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At least 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

THERMAL ANNEALING OF IRRADIATED PREHYDRIDED ZIRCALOY-4 MATERIALS

Hydrogen-assisted irradiation growth is an important issue for reliability and performance of nuclear materials in nuclear fuel cladding and fuel assembly components. The mechanisms of irradiation growth are not entirely clear and the role of hydrogen and hydrides are less so. There is evidence that breakaway irradiation growth is strongly influenced by the formation and dynamics of dislocation loops that vary with alloy composition, heat treatment and hydrogen content. Thermal annealing of eight Zircaloy-4 specimens irradiated in the Advanced Test Reactor (ATR) at the Idaho National Laboratory as part of the joint EPRI Nuclear Fuels Industry Research (NFIR) and Fuel Reliability Program (FRP) AsTeR irradiation growth study were performed by hot-stage transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and dilatometry via NSUF RTE 17-1093. Two fluence levels (6.7 and 12.3 dpa) and three hydrogen contents (~10, 40, 120 ppm) were investigated. TEM with a hot stage was used for the quantification of "a" and "c" dislocation loop population densities and size distributions at a range of temperatures from 25 to 800oC. DSC and dilatometry were performed by dynamically heating and cooling from 30 to 750ºC at rates of 5, 10, and 20 oC/min to determine the behavior of dislocation loop annealing. Dynamic temperature DSC tests were able to detect the temperature ranges and enthalpies of "a" and "c" loop annihilation, hydride dissolution and precipitation, secondary particle precipitation, and a?a + ß phase transformations upon heating and cooling. Dilatometry tests showed that all of the test showed initial linear expansion followed by a decrease in expansion rate from about 400 to 600 ºC associated with annihilation of "a" and "c" loops and showed evidence for the thermal stabilizing effect of hydrogen on dislocation loops. The results are discussed in the context of the larger AsTeR program that will include higher exposure (~20 dpa) and different alloys when they become available in 2021.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Molecular Dynamics Study of Creep Deformation in Nickel-based Superalloy

The present study employs molecular dynamics simulations of Ni-based superalloy to investigate the creep behavior under uniaxial compression test. Dislocation dynamics is analyzed for the nickel-based single crystal superalloy with the presence of void and with varying the distribution of gamma-prime phase The results show that multiple-void systems are more prone to yield than single-void systems and single-void systems are more prone to yield than the system withoutvoid. From the simulations, it has been determined that the creep mechanism in Ni/Ni3Al is subject to change on the applied stress depending on the distribution of gamma-prime phases change. Dislocation behavior is also observed in addition to the time evolution of dislocation nucleation and growth to quantify the propensity and mechanism of creep in nickel-based single crystal superalloy. The studies confirmed that dislocation length prior to plastic deformation decreased as the number of voids increased. In addition, the deformation behavior for finegrained nickel-based polycrystalline superalloy upon grain size variation is also investigated. Our findings show that both the movement mechanisms of the grains and grain boundaries and the relationship between grain size and yield strength are highly affected by the grain size of the materials. All the databases have then been classified to accurately predict the target class vi machine learning using the data mining tool WEKA.

creep, nickel-based superalloy, molecular dynamics↗
DOE OSTI2025

In situ irradiation of spent nuclear fuels

To improve the economics of commercial nuclear reactors, nuclear vendors and utilities are seeking approval for increased burnup usage of the existing nuclear fleet. This is especially critical for meeting the clean energy initiative by the United States Government, Department of Energy (DOE). However, one of the key challenges the nuclear industry faces in this regard is that the regions exposed to high burnup and low temperatures exhibit a fine-grained microstructure with large bubbles known as high-burnup structure (HBS) [1]. The formation of HBS has been correlated to the diminished performance of the reactor, as well as fuel fragmentation and pulverization during transient and accidental conditions [2]. Therefore, it is paramount to understand the mechanisms for HBS formation along with its impact on the properties and performance of nuclear fuels. While existing programs, such as Nuclear Energy Advanced Modeling and Simulation (NEAMS) and Advanced Fuel Campaign (AFC) are focusing on evaluating the performance impact of HBS, the physical mechanisms contributing to HBS formation are still not fully understood. In addition, having predictive capabilities and sound understanding of the microstructural evolution of nuclear fuel is essential for accelerated development, qualification, and deployment of new nuclear materials and novel reactor designs for advanced nuclear reactors. There is a lack of consensus among researchers regarding the mechanisms leading to such restructuring observed in HBS. Grain subdivision due to polygonization versus recrystallization, continuous versus discrete recrystallization occurring in tandem or conjunction, etc., have been proposed and debated. In general, it is hypothesized that defect accumulation and dislocation interaction within the grains cause the realignment of dislocations into grain boundaries, leading to the new subgrain formation, which over time transforms into new grains. However, due to the lack of transient data, the importance of fission rate, irradiation, thermal, and stress history of the fuel on the restructuring could not be assessed. In situ microstructural evolution under various irradiation conditions is desired to bridge this gap. Alternatively, phase-field-based models have been developed to capture HBS formation via discrete recrystallization utilizing the classical nucleation approach [3–5]. However, in these models, the grain nucleation criteria are often defined based on empirical relations for burnup and fission gas density leading to dislocation density change. A mechanistic approach to capture the dislocation interaction with the microstructural features leading to grain subdivision is lacking.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Application of X-ray Technique to Study the Structure of Ultrafine-Grained Ferritic/Martensitic Steel

In this work, x-ray diffraction and transmission electron microscopy (TEM) analyses have been applied to study the structure of ultrafine-grained (UFG) ferritic/martensitic steel as compared to coarse-grained samples subjected to standard treatment. The x-ray phase analysis of diffraction patterns taken in the “transmission” mode allowed determining the volume fraction and phase composition of particles in the UFG samples. The lattice parameter, the size of coherent scattering domains, the averaged dislocation density, and the fraction of edge and screw dislocations of the ferritic phase in steel were determined according to the diffraction patterns taken in the “reflection” mode. The small-angle x-ray scattering (SAXS) technique was used to study the quantitative characteristics of the size, shape, and distribution of precipitates in the UFG samples. The TEM investigations were performed on the same foils to confirm the results of SAXS. The quantitative evaluations of dispersion and dislocation hardenings in ferritic/martensitic steel were conducted on the basis of the results obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2026

A theoretical study of solid solution strengthening in the refractory medium entropy alloy Nb 45 Ta 25 Ti 15 Hf 15

The refractory medium-entropy alloy (RMEA) Nb 45 Ta 25 Ti 15 Hf 15 exhibits exceptional tensile ductility and fracture toughness at ambient temperature, but its engineering applications are limited by a lack of high temperature strength. Using a machine-learning interatomic potential (MLIP) with near-density functional theory (DFT) accuracy, we conducted molecular dynamics (MD) and statics simulations of the behavior of dislocations with both screw and edge characters. We also analyze experimentally measured yield strengths using the Rao-Suzuki model and the Maresca-Curtin model modified to include a temperature-dependent shear modulus and a bulk modulus-dependent misfit volume, thereby uncovering the mechanisms underlying the yielding of this RMEA. Compared with the published experimental yield strength, the models parameterized by the MLIP effectively reproduce the experimental results over a wide temperature range. The models and MD simulations indicate that yielding is governed by screw dislocations, with dipole dragging as the dominant mechanism. In MD simulations, we observed a potential softening mechanism not considered by the Rao-Suzuki screw model: slow migration of interstitial jogs along the dislocation core, which could lead to the annihilation of vacancy and interstitial jog pairs by their combination.

BCC complex concentrated alloys↗
DOE OSTI2014

Atomic-scale studies on the effect of boundary coherency on stability in twinned Cu

The stored energy and hardness of nanotwinned (NT) Cu are related to interaction between dislocations and (111)-twin boundaries (TBs) studied at atomic scales by high-angle annular dark-field scanning transmission electron microscope. Lack of mobile dislocations at coherent TBs (CTBs) provides as-deposited NT Cu a rare combination of stability and hardness. The introduction of numerous incoherent TBs (ITBs) reduces both the stability and hardness. While storing more energy in their ITBs than in the CTBs, deformed NT Cu also exhibits high dislocation density and TB mobility and therefore has increased the driving force for recovery, coarsening, and recrystallization.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Interfacial diffusion aided deformation during nanoindentation

Nanoindentation is commonly used to quantify the mechanical response of material surfaces. Despite its widespread use, a detailed understanding of the deformation mechanisms responsible for plasticity during these experiments has remained elusive. Nanoindentation measurements often show stress values close to a material’s ideal strength which suggests that dislocation nucleation and subsequent dislocation activity dominates the deformation. However, low strain-rate exponents and small activation volumes have also been reported which indicates high temperature sensitivity of the deformation processes. Using an order parameter aided temperature accelerated sampling technique called adiabatic free energy dynamics [J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B, 112, 15742 (2008)], and molecular dynamics we have probed the diffusive mode of deformation during nanoindentation. Localized processes such as surface vacancy and ad-atom pair formation, vacancy diffusion are found to play an important role during indentation. Our analysis suggests a change in the dominant deformation mode from dislocation mediated plasticity to diffusional flow at high temperatures, slow indentation rates and small indenter tip radii.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Temperature dependences of the contact resistivity in ohmic contacts to n{sup +}-InN

The temperature dependences of the contact resistivity (ρ{sub c}) of ohmic contacts based on the Au-Ti-Pd-InN system are measured at an InN doping level of 2 × 10{sup 18} cm{sup −3} in the temperature range of 4.2–300 K. At temperatures T > 150 K, linearly increasing dependences ρ{sub c}(T) are obtained. The dependences are explained within the mechanism of thermionic current flow through metal shunts associated with dislocations. Good agreement between theoretical and experimental dependences is achieved assuming that the flowing current is limited by the total resistance of the metal shunts, and the density of conductive dislocations is ∼5 × 10{sup 9} cm{sup −2}. Using the X-ray diffraction method, the density of screw and edge dislocations in the structure under study is measured: their total density exceeds 10{sup 10} cm{sup −2}.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Unlocking Electrostrain in Plastically Deformed Barium Titanate

Achieving substantial electrostrain alongside a large effective piezoelectric strain coefficient (d 33 *) in piezoelectric materials remains a formidable challenge for advanced actuator applications. Here, in this work, a straightforward approach to enhance these properties by strategically designing the domain structure and controlling the domain switching through the introduction of arrays of ordered {100}<100> dislocations is proposed. This dislocation engineering yields an intrinsic lock-in steady–state electrostrain of 0.69% at a low field of 10 kV cm -1 without external stress and an output strain energy density of 5.24 J cm -3 in single-crystal BaTiO 3 , outperforming the benchmark piezoceramics and relaxor ferroelectric single-crystals. Additionally, applying a compression stress of 6 MPa fully unlocks electrostrains exceeding 1%, yielding a remarkable d 33 * value over 10 000 pm V -1 and achieving a record-high strain energy density of 11.67 J cm -3 . Optical and transmission electron microscopy, paired with laboratory and synchrotron X-ray diffraction, is employed to rationalize the observed electrostrain. Phase-field simulations further elucidate the impact of charged dislocations on domain nucleation and domain switching. These findings present an effective and sustainable strategy for developing high-performance, lead-free piezoelectric materials without the need for additional chemical elements, offering immense potential for actuator technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Grain Boundary Engineering for Achieving High Thermoelectric Performance in n-Type Skutterudites

Grain or phase boundaries play a critical role in the carrier and phonon transport in bulk thermoelectric materials. Previous investigations about controlling boundaries primarily focused on the reducing grain size or forming nanoinclusions. Herein, liquid phase compaction method is first used to fabricate the Yb–filled CoSb 3 with excess Sb content, which shows the typical feature of low–angle grain boundaries with dense dislocation arrays. Seebeck coefficients show a dramatic increase via energy filtering effect through dislocation arrays with little deterioration on the carrier mobility, which significantly enhances the power factor over a broad temperature range with a high room–temperature value around 47 μW cm –2 K –1 . Simultaneously, the lattice thermal conductivity could be further suppressed via scattering phonons via dense dislocation scattering. As a result, the highest average figure of merit ZT of ≈1.08 from 300 to 850 K could be realized, comparable to the best reported result of single or triple–filled Skutterudites. Furthermore, this work clearly points out that low–angle grain boundaries fabricated by liquid phase compaction method could concurrently optimize the electrical and thermal transport properties leading to an obvious enhancement of both power factor and ZT .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Graft Patency in Long-term Survivors after Renovisceral Debranching with VORTEC

PurposeThis study was designed to evaluate retrospectively the long-term stent-graft patency after renovisceral revascularization with Viabahn Open Revascularization Technique (VORTEC) using computed tomography angiography (CTA) and magnetic resonance angiography (MRA).MethodsIn 34 patients (seven women; mean age 72 ± 8 years) with aortic aneurysm, 63 renovisceral vessels (i.e., 54 renal, nine visceral arteries) were revascularized with VORTEC between 2004 and 2009. All patients obtained a pre- and postinterventional CTA and at least one follow-up CTA or MRA after 6 or more months following intervention (median follow up: 43 months). Detection of bypass occlusion, bypass stenosis, infolding, stent-graft fractures and dislocations, and kidney shrinkage were noted by two readers in consensus. Furthermore, mortality during follow-up was assessed using the medical report.ResultsDuring follow-up, 12.6 % of stent-grafts occluded. Cumulative patency rate was 95.2 ± 2.7 % at 12 months, 87.7 ± 4.4 % at 24 and 36 months, and 84.7 ± 5.2 % at 48, 60, 72, 84, and 89 months, respectively. Overall, 19 % of stent-grafts (12/63) developed bypass stenosis (<50 %, 10 stent-grafts; > 50 %, 2 stent-grafts), in one case stenosis (>70 %) was suspected to be hemodynamically significant. No secondary dislocation, no infolding of renovisceral stent-grafts, and no stent-graft fracture occurred. Kidney shrinkage occurred in nine patients, primarily in patients with an occluded Viabahn (n = 7). Eleven patients (32.3 %) died within the follow-up time period.ConclusionsIn long-term survivors after VORTEC cumulative patency rate remained high, and no stent-graft fractures or secondary dislocations occurred.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2017

A review of slip transfer: applications of mesoscale techniques

In this review article, we present and discuss recent mesoscale modeling studies of slip transmission of dislocations through biphase interfaces. Specific focus is given to fcc/fcc material systems. We first briefly review experimental, atomistic, and continuum-scale work that has helped to shape our understanding of these systems. Then several mesoscale methods are discussed, including Peierls–Nabarro models, discrete dislocation dynamics models, and phase field-based techniques. Recent extensions to the mesoscale mechanics technique called phase field dislocation dynamics are reviewed in detail. Results are compiled and discussed in terms of the proposed guidelines that relate composite properties to the critical stress required for a slip transmission event.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Structure and Phase Composition of Heat-Resistant Ni–Al–Co Alloy After Annealing and Creep

The paper presents research into the structure and phase composition of Ni–Al–Co alloy modified by rhenium (~3 аt.%) alloying. Observations are carried out using the transmission electron microscopy. The initial state of the alloy is the state after the directional crystallization. The alloy is further subjected to 900°С annealing during 1143 h. Creep tests are additionally carried out for this alloy at the same temperature and time and 400 MPa load. It is shown that FCC disordered γ- and ordered γ′-phases are major in all alloy states. Secondary phases are found to be σ-phase, χ-phase, Laves and AlRe{sub 2} phases. Experiments show that the high temperature annealing changes the phase composition of the alloy. Thus, the amount of the ordered γ′-phase increases, while that of disordered γ-phase decreases. During the creep process, the amount of the former reduces and the amount of the latter increases. The annealing process modifies the phase composition in secondary phases. It is found that the structural modification caused by the creep process differs from that caused by the annealing process. Thus, the creep-induced modification of the cuboid structure in γ′-phase is stronger than due to annealing. Dislocations are observed in γ- and γ′-phases in all states of the alloy. During the annealing process, the dislocation density in γ-phase is higher than in γ′-phase, and vice versa during the creep process. The experiments show that the behavior of the dislocation structures is different during the annealing and creep processes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of temperature on the nano/microstructure and mechanical behavior of nanotwinned Ag films

In situ and ex situ annealed nanotwinned (NT) Ag thin films have been investigated by TEM and tensile testing to reveal the thermal stability of the twin boundaries, grain boundaries, dislocation densities, and their respective influence of the macroscopic yield stress. The NT Ag films synthesized by magnetron sputtering form both coherent (CTB, Σ3{111}) and incoherent (ITB, Σ3{112}) twin boundaries that are thermally stable up to 473 K (200 Celsius), i.e., no obvious changes in grain size, twin spacing, and yield stress. In situ TEM observations show the dislocations become mobile at 453 K (180 Celsius) resulting in dislocation annihilation primarily at twin and grain boundaries. Rotation of grains with low-angle grain boundaries was observed during in situ heating, resulting in the growth of columnar grains above 453 K (180 Celsius). However, no noticeable changes in the spacings of CTBs were observed during the entire in situ and ex situ annealing [up to 873 K (600 Celsius)]. The increase in grain size and concomitant decrease in yield stress following annealing at various temperatures can be described by the Hall-Petch relationship, demonstrating that grain size rather than twin spacing is most sensitive to thermal annealing and plays a dominant role in the deformation of NT Ag films.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure Stability During Creep of Friction Stir Welded AA2024-T3 Alloy

The poor weldability of the AA2024 aluminum alloy limits its use in industrial applications. Because friction stir welding (FSW) is a non-fusion welding process, it seems to be a promising solution for welding this alloy. In the current study, FSW was applied to butt weld AA2024-T3 aluminum alloy plates. Creep tests were conducted at 250 and at 315 °C on both the parent material and the friction stir welded specimens. The microstructures of the welded and non-welded AA2024-T3 specimens before and after the creep tests were studied and compared. A comprehensive transmission electron microscopy study together with a high-resolution scanning electron microscopy study and energy-dispersive x-ray spectroscopy analysis was conducted to investigate the microstructure stability. The parent material seems to contain two kinds of Cu-rich precipitates—coarse precipitates of a few microns each and uniformly dispersed fine nanosized precipitates. Unlike the parent material, the crept specimens were found to contain the two kinds of precipitates mentioned above together with platelet-like precipitates. In addition, extensive decoration of the grain boundaries with precipitates was clearly observed in the crept specimens. Controlled aging experiments for up to 280 h at the relevant temperatures were conducted on both the parent material and the welded specimens in order to isolate the contribution of exposure to high temperatures to the microstructure changes. TEM study showed the development of dislocation networks into a cellular dislocation structure in the case of the parent metal. Changes in the dislocation structure as a function of the creep strain and the FSW process were recorded. A detailed creep data analysis was conducted, taking into account the instability of the microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Hot Rolling on the Microstructure and Mechanical Properties of Nitrogen Alloyed Austenitic Stainless Steel

In the present investigation, the effect of multi-pass hot rolling in the temperature range of 700–1000 °C on the microstructure and mechanical properties of nitrogen alloyed austenitic stainless steel was studied with the aid of optical microscopy, tensile testing and x-ray diffraction measurements. The microstructural changes that occurred in the hot rolled specimens were elongation of grains in rolling direction, nucleation of new grains at the grain boundaries of elongated grains and growth of nucleated grains to form fully recrystallized grains. Elongated grains formed at lower rolling temperature (700–800 °C) due to inadequate strain/temperature for the initiation of dynamic recrystallization. At higher rolling temperature (900–1000 °C), fine grains formed due to dynamic recrystallization. Tensile properties showed strong dependency on the rolling temperature. Tensile strength increased with the decrease in the rolling temperature at the cost of ductility. Maximum strength was observed in samples hot rolled at 700 °C with yield strength of 917 MPa and ductility of 25%. This variation in the tensile properties with the rolling temperature is attributed to changes in the dislocation density and grain structure. The estimated yield strength from the dislocation density, solid solution and grain boundary strengthening closely matched with experimentally determined yield strength confirming the role of dislocation density and grain size in the strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoengineered Hypereutectoid Steel with Superior Hardness and Wear Resistance

Hypereutectoid SAE 52100 steel is extensively used in bearing applications. Microstructure modification in terms of dislocation martensite and carbide refinement was achieved for SAE 52100 steel through a simple duplex heat treatment. Refinement of prior austenite grains to less than 5 μm resulted in the conversion of conventional high-carbon twinned martensite to dislocation martensite. The concurrent refinement of austenite grains and carbide precipitates was accomplished by high-temperature austenitization followed by low-temperature tempering. This resulted in nanoscale nonstoichiometric ε-carbides within a heavily twinned martensitic structure. These nanoscale carbides acted as grain boundary pinning agents after their transformation to θ-carbides during the final austenitization process. The resulting microstructure was characterized by a fine dispersion of θ-carbides within dislocation martensite and showed roughly 24% increase in tensile strength and 30% better wear resistance compared to conventional structure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Torsion on Precipitation and Hardening Effects during Aging of an Extruded AZ91 Alloy

The effect of torsional deformation by various routes prior to artificial aging treatment on the microstructure of extruded AZ91 rods was investigated. The tension and compression properties along the extruded direction were tested to evaluate the hardening effect. The results showed that torsional deformation can largely enhance the subsequent age-hardening effect. A detailed microstructural characterization revealed that torsional deformation produced a high density of dislocations and {10-12} twins, which can promote continuous precipitation and increase the number density of the Mg{sub 17}Al{sub 12} phase. In addition, part of dislocations and the {10-12} twins were retained after aging treatment. The combination of the increased precipitation hardening by Mg{sub 17}Al{sub 12}, dislocation strengthening and refinement strengthening contributed to the high hardening effect. Moreover, the hardening effect can be further increased by tailoring the torsion route. The torsion route exhibited little influence on the age-hardening effect, while it remarkably affected the texture of the twisted AZ91 rods. Reciprocating torsion had little influence on the texture. In contrast, unidirectional torsion caused the c-axis of the texture to rotate toward the extrusion direction, resulting in a texture softening effect. Therefore, combining reciprocating torsion and aging caused a higher hardening effect than combining unidirectional torsion and aging. The detailed strengthening mechanism was analyzed and is discussed.

36 MATERIALS SCIENCE↗