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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Empirical correlations between the arrhenius' parameters of impurities' diffusion coefficients in CdTe crystals

Understanding of self- and dopant-diffusion in semiconductor devices is essential to our being able to assure the formation of well-defined doped regions. In this paper, we compare obtained in the literature up to date the Arrhenius’ parameters (D=D 0 exp(–ΔE a /kT)) of point-defect diffusion coefficients and the I-VII groups impurities in CdTe crystals and films. We found that in the diffusion process there was a linear dependence between the pre-exponential factor, D 0 , and the activation energy, ΔE a , of different species: This was evident in the self-diffusivity and isovalent impurity Hg diffusivity as well as for the dominant IIIA and IVA groups impurities and Chlorine, except for the fast diffusing elements (e.g., Cu and Ag), chalcogens O, S, and Se, halogens I and Br as well as the transit impurities Mn, Co, Fe. As a result, reasons of the lack of correspondence of the data to compensative dependence are discussed.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Effects of Aluminum and Molybdenum on the Phase Stability of Iron-Chromium Alloys: A First-Principles Study

The interaction between solute atoms is critical to the thermodynamic behavior of Fe-Cr alloys, but the effects of non-dilute Al and Mo on the Fe-Cr phase stability and vacancy formation energy are not clearly understood. In this study, density functional theory, cluster expansion, and Monte Carlo simulation are used to predict the effects of ternary solute elements on the thermodynamic properties in multicomponent Fe-Cr alloys. The machine learning regression approach is applied to train and construct energy models that accurately describe the complex chemical interactions. The computational outcomes include the prediction of the partial ternary phase diagram, mixing enthalpy, and vacancy formation energy for different compositions. The phase boundary calculation predicts a pronounced change of Cr solubility in bcc Fe by the addition of Al and the rejection of Al atoms from a’ precipitates. The mixing enthalpy calculation shows that Mo may also reduce the Cr solubility in bcc Fe. Additionally, the simulations show that non-dilute Cr decreases the vacancy formation energy in bcc Fe, while adding Al results in a less significant effect. Finally, the results demonstrate important applications of using machine learning energy models to study model or commercial alloys with multicomponent solute species and point defects.

36 MATERIALS SCIENCE↗
DOE OSTI2024

1,4-cineole: a bio-derived solvent for highly stable graphene nanoplatelet suspensions and well-dispersed UHMWPE nanocomposite fibers

The exceptional properties of carbon nanoparticles, such as graphene, promise to expand the performance and functionality of many materials. The reinforcement of polymers is of keen interest due to their low density and flexible manufacturing methods. However, dispersing graphene in them has proven to be an enduring challenge due to the particles’ propensity to form performance degrading agglomerations. Furthermore, effective solvents for nanoparticle dispersion are commonly harmful, non-renewable, petrochemicals. In this work, a bio-derived solvent, 1,4-cineole, is demonstrated as a renewable alternative to these solvents that can be used to form highly stable graphene nanoplatelet (GnP) suspensions and used to gel spin well-dispersed UHMWPE/GnP nanocomposite fibers. The GnP concentration in the fibers was varied across three orders of magnitude, 0.01 wt% to 1 wt%, to examine its effect on fiber microstructure and properties. At low concentrations the particles act as point defects without affecting the fiber microstructure, and poor particle/matrix interfacial adhesion results in significantly reduced mechanical properties. At 1 wt% GnPs, a network effect takes hold thereby reinforcing the fibers, but the particles also impede the growth and orientation of crucial load-carrying crystalline structures in the fiber. Furthermore, unveiling the microstructural effects of GnPs on highly oriented and crystalline polymers in this study provides crucial insights for future work developing high-performance polymer nanocomposite fibers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Cavity Swelling in Irradiated Materials

Cavity swelling, due to three-dimensional clustering of vacancies, is an undesirable isotropic volume expansion of materials under irradiation. This phenomenon occurs above stage-III recovery temperature where vacancies are mobile, between ~0.3–0.6 Tm where Tm is the absolute melting temperature. The primary mechanism for cavity swelling is “dislocation bias”, i.e., preferential absorption of interstitials by network dislocations, causing a vacancy supersaturation. Cavity swelling is highly temperature dependent and typically follows a bell-shaped curve with peak swelling occurring at an intermediate temperature. With increasing temperature, cavity size increases, and number density decreases logarithmically. With increasing dose, cavity swelling increases, but follows a hockey-stick type pattern, characterized by a low-swelling transient incubation period and then a steady-state regime. The incubation period, dominant in cavity nucleation, is dependent upon material’s chemistry and irradiation conditions such as dose rate/temperature. Cavity swelling increases linearly with dose in the steady-state regime, that is independent of chemistry and dose rate. This regime is dominated by cavity growth. Transmutation gases like He enhance cavity nucleation such that an early onset of cavity formation occurs. However, swelling magnitudes can be either increased or suppressed when He is present. Results in austenitic steels and Cu indicate that cavity swelling peaks at intermediate He/dpa ratios of ~10 appm He/dpa. The most effective method to mitigate cavity swelling is by increasing the fixed point-defect sink density, by promoting precipitation or dispersion strengthening.

Bhattacharya, Arunodaya↗
DOE OSTI2020

Irradiation Creep in Materials

A knowledge of the dimensional stability of reactor structural components, under irradiation conditions, is of major importance in the design of thermal, fast, and fusion reactors. When subjected to simultaneous mechanical loading and irradiation, structural materials exhibit a visco-plastic deformation phenomenon, referred to as irradiation creep, which can be more rapid than the deformation occurring out of irradiation. In this article, the phenomenology of this peculiar behavior is described after a short history of its discovery. Then, the theoretical mechanisms proposed in the literature during these past 60 years are presented, with a special focus on mechanisms based on stress induced preferred absorption of point defects by dislocation loops, dislocations and grain boundaries and on mechanisms based on climb-enhanced glide of dislocations. These mechanisms are discussed in the light of experimental evidences. Finally, irradiation creep in various materials, such as zirconium alloys, austenitic stainless steels, nickel-based alloys, ferritic-martensitic steels and graphite, is described.

Onimus, Fabien↗
DOE OSTI2013

General Schema for [001] Tilt Grain Boundaries in Dense Packing Cubic Crystals

Atomic resolution Z-contrast images from a series of CeO2 [001] tilt grain boundaries at coincident site lattice (CSL) or near-CSL misorientations can all be explained within a structural unit model. These structural units (which cover all boundaries from 0-90 degrees) show striking similarities to comparable CSL boundaries observed in cubic crystal structures that are also derived from dense packing (fcc metal, rocksalt, perovskite etc). A general model for the structure of grain boundaries in such similarly structured materials systems has been developed that is based on the crystallography of the parent structures. Changes away from these predicted grain boundary symmetries can be interpreted as showing the frustration of symmetry caused by the incorporation of point defects (vacancies and impurities). This general model for grain boundary structures can, in principle, provide a means to infer the structure-property relationships in broad classes of materials.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Segregation and trapping of oxygen vacancies near the SrTiO 3 Σ3 (112) [110] tilt grain boundary

In nanocrystalline materials, structural discontinuities at grain boundaries (GBs) and the segregation of point defects to these GBs play a key role in defining the structural stability of a material, as well as its macroscopic electrical/mechanical properties. In this study, the segregation of oxygen vacancies near the Σ3 (1 1 2) [¯110] tilt GB in SrTiO 3 is explored using density functional theory. We find that oxygen vacancies segregate toward the GB, preferring to reside within the next nearest-neighbor layer. This oxygen vacancy segregation is found to be crucial for stabilizing this tilt GB. Furthermore, we find that the migration barriers of oxygen vacancies diffusing toward the first nearest-neighbor layer of the GB are low, while those away from this layer are very high. Furthermore, the segregation and trapping of the oxygen vacancies in the first nearest-neighbor layer of GBs are attributed to the large local distortions, which can now accommodate the preferred sixfold coordination of Ti. These results suggest that the electronic, transport, and capacitive properties of SrTiO 3 can be engineered through the control of GB structure and grain size or layer thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Irradiation-induced formation of a spinel phase at the FeCr/MgO interface

Oxide dispersion strengthened ferritic/martensitic alloys have attracted significant attention for their potential uses in future nuclear reactors and storage vessels, as the metal/oxide interfaces act as stable high-strength sinks for point defects while also dispersing helium. Here, in order to unravel the evolution and interplay of interface structure and chemistry upon irradiation in these types of materials, an atomically sharp FeCr/MgO interface was synthesized at 500 °C and separately annealed and irradiated with Ni 3+ ions at 500 °C. After annealing, a slight enrichment of Cr atoms was observed at the interface, but no other structural changes were found. However, under irradiation, sufficient Cr diffuses across the interface into the MgO to form a Cr-enriched transition layer that contains spinel precipitates. First-principles calculations indicate that it is energetically favorable to incorporate Cr, but not Fe, substitutionally into MgO. Furthermore, our results indicate that irradiation can be used to form new phases and complexions at interfaces, which may have different radiation tolerance than the pristine structures.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Atomic-scale properties of Ni-based FCC ternary, and quaternary alloys

The aim of our study is to characterize some atomic-scale properties of Ni-based FCC multicomponent alloys. For this purpose, we use Monte Carlo method combined with density functional theory calculations to study short-range order (SRO), atomic displacements, electronic density of states, and magnetic moments in equimolar ternary NiCrCo, and quaternary NiCrCoFe alloys. The salient features for the ternary alloy are a negative SRO parameter between Ni Cr and a positive between Cr Cr pairs as well as a weakly magnetic state. For the quaternary alloy we predict negative SRO parameter for Ni Cr and Ni Fe pairs and positive for Cr Cr and Fe Fe pairs. Atomic displacements for both ternary and quaternary alloys are negligible. In contrast to the ternary, the quaternary alloy shows a complex magnetic structure. The electronic structure of the ternary and quaternary alloys shows differences near the Fermi energy between a random solid solution and the predicted structure with SRO. Despite that, the calculated EXAFS spectra does not show enough contrast to discriminate between random and ordered structures. Finally, the predicted SRO has an impact on point-defect energetics, electron phonon coupling and thermodynamic functions and thus, SRO should not be neglected when studying properties of these two alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Response of 14YWT alloys under neutron irradiation: A complementary study on microstructure and mechanical properties

Nanostructured ferritic alloys (NFAs) having sub-micron grain size with a high density of nano-oxides (NOs) (size of ~2-3 nm) are one of the best candidates for structural components in Generation IV nuclear systems. In this study, 14YWT NFA cladding tubes were irradiated in BOR60 reactor up to 7 dpa at 360-370 °C. Detailed microstructural analysis has been conducted using bright field transmission electron microscopy, bright field scanning transmission electron microscopy, energy filtered transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and transmission Kikuchi diffraction techniques. This revealed cavities, <100> and <111> type dislocation loops, and α' precipitates forming after irradiation with relationships between cavities and NOs, and α' precipitates and NOs. Cavities mostly form on the NOs; whereas, α' precipitates form between the NOs where the point defect concentration is high. Moreover, α' precipitates are distributed homogenously on and around the dislocation loops which is consistent with the observation that there is no Cr segregation on dislocation loops. Grain boundaries were found to be mostly depleted in Cr; however, the characteristics of each grain boundary determines the Cr behavior and the α' denuded zone around the grain boundaries. Mechanical properties of the irradiated tubes have been determined by using both low force and high force nanoindentation techniques, resulting in 1.03±0.33 GPa and 0.82±0.20 GPa hardening, respectively. Dispersed barrier hardening calculations and nanoindentation measurements are in good agreement. In this paper, 14YWT NFA has been systematically studied after neutron irradiation to better understand its superior performance: low α' concentration, low swelling and low radiation-induced hardening.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Hierarchically-structured large superelastic deformation in ferroelastic-ferroelectrics

Large superelastic deformation in ferroelastic-ferroelectrics (FMs) is a complex phenomenon involving multiple mechanisms operating simultaneously. Understanding how these mechanisms contribute corporately is critical to apply this useful property to the intrinsically brittle FMs, which can therefore display both excellent functional and mechanical performance. Here, we have directly observed and quantitatively analyzed in situ in a transmission electron microscope the three main mechanisms of twinning domain, phase transformation and mobile point defect contributing to extremely large superelastic deformation in single-crystal BaTiO 3 (5.0% strain) and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (10.1% strain). Finally, our results reveal the hierarchical origin of large recoverable strain in “brittle” FMs.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Neutron irradiation-induced microstructure damage in ultra-high temperature ceramic TiC

Titanium carbide (TiC) is an ultra-high temperature ceramic with potential as a structural material candidate for advanced reactor concepts. However, the irradiation tolerance of TiC is not well understood. Here, we reveal the key irradiation damage microstructure degradation processes in TiC using mixed spectrum neutron irradiations at dose of ~2 displacements per atom (dpa) at temperatures of ~220, 620, and 1115 °C, combined with state-of-art microstructure characterization using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The helium injection (~65 atomic parts per million) produced by neutron transmutation also occurred in the sample. TiC was observed to form irradiation-induced interstitial-type dislocation loops and He-stabilized cavities. At 220 and 620 °C, the analysis of the electron diffraction patterns, rel-rod imaging and HRTEM revealed that the dislocation loops were faulted Frank loops with Burgers vector b Frank = 1/3<111> lying on {111} planes. A detailed Burgers vector identification performed by the g.b technique revealed that the dislocation loops forming at 1115 °C were unfaulted, edge-type, with Burgers vectors b = a<100> and a/2<110> with corresponding {100} and {110} habit planes. Using continuum mechanics, we estimated the critical radius at which a faulted dislocation loop transitions to a perfect loop to be 9 nm. Further, no amorphization occurred in TiC under-investigated irradiation conditions while macroscopic swelling under point defect swelling regime was observed. He-stabilized cavities were detected at 1115 °C, but not at lower temperature irradiation. These observations indicate the onset of long-range vacancy migration in TiC occurs between 620 and 1115 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Vacancy formation energies and migration barriers in multi-principal element alloys

Multi-principal element alloys (MPEAs) continue to garner interest as structural and plasma-facing materials due to their structural (phase) stability and increased resistance to radiation damage. Despite sensitivity of mechanical behavior to irradiation and point-defect formation, there has been scant attention on understanding vacancy stability and diffusion in refractory-based MPEAs. Using density-functional theory, we examine vacancy stability and diffusion barriers in body-centered cubic (Mo 0.95 W 0.05 ) 0.85 Ta 0.10 (TiZr) 0.05 . The results in this MPEA show strong dependence on environment, originating from local lattice distortion associated with charge-transfer between neighboring atoms that vary with different chemical environments. We find a correlation between degree of lattice distortion and migration barrier: (Ti, Zr) with less distortion have lower barriers, while (Mo, W) with larger distortion have higher barriers, depending up local environments. Under irradiation, our findings suggest that (Ti, Zr) are significantly more likely to diffuse than (Mo, W) while Ta shows intermediate effect. Finally, material degradation caused by vacancy diffusion can be controlled by tuning composition of alloying elements to enhance creep strength at extreme operating temperatures and harsh conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Direct comparison between experiments and dislocation dynamics simulations of high rate deformation of single crystal copper

A long standing challenge in computational materials science is to establish a quantitative connection between the macroscopic properties of plastic deformation with the microscopic mechanisms of dislocations in crystalline materials. Although the discrete dislocation dynamics (DDD) simulation method has been developed for several decades with the goal of addressing this challenge, a one-to-one comparison between the DDD predictions on single crystal stress–strain curves and experimental measurements under identical conditions has not been possible to date. Such a comparison is an essential step towards establishing a dislocation-physics based theory of plasticity and a multiscale framework of the plastic behaviors of crystalline materials. Here we provide direct comparisons between the stress–strain curves of Cu single crystals under high strain rate loading in the [0 0 1] and [0 1 1] directions obtained from miniaturized desktop Kolsky bar experiments and those from DDD simulations under identical loading conditions. With an appropriate set of parameters, DDD simulations can produce stress–strain curves that are in reasonable agreement with the experimental results. However, the dislocation mobility values needed to achieve this agreement are an order of magnitude lower than expected based on previous measurements and atomistic simulations. We hypothesize that this discrepancy could be caused by drag forces from jogs and point defects produced during the plastic deformation. Cross-slip of screw dislocations is also found to be necessary to capture the experimental stress–strain behavior, especially for the [0 1 1] loading direction. Finally, this work provides an example of how direct comparisons between DDD simulations and experimental measurements can provide new insight into the fundamental mechanisms of plastic deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Irradiation damage reduces alloy corrosion rate via oxide space charge compensation effects

Radiation effects in materials often compound and accelerate other detrimental phenomena such as embrittlement, oxidation and creep. However, irradiation can also decrease the oxidation rate, for instance with ZrNb alloys nuclear fuel cladding. In this study, we rationalize this observation on Zr-0.5Nb alloy by introducing a mechanism based on oxide space charge modification, resulting from irradiation enhanced Nb clustering. Here, this mechanism is investigated using a multiscale approach: from the macroscale, to determine post-irradiation oxidation kinetics, to the atomic scale, using in-situ atom probe tomography sample oxidation, to observe elemental solute redistribution across the oxide/metal interface. The mechanism is further supported by high resolution transmission electron microscopy characterization and density functional theory calculations. A point defect model is proposed to account for oxide space charge effects and their changes under irradiation. This integrated, multiscale experimental and modeling approach challenges the current paradigm on irradiation effects and how they can potentially improve materials performance in extreme environments.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Degradation of electrical resistivity of tungsten following shielded neutron irradiation

A major challenge for heat transfer in nuclear materials is to ensure thermal mobility after high amounts of neutron irradiation. Tungsten is widely selected as a heat transfer material in fusion reactors. In metals, thermal conductivity is dominated by electrons’ ability to transfer energy. Neutron irradiation generates point defects, clusters, and solid transmutation (e.g.rhenium and osmium in tungsten), which inhibit electron motion. The purpose of this work is to quantify the irradiation-induced change in electron mobility and deconvolute transmutation and microstructural effects on observed changes to electron mobility. Single and polycrystalline tungsten were fast neutron irradiated in the High Flux Isotope Reactor at Oak Ridge National Laboratory to doses between 0.2 and 0.7 displacements per atom (dpa) and temperatures from 500 °C to 1000 °C. Grain growth was observed in all samples. Microstructure and transmutation were quantified. The geometric orientation of samples with elongated grains has been shown to affect electrical resistivity. A mathematical model was developed and used to deconvolute solid-solution transmutation, grain, and temperature-dependent lattice effects on resistivity. At ~0.4 dpa at ~590 °C, the combined resistivity degradation due to voids, vacancies, interstitials, and dislocations is estimated to be greater than the contribution from solid solution Re transmutation, which is greater than the contribution from grain boundaries. At doses of ~0.7 dpa at ~750 °C, solid solution Re contributions are greater than all other effects combined. As a result, this work establishes a basis to predict the effects of irradiation temperature and transmutation on thermal properties of tungsten and highlights the importance of irradiation temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Elucidating the role of Cr migration in Ni-Cr exposed to molten FLiNaK via multiscale characterization

Structural materials used in nuclear reactor environments are subjected to coupled extremes such as high temperature, irradiation, and corrosion which act in concert to degrade their functional performance. Connecting alloy microstructure such as grain boundaries, and accumulating point defects with corrosion attack and pore morphology is critical to understanding underlying mechanisms. We uncover the compositional variations and morphology at multiple length scales in corrosion-damaged Ni-Cr alloy after exposure to oxidants in molten fluoride salts. A complex network of dense corrosion pores is detected by surface-level SEM observations. The corrosion pores take on a 1-dimensional morphology and are enriched with Ni and depleted of Cr 1–5 µm from the pore surface. STEM-EDS and 4D-STEM strain maps acquired simultaneously highlight the local variations in composition and structure of a ≤ 200 nm Cr-rich layer identified from a cross-section taken at the bottom of an isolated corrosion pore between the Ni-Cr alloy matrix and the embedded salt. However, the absence of an observed interface between the Ni-Cr alloy matrix and the FCC-structured Cr-rich layer suggests that Cr plating from the salt did not transpire. These findings support a proposed Cr lattice diffusion mechanism rather than Cr-precipitation from the salt to accommodate temperature transient conditions during sample cooling. Through the development of a 1D phase field model, these results are rationalized by formation energies for the Ni- and Cr-oxidation into the molten salt. This study reveals the locally altered microstructure caused by high temperature corrosion in non-steady-state molten salt nuclear reactor environments.

36 MATERIALS SCIENCE↗