the lateral composition distribution. The system is considered to be cylindrical system with azimuthal symmetric with a curved melt-solid interface shape which can be expressed as a linear combination of a series of Bessell's functions. The results show that melt-solid interface shape has a dominate effect on lateral composition distribution of these systems. For small values of b, the solute concentration at the melt-solid interface scales linearly with interface shape with a proportional constant of the product of b and (1 - k), where b = VR/D, with V as growth velocity, R as sample radius, D as diffusion constant and k as distribution constant. A detailed theory will be presented. A computer code has been developed and simulations have been performed and compared with experimental results. These will be published in another paper.

Wang, Jai-Ching↗
NASA NTRS1985

Theoretical Problems in Materials Science

Interactions between theoretical physics and material sciences to identify problems of common interest in which some of the powerful theoretical approaches developed for other branches of physics may be applied to problems in materials science are presented. A unique structure was identified in rapidly quenched Al-14% Mn. The material has long-range directed bonds with icosahedral symmetry which does not form a regular structure but instead forms an amorphous-like quasiperiodic structure. Finite volume fractions of second phase material is advanced and is coupled with nucleation theory to describe the formation and structure of precipitating phases in alloys. Application of the theory of pattern formation to the problem of dendrite formation is studied.

Langer, J. S.↗
NASA NTRS1995

Dynamic Instability of Solidification Fronts and Formation of Transverse Layered Structures

A plane front of crystallization in a unidirectional process of solidification of a binary melt or a solution is shown to be unstable with respect to virtual perturbations of the front velocity within some region of physical and operating parameters. This dynamic instability has nothing in common with morphological instability of the front also possible under certain conditions. It gives rise to the origination of a self-oscillating regime of solidification. The oscillations of the front velocity are accompanied by oscillations of other relevant variables, such as the melt concentration and temperature at the front. What is most important is that this provides for a natural explanation of phenomena of the formation of ordered striations commonly observed when producing alloys and withdrawing crystals. A theory of nonlinear instability of the solidification front is developed which also allows for properties of both self-oscillating regimes of solidification and transverse layered structures it allows to be described in an explicit form, without introducing any additional hypotheses and empirical parameters. Apart from other things, the properties are strongly dependent on convective transport originated in consequence of a difference between the densities of the liquid melt and solid alloy. The theory results in a remarkably good agreement with experimental evidence on crystal withdrawal obtained in normal gravity and under space flight conditions. The problem of interplay between modes of the dynamic and morphological types of instability is also discussed. There are parametric regions in which either only one of these types occurs or both of them arise simultaneously.

Buyevich, Yu A.↗
NASA NTRS1986

Containerless processing of hypermonotectic and glass forming alloys using the Marshall Space Flight Center 100 meter drop tube facility

Two separate projects were carried out to study alloys whose solidification structures can be strongly influenced by the presence of a container during melting and solidifications. One project involved containerless solidification of hypermonotectic Au35Rh65 alloys. This alloy exhibits liquid immiscibility over a temperature range. It has been suggested that containerless melting might be one solution to the problem of sedimentation in the dispersions of immiscible liquid phases. However, surface tension driven flows could also lead to accumulation of the minority liquid phase at the external surface of a containerlessly melted alloy. The research underway is a first step in determining the influence of containerless, microgravity processing on immiscible alloys. Nickel-niobium alloys were studied using the drop tube facility. One alloy in this system, a Ni60Nb40 alloy, is a good candidate for the formation of a bulk metallic glass. Amorphous alloys of this composition were produced using thin film and mechanical alloying techniques. However, theory indicates that if heterogeneous nucleation can be avoided, it should be possible to produce an amorphous structure in this system using a moderate cooling rate from the melt. The containerless melting and solidification capabilities of the drop tube faciltiy provide ideal conditions for a study of this type. To date, several Ni60Nb40 samples have been levitated, melted and cooled during 4.6 seconds of free fall in the 100 meter drop tube. Structures obtained are discussed.

Andrews, J. B.↗
NASA NTRS1987

High-temperature ordered intermetallic alloys II; Proceedings of the Second Symposium, Boston, MA, Dec. 2-4, 1986

The papers presented in this volume provide on overview of recent theoretical and experimental research in the field of high-temperature ordered intermetallic alloys. The papers are gouped under the following headings: ordering behavior and theory, microstructures, mechanical behavior, alloy design and microstructural control, and metallurgical properties. Specific topics discussed include antiphase domains, disordered films and the ductility of ordered alloys based on Ni3Al; kinetics and mechanics of formation of Al-Ni intermetallics; deformability improvements of L1(2)-type intermetallic compounds; B2 aluminides for high-temperature applications; and rapidly solidified binary TiAl alloys.

Stoloff, N. S.↗
NASA NTRS2010

Bonding and Microstructural Stability in Ni55Ti45 Studied by Experimental and Theoretical Methods

Spiral orbit tribometry friction tests performed on Ni-rich Ni55Ti45 titanium ball bearings indicate that this alloy is a promising candidate for future aerospace bearing applications. Microstructural characterization of the bearing specimens was performed using transmission electron microscopy and energy dispersive spectroscopy, with NiTi, Ni4Ti3, Ni3Ti, and Ni2Ti4Ox phases identified within the microstructure of the alloy. Density functional theory was applied to predict the electronic structure of the NixTiy phases, including the band structure and site projected density of states. Ultraviolet photoemission spectroscopy was used to verify the density of states results from the density functional theory calculations, with good agreement observed between experiment and theory.

Stott, Amanda C.↗
NASA NTRS1986

New vistas in the determination of hydrogen in aerospace engine metal alloys

The application of diffusion theory to the analysis of hydrogen desorption data has been studied. From these analyses, important information concerning hydrogen solubilities and the nature of the hydrogen distributions in the metal has been obtained. Two nickel base alloys, Rene' 41 and Waspaloy, and one ferrous alloy, 4340 steel, were studied in this work. For the nickel base alloys, it was found that the hydrogen distributions after electrolytic charging conformed closely to those which would be predicted by diffusion theory. The hydrogen distributions in electrolytically charged 4340 steel, on the other hand, were essentially uniform in nature, which would not be predicted by diffusion theory. Finally, it has been found that the hydrogen desorption is completely explained by the nature of the hydrogen distribution in the metal, and that the 'fast' hydrogen is not due to surface and subsurface hydride formation, as was originally proposed.

Danford, M. D.↗
NASA NTRS1985

High temperature static strain gage alloy development program

The literature, applicable theory and finally an experimental program were used to identify new candidate alloy systems for use as the electrical resistance elements in static strain gages up to 1250K. The program goals were 50 hours of use in the environment of a test stand gas turbine engine with measurement accuracies equal to or better than 10 percent of full scale for strains up to + or - 2000 microstrain. As part of this effort, a computerized electrical resistance measurement system was constructed for use at temperatures between 300K and 1250K and heating and cooling rates of 250K/min and 10K/min. The two best alloys were an iron-chromium-aluminum alloy and a palladium base alloy. Although significant progress was made, it was concluded that a considerable additional effort would be needed to fully optimize and evaluate these candidate systems.

Hulse, C. O.↗
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 NTRS1991

Theoretical modelling of AFM for bimetallic tip-substrate interactions

Recently, a new technique for calculating the defect energetics of alloys based on Equivalent Crystal Theory was developed. This new technique successfully predicts the bulk properties for binary alloys as well as segregation energies in the dilute limit. The authors apply this limit for the calculation of energy and force as a function of separation of an atomic force microscope (AFM) tip and substrate. The study was done for different combinations of tip and sample materials. The validity of the universality discovered for the same metal interfaces is examined for the case of different metal interactions.

Bozzolo, Guillermo↗
NASA NTRS2002

A Nonlinear Theory for Smart Composite Structures

The paper discusses the following: (1) Development of a completely coupled thermo-piezoelectric-mechanical theory for the analysis of composite shells with segmented and distributed piezoelectric sensor/actuators and shape memory alloys. The higher order displacement theory will be used to capture the transverse shear effects in anisotropic composites. The original theory will be modified to satisfy the stress continuity at ply interfaces. (2) Development of a finite element technique to implement the mathematical model. (3) Investigation of the coupled structures/controls interaction problem to study the complex trade-offs associated with the coupled problem.

Chattopadhyay, Aditi↗