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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Cluster dynamics models of irradiation damage accumulation in ferritic iron. II. Effects of reaction dimensionality

The black dot damage features which develop in iron at low temperatures exhibit significant mobility during in situ irradiation experiments via a series of discrete, intermittent, long range hops. By incorporating this mobility into cluster dynamics models, the temperature dependence of such damage structures can be explained with a surprising degree of accuracy. Such motion, however, is one dimensional in nature. This aspect of the physics has not been fully considered in prior models. This article describes one dimensional reaction kinetics in the context of cluster dynamics and applies them to the black dot problem. This allows both a more detailed description of the mechanisms by which defects execute irradiation-induced hops while allowing a full examination of the importance of kinetic assumptions in accurately assessing the development of this irradiation microstructure. Results are presented to demonstrate whether one dimensional diffusion alters the dependence of the defect population on factors such as temperature and defect hop length. Finally, the size of interstitial loops that develop is shown to depend on the extent of the reaction volumes between interstitial clusters, as well as the dimensionality of these interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Computationally-efficient stochastic cluster dynamics method for modeling damage accumulation in irradiated materials

An improved version of a recently developed stochastic cluster dynamics (SCD) method (Marian and Bulatov, 2012) [6] is introduced as an alternative to rate theory (RT) methods for solving coupled ordinary differential equation (ODE) systems for irradiation damage simulations. SCD circumvents by design the curse of dimensionality of the variable space that renders traditional ODE-based RT approaches inefficient when handling complex defect population comprised of multiple (more than two) defect species. Several improvements introduced here enable efficient and accurate simulations of irradiated materials up to realistic (high) damage doses characteristic of next-generation nuclear systems. The first improvement is a procedure for efficiently updating the defect reaction-network and event selection in the context of a dynamically expanding reaction-network. Next is a novel implementation of the τ-leaping method that speeds up SCD simulations by advancing the state of the reaction network in large time increments when appropriate. Lastly, a volume rescaling procedure is introduced to control the computational complexity of the expanding reaction-network through occasional reductions of the defect population while maintaining accurate statistics. The enhanced SCD method is then applied to model defect cluster accumulation in iron thin films subjected to triple ion-beam (Fe{sup 3+}, He{sup +} and H{sup +}) irradiations, for which standard RT or spatially-resolved kinetic Monte Carlo simulations are prohibitively expensive.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

RBS/C, HRTEM and HRXRD study of damage accumulation in irradiated SrTiO3

Damage accumulation in argon-irradiated SrTiO3 single crystals has been studied by using combination of Rutherford Backscattering/Channeling (RBS/C), High Resolution Transmission Electron Microscopy (HRTEM) and High Resolution X-Ray Diffraction (HRXRD) techniques. The RBS/C spectra were fitted using McChasy, a Monte Carlo simulation code allowing the quantitative analysis of amorphous-like and dislocation-like types of defects. The results were interpreted by using a Multi-Step Damage Accumulation model which assumes, that the damage accumulation occurs in a series of structural transformations, the defect transformations are triggered by a stress caused by formation of a free volume in the irradiated crystal. This assumption has been confirmed by High Resolution Transmission Electron Microscopy and High Resolution X-Ray Diffraction analysis.

damage accumulation, radiation defects, oxides, ir↗
DOE OSTI2012

On the Effect of Ramp Rate in Damage Accumulation of the CPV Die-Attach: Preprint

It is commonly understood that thermal cycling at high temperature ramp rates may activate unrepresentative failure mechanisms. Increasing the temperature ramp rate of thermal cycling, however, could dramatically reduce the test time required to achieve an equivalent amount of thermal fatigue damage, thereby reducing overall test time. Therefore, the effect of temperature ramp rate on physicaldamage in the CPV die-attach is investigated. Finite Element Model (FEM) simulations of thermal fatigue and thermal cycling experiments are made to determine if the amount of damage calculated results in a corresponding amount of physical damage measured to the die-attach for a variety of fast temperature ramp rates. Preliminary experimental results are in good agreement with simulations andreinforce the potential of increasing temperature ramp rates. Characterization of the microstructure and resulting fatigue crack in the die-attach suggest a similar failure mechanism across all ramp rates tested.

14 SOLAR ENERGY↗
DOE OSTI2015

Atomistic simulation of damage accumulation and amorphization in Ge

Damage accumulation and amorphization mechanisms by means of ion implantation in Ge are studied using Kinetic Monte Carlo and Binary Collision Approximation techniques. Such mechanisms are investigated through different stages of damage accumulation taking place in the implantation process: from point defect generation and cluster formation up to full amorphization of Ge layers. We propose a damage concentration amorphization threshold for Ge of ∼1.3 × 10{sup 22} cm{sup −3} which is independent on the implantation conditions. Recombination energy barriers depending on amorphous pocket sizes are provided. This leads to an explanation of the reported distinct behavior of the damage generated by different ions. We have also observed that the dissolution of clusters plays an important role for relatively high temperatures and fluences. The model is able to explain and predict different damage generation regimes, amount of generated damage, and extension of amorphous layers in Ge for different ions and implantation conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Effect of composition on damage accumulation in ternary ZnO-based oxides implanted with heavy ions

Thin films of wurtzite Mg{sub x}Zn{sub 1-x}O (x{<=}0.3) grown by molecular beam epitaxy and wurtzite Cd{sub x}Zn{sub 1-x}O (x{<=}0.05) grown by metal organic chemical vapor deposition were implanted at room temperature with 150 keV Er{sup +} ions and 200 keV Au{sup +} ions in a wide dose range. Damage accumulation was studied by Rutherford backscattering/channeling spectrometry. Results show that the film composition affects the damage accumulation behavior in both MgZnO and CdZnO dramatically. In particular, increasing the Mg content in MgZnO results in enhanced damage accumulation in the region between the bulk and surface damage peaks characteristically distinguished in the pure ZnO. However, the overall damage accumulation in MgZnO layers, as well as in pure ZnO, exhibits saturation with increasing ion dose and MgZnO cannot be amorphized even at the highest ion dose used (3x10{sup 16} Er/cm{sup 2}). Increasing the Cd content in CdZnO affects the saturation stage of the damage accumulation and leads to an enhancement of damage production in both Cd and Zn sublattices.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Radiation tolerance of ceramics—Insights from atomistic simulation of damage accumulation in pyrochlores

We have used molecular dynamics simulations to examine the effects of radiation damage accumulation in two pyrochlore-structured ceramics, namely Gd2Ti2O7 and Gd2Zr2O7. It is well known from experiment that the titanate is susceptible to radiation-induced amorphization, while the zirconate does not go amorphous under prolonged irradiation. Our simulations show that cation Frenkel pair accumulation eventually leads to amorphization of Gd2Ti2O7. Anion disorder occurs with cation disorder. The amorphization is accompanied by a density decrease of about 12.7% and a decrease of about 50% in the elastic modulus. In Gd2Zr2O7, amorphization does not occur, because the residual damage is not sufficiently energetic to drive the material amorphous. Subtle differences in damage accumulation and annealing between the two pyrochlores lead to drastically different radiation response as the damage accumulates.

ceramic↗
DOE OSTI2015

Comparison of Two Models for Damage Accumulation in Simulations of System Performance

A comprehensive simulation study of system performance needs to address variations in component behavior, variations in phenomenology, and the coupling between phenomenology and component failure. This paper discusses two models of this: 1. damage accumulation is modeled as a random walk process in each time history, with component failure occurring when damage accumulation reaches a specified threshold; or 2. damage accumulation is modeled mechanistically within each time history, but failure occurs when damage reaches a time-history-specific threshold, sampled at time zero from each component’s distribution of damage tolerance. A limiting case of the latter is classical discrete-event simulation, with component failure times sampled a priori from failure time distributions; but in such models, the failure times are not typically adjusted for operating conditions varying within a time history. Nowadays, as discussed below, it is practical to account for this. The paper compares the interpretations and computational aspects of the two models mentioned above.

42 ENGINEERING↗
DOE OSTI2018

Ion mass dependence of irradiation-induced damage accumulation in KTaO 3

Damage production and amorphization resulting from the interaction of medium-energy (from 40 to 480 keV) noble-gas ions (from He to Kr) with potassium tantalate (KTaO 3 ) are determined using ion channeling measurements. A disorder accumulation model has been fit to the maximum damage concentration versus ion fluence to extract the cross sections for direct-impact and defect-stimulated amorphization, and the results indicate that defect-stimulated amorphization is the dominant mechanism. These cross sections exhibit a strong dependence on the calculated cross sections for displacing lattice atoms, indicating a dominant contribution of nuclear interactions to the defect production and amorphization processes under the irradiation conditions used in this study. Furthermore, these experimental findings, along with the model fits, suggest that the difference in recoil spectra between He and the other heavier ions may be the main driving force for the decreased damage efficiency observed for He ions, which results in a reduced rate of damage accumulation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Delayed damage accumulation by athermal suppression of defect production in concentrated solid solution alloys

A combined experimental and computational evaluation of damage accumulation in ion-irradiated Ni, NiFe, and NiFeCoCr is presented. Furthermore, a suppressed damage accumulation, at early stages (low-fluence irradiation), is revealed in NiFeCoCr, with a linear dependence as a function of ion fluence, in sharp contrast with Ni and NiFe. This effect, observed at 16 K, is attributed to the complex energy landscape in these alloys that limits defect mobility and therefore enhances defect interaction and recombination. Our results, together with previous room-temperature and high-temperature investigations, suggest "self-healing" as an intrinsic property of complex alloys that is not a thermally activated process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Coupled effects of electronic and nuclear energy deposition on damage accumulation in ion-irradiated SiC

Coupling between electronic and nuclear energy dissipation in ion-irradiated, single crystal 4H-SiC has been investigated using Si, Ti, and Ni ions over a range of energies at 300 K, and irradiation damage accumulation is characterized using Rutherford backscattering spectroscopy in channeling geometry. The damage production rate from nuclear energy loss (S n ) is observed to decrease with increasing electronic energy loss (S e ) of the incident ions. A dynamic threshold (S e,th ) in electronic energy loss is determined for each ion species, which defines two regions: i) S e > S e,th , where electronic energy dissipation fully suppresses damage production due to nuclear energy loss along incident ion paths, and ii) S e < S e,th , where simultaneous damage recovery due to Se competes with damage production processes. Here, the electronic energy loss threshold (S e,th ) increases sublinearly with incident ion atomic number. Here, the assessment of S e,th and how it affects damage accumulation is important to advance the understanding of complex processes occurring under ion-solid interactions, as well as in the design of functional materials for opto-electronics and novel structural materials and devices tolerant to harsh thermal and radiation environments.

36 MATERIALS SCIENCE↗
DOE OSTI2014

PBX 9502 Multimode Damage Accumulation Cycles-to-Failure Study

In FY13, we performed a detailed cycles-to-failure study of PBX 9502 using 3- point bend specimens. In that study we examined the effect of temperature and cyclic frequency on the failure behavior of PBX 9502. We identified cycles-to-failure curves for samples tested at -20°C, ~19°C and 50°C and at cyclic frequencies of 0.0001 up to 1-Hz. We also did cyclic conditioning of parts in an effort to establish damage accumulation criteria that could then be used in predictive models. As follow-on to that work, in FY14 we have added multimode cycles-to-failure to our test suite. Work this year re-established the baseline cycles-to-failure behavior of the PBX 9502 material tested round ~19°C at 1-Hz and also looked at damage accumulation under a multimode loading environment. This fresh baseline development was performed to remove any time related or test rig related variables. The multimode loading was done because real-world applications are not mono-modal and we desire for our predictive capability to be robust and include more complex, realistic scenarios. In the following sections we briefly describe the test setup, we provide a summary of the new baseline results and we show the multimode cycles-to-failure data and how it compares to the mono-modal test data.

42 ENGINEERING↗
DOE OSTI2024

Effect of Hydrogen and Helium on Irradiation Damage Accumulation in Nickel

We report our results on the study of the effects of hydrogen and helium on irradiation damage accumulation in nickel, aiming to understand the TEM observations made by Edwards et al. regarding the Ni-coatings protecting the getter tube in a TPBAR. We hypothesized that differences in the fluxes of 3 H and/or He between the coatings might explain these microstructural variations. However, our simulation results show that during the initial stages of irradiation, the presence of H or He enhances void nucleation by stabilizing small vacancy clusters against thermal dissociation and does not enhance void growth. Nevertheless, we think that helium’s higher diffusivity, stronger binding with vacancy clusters, and ability to displace a nearby lattice atom—either to create a new vacancy or to enlarge an existing vacancy cluster—could result in a larger average vacancy cluster size. Therefore, it is likely that void growth will occur post-nucleation phase as more He accumulates at higher doses than those simulated in this study.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Hybrid Physics-Based, Data-Driven Approach to Model Damage Accumulation in Corrosion of Polymeric Adhesives

The degradation of polymeric adhesives in dissimilar joints is a menace to electric and structural components of the automotive industry. In polymeric adhesives, failure often occurs due to damage accumulated from discrete environmental sources—thermal degradation, oxidation, hydrolysis, radiation, and moisture—that are combined with progressive damage mechanisms, such as aging and fatigue. Because most damage mechanisms that lead to degradation act in parallel, the estimation of the lifetime of a polymeric material becomes a significant reliability issue, no study currently exists that can simultaneously consider even two of these phenomena. Most current efforts are directed toward a single-phenomenon, single-model approach in which a model is developed to cover one specific damage mechanism. While this approach is advantageous in exploring single mechanisms, it is strongly inconclusive for degradation that occurs by multiple damage mechanisms. Our team plans to take a fresh perspective by designing a hybrid constitutive/data-driven approach that allows models of different damage mechanisms to be integrated into one model and then used to explore the degradation-induced failure of polymeric adhesives. The theoretical-computational part of this work is divided into four research components as described in the Approach section below.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of electron–phonon coupling on damage accumulation in molecular dynamics simulations of irradiated nickel

The role of the electronic system in high energy displacement cascades is explored. The energy exchange between the electronic and the atomic subsystem is described by the electron–phonon coupling. The electronic effects on the damage accumulation due to 100 keV Ni ion cascades in nickel, a prototype system to a large group of nickel-based high entropy alloys, are investigated for overlapping cascades. It is shown that the energy exchange between the two subsystems affects microstructure evolution, resulting in the formation of smaller clusters and more isolated defects. This effect is more significant for the vacancy cluster formation and size distribution.

36 MATERIALS SCIENCE↗