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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Microstructural and electrical changes in nickel manganite powder induced by mechanical activation

Highlights: {yields} The influence of mechanical activation on microstructure evolution in the nickel manganite powder was investigated as well as electrical properties of the sintered samples. {yields} Structural refinement obtained by Topas-Academic software based on Rietveld analysis showed that the milling process remarkably changed the powder morphology and microstructure. {yields} SEM studies of sintered samples also revealed the strong influence of milling time on ceramics density (increases with milling time). {yields} The electrical properties of ceramic samples are clearly conditioned by terms of synthesis, in our case the time of mechanical activation. {yields} The highest density and higher values of dielectric constant were achieved at the sample activated for 45 min. -- Abstract: Nickel manganite powder synthesized by calcination of a stoichiometric mixture of manganese and nickel oxide was additionally mechanically activated in a high energy planetary ball mill for 5-60 min in order to obtain a pure NiMn{sub 2}O{sub 4} phase. The as-prepared powders were uniaxially pressed into disc shape pellets and then sintered for 60 min at 1200 {sup o}C. Changes in the particle morphology induced by mechanical activation were monitored using scanning electron microscopy, while changes in powder structural characteristics were followed using X-ray powder diffraction. The ac impedance spectroscopy was performed on sintered nickel manganite samples at 25 {sup o}C, 50 {sup o}C and 80 {sup o}C. It was shown that mechanical activation intensifies transport processes causing a decrease in the average crystallites size, while longer activation times can lead to the formation of aggregates, defects and increase of lattice microstrains. The observed changes in microstructures were correlated with measured electrical properties in order to define optimal processing conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Analyzing the effect of pressure on the properties of point defects in $\gamma\mathrm{U–Mo}$ through atomistic simulations

Uranium–molybdenum (U–Mo) alloys in monolithic fuel foil are the primary candidate for the conversion of high-performance research reactors in the USA. Monolithic fuel is utilized in a plate-type design with a zirconium diffusion barrier and aluminum cladding. These fuel types are unique in that they contain no plenum for the release of fission gases, which, in conjunction with the aluminum cladding, can lead to large stress states within the fuel. The nature of how fundamental processes of radiation damage, including the evolution of point defects, under such stresses occur is unknown. In this work, we present molecular dynamics simulations of the formation energy of point defects under applied stress. We report this work will allow for the implementation of stress-dependent microstructural evolution models of nuclear fuels, including those for both fission gas bubble growth and creep, which are critical to ensure the stable and predictable behavior of research reactor fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Evaluating Effects of Shear Processing on 2D Crystalline Materials in 3D Metal Matrices: Atomistic Understanding of High Shear Deformation of Copper Graphene Composites

Composites made of copper and graphene demonstrate high strength, lubricity and enhanced electrical and thermal conductivity compared pure copper. However, manufacturing these composites at bulk volumes for industrial applications has been a big challenge. Shear deformation assisted processing is an effective method for manufacturing materials such as copper-graphene composites demonstrating ultra-fine grain structures and compositional homogeneity. Nevertheless, microstructural evolution of the composites and their property development under such conditions is not clearly understood currently. To rectify this gap in literature, high strain shear deformation of copper coated graphene foils was performed using a tribometer pin in this study. Changes in microstructure of the composite as well as the constituent components under shear deformation was correlated to process conditions. A sharp increase in the coefficient of friction attributed to rupture and smearing of graphene layer into copper substrate was observed during the shear processing. The coefficient of friction of the sheared copper/graphene composite was lower than that of pure copper, suggesting that partially worn graphene is effectively lubricious at the macroscale. A multimodal characterization of the processed region further revealed a shear deformation-induced ultrafine two-phase grain-structure consisting of copper and graphitic domains. Shear deformation reduced the copper grain size from around 50 – 100 µm to ~200 nm on an average and ~2 – 5 nm in some locations. The semicrystalline graphene films were observed to fracture into 10 – 50 µm size flakes. Oxygen enrichment was observed in the processed region. Finally, graphitic domain were identified for the first time in the copper matrix and not just at the grain boundaries providing evidence for a metastable composite microstructure as a result of solid phase processing at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2025

SEM Milestone Report for MARMOT Validation Project

MARMOT is a meso-scale fuel code under development by NEAMS designed to predict the microstructure evolution of fuel materials to determine updated thermal and mechanical property models. These updated models can be used by engineering scale codes such as BISON. To support its development and validation the MARMOT Validation Project (MVP) was initiated and consists of extensive microstructure and thermal characterization of U-10Zr fuel material before and at discreet levels of irradiation. This report summarizes the SEM and EDS characterization of the lowest burnup samples (1.5% FIMA) and how they compare to pre-irradiation SEM and EDS.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Impact of Cryogenic Treatment on Mechanical Behavior and Microstructure of Ti-6Al-4V ELI Biomaterial

Ti-6Al-4V with its ELI variant is a potential biomaterial for structural implants. However, its poor resistance to plastic shearing results in poor tribological properties that hinders its widespread use in biomedical implants. Cryogenic treatment being an established technique for improving wear resistance and other mechanical properties of ferrous materials is applied on Ti-6Al-4V ELI alloy. To ensure comprehensive investigations, the widely studied microstructures of Ti-6Al-4V ELI, i.e., lamellar, martensite, two variants of bimodal and mill annealed are considered in this work. Deep cryogenic treatment at the soaking temperature (− 196 °C) for 24 h soaking time followed by tempering has been applied. The impact of cryogenic treatment has been assessed in terms of changes in the mechanical behavior including wear resistance, hardness, tensile properties and impact strength. The alterations in mechanical behavior due to cryogenic treatment are correlated with microstructure evolution. Cryogenic treatment has caused significant improvement in mechanical behavior of Ti-6Al-4V ELI alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Magnetic Processing – A Pervasive Energy Efficient Technology for Next Generation Materials for Aerospace and Specialty Steel Markets

Thermomagnetic Magnetic Processing is an exceptionally fertile, pervasive and cross-cutting technology that is just now being recognized by several major industry leaders for its significant potential to increase energy efficiency and materials performance for a myriad of energy intensive industries in a variety of areas and applications. ORNL has pioneered the use and development of large magnetic fields in thermomagnetically processing (T-MP) materials for altering materials phase equilibria and transformation kinetics. ORNL has discovered that using magnetic fields, we can produce unique materials responses. T-MP can produce unique phase stabilities & microstructures with improved materials performance for structural and functional applications not achieved with traditional processing techniques. These results suggest that there are unprecedented opportunities to produce significantly enhanced materials properties via atomistic level (nano-) microstructural control and manipulation. ORNL (in addition to others) have shown that grain boundary chemistry and precipitation kinetics are also affected by large magnetic fields. This CRADA has taken advantage of ORNL’s unique, custom-designed thermo-magnetic, 9 Tesla superconducting magnet facility that enables rapid heating and cooling of metallic components within the magnet bore; as well as ORNL’s expertise in high magnetic field (HMF) research. Carpenter Technologies, Corp., is a a US-based industrial company, that provides enhanced performance alloys for the Aerospace and Specialty Steel products. In this CRADA, Carpenter Technologies, Corp., is focusing on applying ORNL’s Thermomagnetic Magnetic Processing (TMP) technology to improve their current and future proprietary materials’ product performance and open up new markets for their Aerospace and Specialty Steel products. Unprecedented mechanical property performance improvements have been demonstrated for a high strength bainitic alloy industrial/commercial alloy that is envisioned to provide the potential for new markets for this alloy. These thermomechanical processing results provide these alloys with a major breakthrough demonstrating that simultaneous improvements in yield strength and ductility are achieved: 12 %, 10%, 13%, and 22% increases in yield strength, elongation, reduction-in-area, and impact energy respectively. In addition, TMP appears to overcome detrimental chemical homogeneity impacts on uniform microstructure evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modeling of intragranular misorientation and grain fragmentation in polycrystalline materials using the viscoplastic self-consistent formulation

In this paper, the recently established methodology to use known algorithmic expressions of the second moments of the stress field in the grains of a polycrystalline aggregate for calculating average fluctuations of lattice rotation rates and the associated average intragranular misorientation distributions using the mean-field viscoplastic self-consistent (VPSC) formulation is extended to solve the coupled problem of considering the effect of intragranular misorientations on stress and rotation rate fluctuations. In turn, these coupled expressions are used to formulate and implement a grain fragmentation (GF) model in VPSC. Case studies, including tension and plane-strain compression of face-centered cubic polycrystals are used to illustrate the capabilities of the new model. GF-VPSC predictions of intragranular misorientation distributions and texture evolution are compared with experiments and full-field numerical simulations, showing good agreement. In particular, the inclusion of misorientation spreads reduced the intensity of the deformed texture and thus improved the texture predictions. Finally and moreover, considering that intragranular misorientations act as driving forces for recrystallization, the new GF-VPSC formulation is shown to enable modeling of microstructure evolution during deformation and recrystallization, in a computationally efficient manner.

36 MATERIALS SCIENCE↗
DOE OSTI2025

The microscopic mechanisms of high temperature oxidation of Haynes 282

Nickel-based superalloy finds widespread applications in aerospace and extreme environments. They are known for their high temperature oxidation resistance under extended periods. However, the oxide formation and evolution which sets the stages for the later parabolic oxidation kinetics is not fully understood. This paper aims to provide new insights into the transient stage oxidation mechanism and kinetics of a typical Ni-based superalloy (Haynes 282). While tracking the chemical and microstructural evolution under micrometer scale at 800°C, we show that the oxide scale and its grain boundary species change significantly during the initial stages of oxidation and can have a profound impact on the oxidation kinetics. Cr 2 O 3 forms initially at the grain boundary along with minor amount of Al 2 O 3 . Then, the grain boundary region is enriched with copious amounts TiO 2 while Cr migrates away from the grain boundary. A noticeable change in the isothermal oxidation kinetics observed likely results from a mechanistic change from uniform surface oxidation to preferential outward diffusion of Ti 4+ ions through the grain boundary. Through first-principles calculations combined with energy dispersive spectroscopy, we confirmed the preferential outward diffusion of Ti 4+ ions as the diffusion barrier is lower for Ti than Cr along the grain boundaries. In conclusion, these findings highlight the critical role of early-stage grain boundary oxidation dynamics in dictating the long-term oxidation resistance of Ni-based superalloys and provide a foundation for future strategies to enhance their performance in extreme environments.

DFT↗
DOE OSTI2016

Atomistic Simulations in Support of UO{sub 2} Fuel Performance Modeling

Large fission gas atoms such as Xe have very low solubility in the UO{sub 2} fuel matrix, which drives precipitation and growth of intra-granular fission gas bubbles, segregation of gas atoms to grain boundaries and evolution of intergranular bubbles. The inter-granular bubbles develop percolation networks leading to release of the fission gas. Fission gas bubbles induce fuel swelling and the fuel thermal conductivity is reduced by both fission gas bubbles and dispersed gas atoms. When fission gas is released from the fuel to the plenum, the pressure on the clad walls increases and the gap thermal conductivity decreases. All of these detrimentally affect fuel performance. The thermal conductivity of UO{sub 2} is low across a wide temperature range compared to iso-structural materials such as ThO{sub 2}, which is caused by the coupling between phonons carrying heat and uranium spins or magnetism providing a scattering mechanism with very low mean free path. For in-pile conditions the thermal conductivity is further reduced due to, for example, radiation damage, generation of fission products and fission gases and fuel microstructure evolution. This results in a complex burn-up dependent thermal conductivity. Thermal conductivity and fission gas evolution are two of the most important properties determining UO{sub 2} nuclear fuel performance. Both properties have significant uncertainty. In order to reduce this uncertainty, new models are being developed based on multi-scale simulations that attempt to improve the description of material properties and also couple them to the microstructure state of the material. In this paper we highlight atomistic calculations and simulations used to develop meso-scale models of the evolution of fission gases and thermal conductivity in UO{sub 2}. Literature examples of the application of these results and the corresponding models to fuel performance simulations will also be given. (author)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Simulation Kinetics of Austenitic Phase Transformation in Ti+Nb Stabilized IF and Microalloyed Steels

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Thermal transport across nanoscale damage profile in sapphire irradiated by swift heavy ions

In this work, we studied the degradation of thermal conductivity in single crystal sapphire (α-Al 2 O 3 ) irradiated by 167 MeV Xe swift heavy ions (SHIs) over the multiple fluences in the range of 10 12 –10 14 ions/cm 2 . Thermal conductivity was measured primarily in the cross-plane direction using a noncontact ultrafast optical pump-probe technique called picosecond time domain thermoreflectance (TDTR). Multiple samples with variable ion fluences allowed us to probe distinct regions resulting from different regimes of microstructure evolution caused by electronic energy loss. By tuning the penetration depth of the thermal waves using different modulation frequencies, two regions with distinct conductivities were identified and the values of which were found to be consistent with phonon-mediated thermal transport models while the microstructure was confirmed by electron microscopy characterization. These damaged regions were determined to be a several micrometer thick ion track region and several tens of nanometer-thick amorphous layer present only above 5.0 × 10 13 ions/cm 2 . These results demonstrate the applicability of TDTR to resolve thermal transport behavior in SHI irradiated oxides having nonhomogeneous damage profile on a nanometer scale. The presented approach facilitates future studies aiming at resolving the impact of distinct damage resulting from electronic and nuclear stopping regimes under irradiation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Microstructures and Rheology of the Earth Upper Mantle Inferred from a Multiscale Approach

The strongly anisotropic rheology of olivine polycrystals, associated to their microstructure, constitutes a key feature affecting the dynamics of the Earth's upper mantle. High pressure deformation experiments carried out on olivine single crystals under synchrotron radiation, together with estimations of lattice friction based on first-principle calculations, show a transition from easy [100] to easy [001] slips as pressure and temperature (thus depth) increases. We input these data at the slip system level into the second-order extension of the self-consistent scheme to assess microstructure evolution along a typical flow pattern beneath an oceanic spreading center.

58 GEOSCIENCES↗
DOE OSTI2012

In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries

It is well known that upon lithiation both crystalline and amorphous Si transform to a armorphous LixSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li15Si or Li22Si5. Up to date, the detailed atomistic mechanism of this phase transformation, and degradation process in nanostructured Si, are not fully understood. Here, we report the microstructural evolution and phase transformation characteristic of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during charge/discharge process using in-situ TEM and density function theory molecular dynamic (DFT-MD) calculation. We found that the crystallization of Li15Si4 from amorphous LixSi is a spontaneous, congruent phase transition process without phase separation or large scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long term charge/discharge cycling.

25 ENERGY STORAGE↗
DOE OSTI2019

A New Efficient Quantitative Multi-component Phase Field: Lattice Boltzmann Model for Simulating Ti6Al4V Solidified Dendrite Under Forced Flow

Ti6Al4V is a widely used, multi-component alloy in additive manufacturing, during which the fluid flow in the molten pool significantly affects the solidified dendrites. To predict and further control the microstructure, modeling and simulating the microstructure evolution play a critical role. In this study, a newly developed, efficient, quantitative multi-component phase-field (PF) model is coupled with a lattice Boltzmann (LB) model to simulate Ti6Al4V solidified dendrite evolution under fluid flow. The accuracy and convergence behavior of the model is validated by the Gibbs–Thomson relation at the dendrite tip. Single and multiple two-dimensional (2D) equiaxed dendrite evolution cases under forced flow were simulated. Results show that the dendrite pattern is influenced remarkably by the fluid flow. Underlying mechanisms of the asymmetrical evolution are revealed by discussing the interaction among the flow, composition distribution and dendrite morphology, quantitatively. The dendrite kinetics are also derived, which ascertains the relationship between tip velocity and undercooling and inlet velocity and is the foundation for larger-scale simulation. We believe that the coupled quantitative multi-component PF–LB framework employed in this study helps in investigating the solidified dendrite morphology evolution in a deep and quantitate manner.

36 MATERIALS SCIENCE↗
DOE OSTI2016

High pressure FAST of nanocrystalline barium titanate

Here, this work studies the microstructural evolution of nanocrystalline (<1 µm) barium titanate (BaTiO 3 ), and presents high pressure in field-assisted sintering (FAST) as a robust methodology to obtain >100 nm BaTiO 3 compacts. Using FAST, two commercial ~50 nm powders were consolidated into compacts of varying densities and grain sizes. Microstructural inhomogeneities were investigated for each case, and an interpretation is developed using a modified Monte Carlo Potts (MCP) simulation. Two recurrent microstructural inhomogeneities are highlighted, heterogeneous grain growth and low-density regions, both ubiqutously present in all samples to varying degrees. In the worst cases, HGG presents an area coverage of 52%. Because HGG is sporadic but homogenous throughout a sample, the catalyst (e.g., the local segregation of species) must be, correspondingly, distributed in a homogenous manner. MCP demonstrates that in such a case, a large distance between nucleating abnormal grains is required—otherwise abnormal grains prematurely impinge on each other, and their size is not distinguishable from that of normal grains. Compacts sintered with a pressure of 300 MPa and temperatures of 900 °C, were 99.5% dense and had a grain size of 90±24 nm. These are unprecedented results for commercial BaTiO 3 powders or any starting powder of 50 nm particle size—other authors have used 16 nm lab-produced powder to obtain similar results.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Material flow behavior and microstructural refinement of AA6061 alloy during friction extrusion

In this work we used friction extrusion (FE), a solid phase processing technique, to produce dense, fully consolidated 5?mm rods of aluminum alloy 6061 (AA6061). The combination of large shear stresses and high temperatures at the tool-billet interface during extrusion produced equiaxed, dynamically recrystallized grains and finely distributed precipitates. Texture and microstructure evolution during extrusion was investigated in detail using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Smoothed particle hydrodynamics simulation was performed to study complex material flow during extrusion. Simulation results suggest spiral material flow during extrusion which corroborated experimental results. Advantages of friction extruded microstructure over conventionally extruded counterparts are also explored using flash annealing for solution treatment followed by artificial aging. Mechanical properties of the as-friction extruded, and the artificially aged specimens were evaluated using tensile testing and compared with conventional extruded material properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Additive manufacturing of a metastable high entropy alloy: Metastability engineered microstructural control via process variable driven elemental segregation

Compositional paradigm shift in high entropy alloys (HEAs) provided new opportunities for microstructural engineering, whereas process control during laser powder bed fusion (LPBF) additive manufacturing (AM) enable fine microstructure tailoring. Metastability engineering in transformation induced plasticity (TRIP) HEAs by addition of minor alloying elements is an attractive strategy for fine microstructural tuning. This study explored in detail the microstructural evolution during LPBF AM of a metastable Fe 40 Mn 20 Co 20 Cr 15 Si 5 (at.%) dual phase HEA (CS-HEA). LPBF processing window for CS-HEA was established based on quantitative analysis and experiments. Based on melt pool overlap lack of fusion pores were observed at lower energy densities (J) of $\textit{J}$ ≤ 31.25 J/mm 3 and key-hole formation by melt pool destabilization in case of $\textit{J}$ ≥ 75 J/mm 3 . The microstructure of CS-HEA consists of metastable FCC-γ and HCP-ε phases; LPBF process parameters governed the final phase fraction in the alloy which has been correlated to metastability alteration of the high temperature γ phase. Final microstructural engineering was devised by LPBF process control which enabled cooling rate manipulation to guide Mn and Si segregation at the cell boundaries, thereby controlling the matrix metastability and final phase fraction. Additionally, high resolution transmission electron microscopy (TEM) revealed disparity in stacking fault morphology in CS-HEA with LPBF process variable alterations associated with local variations in chemical composition and stacking fault energy. Further, the phase evolution with process parameters also affected the nanomechanical behavior of the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A machine learning approach to thermal conductivity modeling: A case study on irradiated uranium-molybdenum nuclear fuels

A deep neural network was developed for the purpose of predicting thermal conductivity with a case study performed on irradiated nuclear fuel. Traditional thermal conductivity modeling approaches rely on existing theoretical frameworks that describe known, relevant phenomena that govern the microstructural evolution processes during neutron irradiation (such as recrystallization, and pore size, distribution and morphology). Current empirical modeling approaches, however, do not represent all irradiation test data well. We develop a machine learning approach to thermal conductivity modeling that does not require a priori knowledge of a specific material microstructure and system of interest. Our approach allows re-searchers to probe dependency of thermal conductivity on a variety of reactor operating and material conditions. The purpose of building such a model is to allow for improved predictive capabilities linking structure-property-processing-performance relationships in the system of interest (here, irradiated nuclear fuel), which could lead to improved experimental test planning and characterization. The uranium-molybdenum system is the fuel system studied in this work, and historic irradiation test data is leveraged for model development. Our model achieved a mean absolute percent error of approximately 10%. Results indicate our model generalizes well to never before seen data, and thus use of deep learning methods for material property predictions from limited, historic irradiation data is a viable approach. This work challenges the current paradigm in materials science, where material property models are based on limited experimental data fitted to low-dimensionality phenomenological models, or by simulations in which fundamental equations are explicitly solved. The work presented here aims to demonstrate the potential and limitations of machine learning in the field of materials science and material property modeling.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗