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Results for “Alloy theory”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Equivalent crystal theory of alloys

Equivalent Crystal Theory (ECT) is a new, semi-empirical approach to calculating the energetics of a solid with defects. The theory has successfully reproduced surface energies in metals and semiconductors. The theory of binary alloys to date, both with first-principles and semi-empirical models, has not been very successful in predicting the energetics of alloys. This procedure is used to predict the heats of formation, cohesive energy, and lattice parameter of binary alloys of Cu, Ni, Al, Ag, Au, Pd, and Pt as functions of composition. The procedure accurately reproduces the heats of formation versus composition curves for a variety of binary alloys. The results are then compared with other approaches such as the embedded atom and lattice parameters of alloys from pure metal properties more accurately than Vegard's law is presented.

Bozzolo, Guillermo↗
NASA NTRS1991

Effect of small scattering centers on the thermoelectric properties of p-type SiGe alloys

Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity. To investigate this prediction, ultrafine particulates (20 to 120 A) of silicon nitride have been added to boron-doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultrafine powder have lower thermal conductivities than standard SiGe, but high-temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25 percent in thermal conductivity has been achieved in these samples. The magnitude of the reduction agrees with theoretical predictions.

Beaty, John S.↗
NASA NTRS1989

High-temperature ordered intermetallic alloys III; Proceedings of the Third Symposium, Boston, MA, Nov. 29-Dec. 1, 1988

The present conference on high-temperature ordered intermetallic alloys discusses alloy theory and phase stability, defects and microstructures, mechanical behavior, alloy design and material processing, multiphase and composite materials, nickel aluminides, titanium aluminides, and other ordered intermetallics. Attention is given to compositional effects on processing and properties of nickel aluminides, dynamic compaction and hot-isostatic-pressing of nickel aluminides, improvement of elevated-temperature material properties in Ni3Al, the effect of microalloying B on the high-temperature mechanical properties of Ti3Al, the effects of structure on creep of Ti-53.4 mol pct Al intermetallics, the deformation microstructure in Ni3Si polycrystals strained over the range of temperature of flow stress anomaly, and the microstructure and mechanical properties of dual phase alloys consisting of the intermetallic phases.

Liu, C. T.↗
NASA NTRS1982

High temperature static strain sensor development program

The development of resistance strain gages which are useful for static strain measurements on nickel or cobalt superalloy parts inside a gas turbine engine on a test stand were examined. These measurements of a strain gage alloy development program which to be followed by an optional investigation of complete strain gage systems which will use the best of the alloys developed together with other system improvements is reviewed. The specific goal for the complete system is to make measurements to 2,000 micro epsilon with error of only + or - 10% over a 50 hour period. In addition to simple survival and stability, attaining a low thermal coefficient to resistivity, of order 100 ppm/K or less, is also a major goal. The first task was to select candidate alloys or alloy systems using a search of the literature and the available metallurgical theory. Alloy candidates were evaluated and compared by a grading system. Equipment and techniques were developed which are suitable for iterative studies of a variety of compositions. Many compositions were examined and significantly improved alloys were identified.

Hulse, C.↗
NASA NTRS1992

Electronic and total energy properties of ternary and quaternary semiconductor compounds, alloys and superlattices

This proposal was mainly concerned with the theoretical study of semiconductor compounds, alloys, and superlattices of interest for photovoltaic applications. In the last year (1991) a study was devoted to metal/graphite bonding in relation to use of graphite fiber reinforcement of Cu for high thermal conductivity applications. The main research topics addressed during the full period of the grant are briefly described: studies of the In-Ga-As ternary system; band-offsets at common anion and InAs/GaSb/AlSb heterojunctions; alloy theory (cluster variation method); and Cu/graphite bonding. Most of the work was described more extensively in previous yearly reports and renewal applications and in publications. The last topic is described more fully in a separate report attached. A list of publications resulting directly from this grant or from other grants but related to this work and of conference presentations is given at the end.

Lambrecht, Walter R. L.↗
DOE OSTI2025

Hydrogen and water interactions with CrMnFeCoNi alloy from density functional theory calculations

High entropy alloys (HEAs) are a promising class of materials with remarkable mechanical and catalytic properties. Among these, the quinary CrMnFeCoNi alloy (also called “Cantor alloy”) has attracted considerable attention given its thermodynamic stability and remarkable mechanical properties under different temperatures. Given that various degradation mechanisms involve multiple contaminants, such as hydrogen and water in hydrogen embrittlement and surface poisoning, respectively, understanding their interactions with the Cantor alloy is critical for its practical applications as structural, nuclear, or hydrogen storage material. In this work, we perform first-principles calculations based on Density Functional Theory (DFT) to investigate such interactions when considering various microstructures, including bulk materials and those containing certain defects, such as grain boundaries, stacking faults, and vacancies. We also employ Global Sensitivity Analysis to identify the importance of different factors in the stability of the impurities. We find that the accuracy of the H formation energy is significantly affected by spin polarization and chemical short-range order. The study also identifies a strong tendency for hydrogen interstitials to segregate to Σ5(210)/[001] symmetric tilt grain boundary, even when H concentrations are high, suggesting that a certain type of grain boundaries acts as H sinks within the alloy. Further, this result is reinforced by the low formation energy of vacancy-hydrogen complexes, which can contain multiple hydrogen atoms. Finally, the surface reactivity analysis reveals that the adsorption energy of oxygen and hydroxyl groups is highly sensitive to the specific metal atom involved in the binding, with a clear preference for chromium atoms, which could have implications for the alloy’s oxidation and corrosion behavior.

36 MATERIALS SCIENCE↗
NASA NTRS1990

High temperature inelastic deformation of the B1900 + Hf alloy under multiaxial loading - Theory and experiment

The multiaxial deformation behavior of the Ni-based alloy B1900 + Hf has been studied at elevated temperatures in the range of 649-982 C. Combined tension/torsion cyclic tests were performed on thin-wall tubular specimens under both in-phase and out-of-phase strain-controlled loading cycles. Both straining conditions resulted in stress loci of comparable magnitude, exhibiting no difference in cyclic hardening response. A phase angle was observed between the deviatoric stress and the incremental plastic strain vectors during 90-deg out-of-phase strain cycling, and nonproportional stress relaxation occurred under biaxial strain hold. The overall results have been used to assess the flow law, the hardening equations, and the applicability of the J2-based, elastic-viscoplastic model of Bodner-Partom (1979) for multiaxial loading conditions. The overall agreement between theory and experiment is good. Discrepancies are discussed in relation to micromechanical considerations.

Chan, K. S.↗
NASA NTRS1987

Dendritic solidification in a binary alloy melt - Comparison of theory and experiment

A simple model for 'constrained' growth of an 'array' of cells or dendrites in a binary alloy melt, in the presence of a positive temperature gradient in the liquid ahead of the tips, is presented. The cell or dendrite tip radius is calculated by adopting both the 'ad hoc' assumption of minimum undercooling at the tips as well as the more recently proposed hypothesis of dendritic growth under conditions of 'marginal stability'. Theoretical predictions of the model have been compared with experimental data in binary succinonitrile-acetone alloys. It has been shown that, according to the present model, the 'marginally stable' state may be virtually indistinguishable from the 'minimum undercooled' state, under the usual conditions of dendritic growth.

Laxmanan, V.↗
NASA NTRS1990

The effect of an electric field on the morphological stability of the crystal-melt interface of a binary alloy. III - Weakly nonlinear theory

The effect of a constant electric current on the crystal-melt interface morphology during directional solidification at constant velocity of a binary alloy is considered. A linear temperature field is assumed, and thermoelectric effects and Joule heating are neglected; electromigration and differing electrical conductivities of crystal and melt are taken into account. A two-dimensional weakly nonlinear analysis is carried out to third order in the interface amplitude, resulting in a cubic amplitude equation that describes whether the bifurcation from the planar state is supercritical or subcritical. For wavelengths corresponding to the most dangerous mode of linear theory, the demarcation between supercritical and subcritical behavior is calculated as a function of processing conditions and material parameters. The bifurcation behavior is a sensitive function of the magnitude and direction of the electric current and of the electrical conductivity ratio.

Wheeler, A. A.↗
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↗
NASA NTRS1982

Constitutive relationships for anisotropic high-temperature alloys

A constitutive theory is presented for representing the anisotropic viscoplastic behavior of high temperature alloys that posses directional properties resulting from controlled grain growth or solidification. The theory is an extension of a viscoplastic model that was applied in structural analyses involving isotropic metals. Anisotropy is introduced through the definition of a vector field that identifies a preferential (solidification) direction at each material point. Following the development of a full multiaxial theory, application is made to homogeneously stressed elements in pure shear and to a uniaxially stressed rectangular block in plane stress with the stress direction oriented at an arbitrary angle with the material direction. It is shown that an additional material parameter introduced to characterize the degree of anisotropy can be determined on the basis of simple creep tests.

Robinson, D. N.↗
NASA NTRS1983

Constitutive relationships for anisotropic high-temperature alloys

A constitutive theory is presented for representing the anisotropic viscoplastic behavior of high-temperature alloys that possess directional properties resulting from controlled grain growth or solidification. The theory is an extension of a viscoplastic model that has been applied in structural analyses involving isotropic metals. Anisotropy is introduced through the definition of a vector field that identifies a preferential (solidification) direction at each material point. Following the development of a full multiaxial theory, application is made to homogeneously stressed elements in pure shear and to a uniaxially stressed rectangular block in plane stress with the stress direction oriented at an arbitrary angle with the material direction. It is shown that an additional material parameter introduced to characterize the degree of anisotropy can be determined on the basis of simple creep tests.

Robinson, D. N.↗
NASA NTRS1988

Phase diagrams and microscopic structures of (Hg,Cd)Te, (Hg,Zn)Te, and (Cd,Zn)Te alloys

A cluster theory based on the quasi-chemical approximation has been applied to study the local correlation bond-length distribution, and phase diagrams of the II-VI pseudobinary alloys Hg(1 - x)Cd(x)Te, Hg(1 - x)Zn(x)Te, and Cd(1 - x)Zn(x)Te. The cluster energy is calculated by letting it relax in some effective alloy medium and then considering the contributions from the strain and chemical energies. Two different models are presented to simulate the alloy medium. While both models show that all three alloys have nearly random distributions, the signs of the local correlation prove to be sensitive to the alloy medium chosen for the energy calculation. Good agreement is found between experiment and the bond lengths and phase diagrams in both models.

Patrick, R. S.↗
DOE OSTI2025

Bayesian prior construction for uncertainty quantification in first-principles statistical mechanics

First-principles statistical mechanics enables the prediction of thermodynamic and kinetic properties of materials, but is computationally expensive. Many approaches require surrogate models to calculate energies within Monte Carlo or molecular dynamics simulations. Inexpensive surrogates such as cluster expansions enable otherwise intractable calculations by interpolating data from higher accuracy methods, such as Density Functional Theory (DFT). Surrogate models introduce uncertainty into downstream calculations, in addition to any uncertainty inherent to DFT calculations. Bayesian frameworks address this by quantifying uncertainty and incorporating expert knowledge through priors. However, constructing effective priors remains challenging. This work introduces and describes practical strategies for building Bayesian cluster expansions, focusing on basis truncation, hyperparameter selection, and ground state replication. We analyze multiple basis truncation schemes, compare cross-validation to the evidence-approximation for hyperparameter optimization, and provide methods to find and enforce ground-state-preserving models through priors. Additionally, we compare the uncertainties between different approximations to DFT (LDA, PBE, SCAN) against the uncertainty introduced with the use of cluster expansion surrogate models. These approaches are demonstrated on the BCC Li x Mg 1-x and Li x Al 1-x alloys, which are both of interest for solid-state Li batteries. Our results provide guidelines for constructing and utilizing Bayesian cluster expansions, thereby improving the transparency of materials modeling. Furthermore, the approaches and insights developed in this work can be transferred to a wide range of cluster expansion surrogate models, including the atomic cluster expansion and related machine-learned interatomic potential architectures.

Alloy theory↗
DOE OSTI2025

Accurate and uncertainty-aware multi-task prediction of HEA properties using prior-guided deep Gaussian processes

Surrogate modeling techniques have become indispensable in accelerating the discovery and optimization of high-entropy alloys (HEAs), especially when integrating computational predictions with sparse experimental observations. This study systematically evaluates the training and testing performance of four prominent surrogate models—conventional Gaussian processes (cGP), Deep Gaussian processes (DGP), encoder-decoder neural networks for multi-output regression and eXtreme Gradient Boosting (XGBoost)—applied to a hybrid dataset of experimental and computational properties of the 8-component HEA system Al-Co-Cr-Cu-Fe-Mn-Ni-V. We specifically assess their capabilities in predicting correlated material properties, including yield strength, hardness, modulus, ultimate tensile strength, elongation, and average hardness under dynamic/quasi-static conditions, alongside auxiliary computational properties. The comparison highlights the strengths of hierarchical deep modeling approaches in handling heteroscedastic, heterotopic, and incomplete data commonly encountered in materials science. Our findings illustrate that combined surrogate models such as DGPs infused with machine-learned priors outperform other surrogates by effectively capturing inter-property correlations and by assimilating prior knowledge. This enhanced predictive accuracy positions the combined surrogate models as powerful tools for robust and data-efficient materials design.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Coherent potential theory of a random binary alloy Effects of scattering from two-sites clusters and off-diagonal randomness

Extension of the single-site coherent potential approximation for random binary alloys to include the effect of off-diagonal randomness and pair scattering. This extension is achieved by analyzing a one-band model of a random binary alloy in terms of a two-sites coherent potential approximation. Numerical results are presented for a number of different alloys. In the overlapping-band case, the presence of off-diagonal randomness is shown to modify the bandwidths to values larger than those obtained from the virtual-crystal approximation. A simple iterative procedure is described for overcoming the convergence difficulties in the split-band case. In this limit, the inclusion of pair scattering and off-diagonal randomness is found to lead to the appearance of structure in the density of states of the minority component band.

Moorjani, K.↗