Damage In Structural Materials Research Articles

Data Driven In Situ TEM: A Path Towards Accurate Characterization of Radiation Damage in Structural Materials.

Data Driven In Situ TEM: A Path Towards Accurate Characterization of Radiation Damage in Structural Materials.

Theoretical studies on the stabilization and diffusion behaviors of helium impurities in 6H-SiC by DFT calculations

In fusion environments, large scales of helium (He) atoms are produced by a radical transformation along with structural damage in structural materials, resulting in material swelling and degradation of physical properties. To understand its irradiation effects, this paper investigates the stability, electronic structure, energetics, charge density distribution, PDOS and TDOS, and diffusion processes of He impurities in 6H-SiC materials. The formation energy indicates that a stable, favorable position for interstitial He is the HR site with the lowest energy of 2.40 eV. In terms of vacancy, the He atom initially prefers to substitute at pre-existing Si vacancy than C vacancy due to lower substitution energy. The minimum energy paths (MEPs) with migration energy barriers are also calculated for He impurity by interstitial and vacancy-mediated diffusion. Based on its calculated energy barriers, the most possible diffusion path includes the exchange of interstitial and vacancy sites with effective migration energies ranging from 0.101 eV to 1.0 eV. Our calculation provides a better understanding of the stabilization and diffusion behaviors of He impurities in 6H-SiC materials.

Assessment of the degree of damage in structural materials using the parameters of structural acoustic noise

ABSTRACT The study considers the features of controlling the degree of structural metallic material degradation at the stage of scattered damage accumulation before the appearance of macrocracks using an approach based on the analysis of structural acoustic noise. Various methods of measuring the parameters of structural noise have been analysed from the point of view of their information content and the stability of the used calculation algorithms. Refined calculation algorithms for determining the spectral-energy parameters of structural noise are proposed, and the results of their experimental verification are presented. Comparative results of the stability of these algorithms are given. On the example of API X70 steel samples, cut from the main gas pipeline after long-term operation, and the sample of 08Cr18Ni10Ti steel, tested for low-cycle fatigue, the effectiveness of the proposed algorithm application for assessing the degree of critical material technical objects damage and the possibility of constructing a method for diagnosing products from these materials is shown.

Impact of materials technology on the breeding blanket design – Recent progress and case studies in materials technology

A major part in the EUROfusion materials research program is dedicated to characterize and quantify nuclear fusion specific neutron damage in structural materials. While the majority of irradiation data gives a relatively clear view on the displacement damage, the effect of transmutation – i.e. especially hydrogen and helium production in steels – is not yet explored very well. However, few available results indicate that EUROFER-type steels will reach their operating limit as soon as the formation of helium bubbles reaches a critical amount or size. At that point, the material would fail due to embrittlement at the considered load.This paper presents a strategy for the mitigation of the before-mentioned problem using the following facts:•the neutron dose and related transmutation rate decreases quickly inside the first wall, that is, only a plasma-near area is extremely loaded•nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium and hydrogen, which would suppress the formation of large helium bubbles•compared to conventional steels, ODS steels show improved irradiation tensile ductility and creep strengthIn summary, producing the plasma facing, highly neutron and heat loaded part of blankets by an ODS steel, while using EUROFER97 for everything else, would allow a higher heat flux as well as a longer operating period.Consequently, we (1) developed and produced 14 % Cr ferritic ODS steel plates. (2) We fabricated a mockup with 5 cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. And finally, (3) we performed high heat flux tests in the HELOKA facility (Helium Loop Karlsruhe at KIT) applying short and up to 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e., 550 °C) was finally exceeded by 100 K. Thereafter, microstructure and defect analyses did not reveal defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. This demonstrates that the use of ODS steel could make a decisive difference in the future design and performance of breeding blankets.

Quantification of the temperature threshold of hydrogen embrittlement in X90 pipeline steel

In the current research, the dependence of hydrogen embrittlement (HE) on the temperature in X90 steel is studied through experimental tests and molecular dynamics simulations. Slow strain rate tests (SSRT) were applied to X90 specimens in the air and the simulated groundwater solution (called NS4), respectively. The fracture morphologies of the specimens were observed by scanning electron microscope (SEM). The results indicate that the temperature threshold of hydrogen embrittlement (HE) in X90 steel is 313 K, beyond which the HE would be weakened with the rise of temperature, and below which the HE would be enhanced with the rise of temperature. To illustrate the underlying mechanism of this phenomenon, molecular dynamics simulations were applied to quantify the correlation of temperature with hydrogen diffusivity, and the results of Devanathan-Stachurski tests were used to quantify the bulk hydrogen concentration at different temperatures. A theoretical model was thus developed based on hydrogen potential thermodynamics to predict the threshold temperature. The predictive model matches well with experimental results, revealing the promoting effect of hydrogen diffusion and accumulation on the crack growth, which is fundamental for understanding hydrogen-induced damage in structural materials.

Real-time quantification of damage in structural materials during mechanical testing.

A novel methodology is introduced for quantifying the severity of damage created during testing in composite components. The method uses digital image correlation combined with image processing techniques to monitor the rate at which the strain field changes during mechanical tests. The methodology is demonstrated using two distinct experimental datasets, a ceramic matrix composite specimen loaded in tension at high temperature and nine polymer matrix composite specimens containing fibre-waviness defects loaded in bending. The changes in the strain field owing to damage creation are shown to be a more effective indicator that the specimen has reached its proportional limit than using load-extension diagrams. The technique also introduces a new approach to using experimental data for creating maps indicating the spatio-temporal distribution of damage in a component. These maps indicate where damage occurs in a component, and provide information about its morphology and its time of occurrence. This presentation format is both easier and faster to interpret than the raw data which, for some tests, can consist of tens of thousands of images. This methodology has the potential to reduce the time taken to interpret large material test datasets while increasing the amount of knowledge that can be extracted from each test.

Assessment of Scattered Damage in Structural Materials

Methods for assessing scattered damage to materials are considered. Ultrasonic methods based on recording backscattered ultrasonic signals are among the most practically feasible methods for assessing scattered damage to materials at the mesolevel (the size of the probing wavelength). We describe methods for determining scattered damage in the volume of a material based on scanning the object surface with a normal double-crystal piezoelectric transducer, recording and statistically processing the backscattered signal in the form of an A-scan, constructing the spatial distribution of backscattering cross section in the form of a B-scan or tomographic image, and assessing damage in the material volume based on the relative change in the backscattering cross section or the “disorder” of its spatial image.

Ion irradiation combined with nanoindentation as a screening test procedure for irradiation hardening

Ion irradiation has long been recognized as a means to efficiently approximate neutron damage in structural materials. Likewise, nanoindentation has long been recognized as a tool to probe the mechanical behaviour of thin layers. The combination of both techniques in order to establish a screening test procedure for the resistance of ferritic/martensitic (f/m) steels to neutron damage in terms of hardening requires consideration of a number of details. The objective is to specify one among several possible variants of such a screening test. Important constituents of the approach include: (1) the design of the ion irradiation experiments, e.g. using the Monte Carlo binary collision code SRIM, (2) nanoindentation testing over a large range of indentation depths, and (3) proper consideration of the indentation size effect, the substrate effect and the pile-up effect. An elastic-modulus-based correction of the contact area was rationalized. A version of the overall approach sketched above was applied to unirradiated, self-ion-irradiated and neutron-irradiated samples of the 9% Cr f/m steel T91. Apparently, this is the first direct comparison of nanoindentation results obtained for samples of the same f/m steel irradiated with ions and neutrons at the same temperature (200 °C) and up to about the same fluence (2.5 dpa versus 2.31 dpa). The findings indicate that the indentation hardness increase is significant and agrees within the range of errors.

Emulation of reactor irradiation damage using ion beams

Progress in understanding radiation damage in structural materials is hampered by the lack of test reactors, long irradiations and high cost. Here we show that through strict control of experimental parameters and accounting for He production and damage-rate differences, the microstructure of ion-irradiated ferritic–martensitic steel closely resembles that created in-reactor across the full range of microstructure features. The level of agreement establishes for the first time the capability to tailor ion irradiation to emulate in-reactor radiation damage.

Calculation of displacement cross-sections for structural materials in accelerators using PHITS event generator and its applications to radiation damage

The displacement cross-sections, implemented in the particle and heavy ion transport code system (PHITS), have been calculated to estimate the radiation damage in structural materials used at accelerator facilities. The event generator in PHITS was used to calculate displacement cross-sections for 14 elemental targets and two practical alloys irradiated with neutrons at energies from 10−10 MeV to 3 GeV, and protons and deuterons at energies from several keV to 3 GeV. These displacement cross-sections were used to estimate the displacement per atom (DPA) in the target assembly at accelerator driven system (ADS) Target Test Facility (TEF-T) using 400-MeV protons, and in the Type 316 stainless steel target using 40-MeV deuterons. For the target assembly at TEF-T using 400-MeV protons, the DPA value for the proton component was approximately twice higher than that for the neutron component. For the Type 316 target using 40-MeV deuterons, radiation damage took place near the surface of Type 316 because of the Coulomb scattering effect between the incident deuterons and the material.

Solving the Puzzle of⟨100⟩Interstitial Loop Formation in bcc Iron

The interstitial loop is a unique signature of radiation damage in structural materials for nuclear and other advanced energy systems. Unlike other bcc metals, two types of interstitial loops, 1/2<111> and <100>, are formed in bcc iron and its alloys. However, the mechanism by which <100> interstitial dislocation loops are formed has remained undetermined since they were first observed more than fifty years ago. We describe our atomistic simulations that have provided the first direct observation of <100> loop formation. The process was initially observed using our self-evolving atomistic kinetic Monte Carlo method, and subsequently confirmed using molecular dynamics simulations. Formation of <100> loops involves a distinctly atomistic interaction between two 1/2<111> loops, and does not follow the conventional assumption of dislocation theory, which is Burgers vector conservation between the reactants and the product. The process observed is different from all previously proposed mechanisms. Thus, our observations might provide a direct link between experiments and simulations and new insights into defect formation that may provide a basis to increase the radiation resistance of these strategic materials.

The nature of high-energy radiation damage in iron

Understanding and predicting a material’s performance in response to high-energy radiation damage, as well as designing future materials to be used in intense radiation environments, requires knowledge of the structure, morphology and amount of radiation-induced structural changes. We report the results of molecular dynamics simulations of high-energy radiation damage in iron in the range 0.2–0.5 MeV. We analyze and quantify the nature of collision cascades both at the global and the local scale. We observe three distinct types of damage production and relaxation, including reversible deformation around the cascade due to elastic expansion, irreversible structural damage due to ballistic displacements and smaller reversible deformation due to the shock wave. We find that the structure of high-energy collision cascades becomes increasingly continuous as opposed to showing sub-cascade branching as reported previously. At the local length scale, we find large defect clusters and novel small vacancy and interstitial clusters. These features form the basis for physical models aimed at understanding the effects of high-energy radiation damage in structural materials.

Effects of helium and deuterium on irradiation damage in pure iron

Productions of transmute elements (hydrogen and helium) have great influences on the resistance to irradiation damage in structural materials for fusion reactor. The evolution of irradiation damage in bcc iron is investigated with ion implantation and electron irradiation. Pure iron implanted by He+ or D+ ions at room temperature are aged at 500℃ for 1 h, then irradiated by electrons under high voltage electron microscope. The results show that interstitial loops (i-loop) and vacancy loops (v-loop) are formed in He+-implanted iron and D+-implanted iron respectively. Under electron irradiation, due to the absorption of interstitials atom, i-loop grows up while v-loop shrinks. According to the rate of variation of dislocation loop, v-loop absorbs more interstitial atoms, i.e., the dislocation bias of D+-implanted iron is larger than that of He+-implanted iron, which means that the v-loop has the more contributions to irradiation swelling than i-loop. The causes of the different natures of dislocation loops formed in D+-implanted iron and He+-implanted iron are analyzed by the structures of He-V and D-V complexes.

On sequencing the feature extraction techniques for online damage characterization

The current state of the health-monitoring technology lacks a generalized and definitive approach to the identification and localization of mechanical damage in structural materials. In past decades, several signal-processing tools have been used for solving different health-monitoring problems but the commutability of the tools between different problems has been restricted. The fundamental reasons for this shortcoming have never been investigated in detail. A thorough study is presented in this article employing almost all promising feature extraction tools on a representative problem—a plate with rivet holes. The cracks around rivet holes in a joint panel of a steel truss bridge are very difficult to detect. Although well established, Lamb wave–based nondestructive evaluation techniques are revisited and new tools are developed to address this issue. The simulation of scattered ultrasonic wave field is carried out using the finite element method. This ultrasonic wave field is further analyzed to evaluate the integrity of the structure using various feature extraction techniques. The joint time–frequency–energy representation is obtained from ultrasonic signals recorded at various locations on the plate (joint panel) and used to extract damage-sensitive features. Those features were then used to formulate a new damage parameter for better visualization of the crack. The results are shown to demonstrate the comparative effectiveness of these techniques. It is concluded that any particular feature extraction technique cannot detect all possible sizes and orientations of the crack. It is suggested that the statistical occurrence and pattern of the crack must be visualized through few selective feature extraction techniques in a sequence.

Fatigue and Damage in Structural Materials Studied by X-Ray Tomography

This paper reviews progress using X-ray computed tomography to study damage accumulation. Since its introduction, X-ray microtomography has been used to diagnose the presence of damage. In this review, a wide range of damage-accumulation mechanisms are covered including cavitation, fracture, microcracking, fatigue cracking, and stress corrosion cracking. In this regard, the advantages of attenuation and phase contrast imaging are discussed. This review includes both measurements of damage accumulation, taken postmortem, and the incremental monitoring of damage-accumulation processes during life (sometimes termed four-dimensional tomography). In addition to the qualitative diagnostic studies, the quantitative analysis of tomography images to extract key failure parameters is examined.

Combining in situ transmission electron microscopy irradiation experiments with cluster dynamics modeling to study nanoscale defect agglomeration in structural metals

We present a combinatorial approach that integrates state-of-the-art transmission electron microscopy (TEM) in situ irradiation experiments and high-performance computing techniques to study irradiation defect dynamics in metals. Here, we have studied the evolution of visible defect clusters in nanometer-thick molybdenum foils under 1MeV krypton ion irradiation at 80°C through both cluster dynamics modeling and in situ TEM experiments. The experimental details are reported elsewhere; we focus here on the details of model construction and comparing the model with the experiments. The model incorporates continuous production of point defects and/or small clusters, and the accompanying interactions, which include clustering, recombination and loss to the surfaces that result from the diffusion of the mobile defects. To account for the strong surface effect in thin TEM foils, the model includes one-dimensional spatial dependence along the foil depth, and explicitly treats the surfaces as black sinks. The rich amount of data (cluster number density and size distribution at a variety of foil thickness, irradiation dose and dose rate) offered by the advanced in situ experiments has allowed close comparisons with computer modeling and permitted significant validation and optimization of the model in terms of both physical model construct (damage production mode, identities of mobile defects) and parameterization (diffusivities of mobile defects). The optimized model exhibits good qualitative and quantitative agreement with the in situ TEM experiments. The combinatorial approach is expected to bring a unique opportunity for the study of radiation damage in structural materials.

Monitoring mechanical damage in structural materials using complimentary NDE techniques based on thermography and acoustic emission

This work deals with nondestructive evaluation (NDE) of the fracture behavior of metallic materials by combining thermographic and acoustic emission (AE) characterization. A new procedure, based on lock-in infrared (IR) thermography, was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermography results on crack growth rate were found to be in agreement with measurements obtained by the conventional compliance method. Furthermore, acoustic emission was used to record different cracking events. The rate of incoming signals, as well as qualitative features based on the waveform shape, was correlated with macroscopically measured mechanical parameters, such as load and crack propagation rate. Additionally, since the failure modes have distinct AE signatures, the dominant active fracture mode was identified in real time. The application of combined NDE techniques is discussed for characterizing the damage process which leads to catastrophic failure of the material, thereby enabling life prediction in both monolithic aluminum alloys and aluminum alloy/SiC particle (SiCp) reinforced composites.

Characterization of creep and creep damage by in-situ microtomography

Application of in-situ microtomography to characterization of power law creep and creep damage in structural materials is presented. It is shown first that the successively reconstructed volumes are adequately monitoring the macroscopic sample shape and that microtomography is an optimal tool to characterize inhomogeneous specimen deformation. Based on a two-step image correlation technique the evolution of single voids is revealed and the basis of a pioneering approach to creep damage studies is presented. The method allows the unequivocal separation of three concurrent damage mechanisms: nucleation, growth, and coalescence of voids. The results indicate that growth rate of voids with equivalent diameters in the range of 2–5 mm is of about one order of magnitude higher than the prediction of continuum solid mechanics. Analysis of void coalescence points out the presence of two stable growth regimes related to coalescence between primary and secondary voids, respectively.

Studying Radiation Damage in Structural Materials by Using Ion Accelerators

Radiation damage in structural materials is of major concern and a limiting factor for a wide range of engineering and scientific applications, including nuclear power production, medical applications, or components for scientific radiation sources. The usefulness of these applications is largely limited by the damage a material can sustain in the extreme environments of radiation, temperature, stress, and fatigue, over long periods of time. Although a wide range of materials has been extensively studied in nuclear reactors and neutron spallation sources since the beginning of the nuclear age, ion beam irradiations using particle accelerators are a more cost-effective alternative to study radiation damage in materials in a rather short period of time, allowing researchers to gain fundamental insights into the damage processes and to estimate the property changes due to irradiation. However, the comparison of results gained from ion beam irradiation, large-scale neutron irradiation, and a variety of experimental setups is not straightforward, and several effects have to be taken into account. It is the intention of this article to introduce the reader to the basic phenomena taking place and to point out the differences between classic reactor irradiations and ion irradiations. It will also provide an assessment of how accelerator-based ion beam irradiation is used today to gain insight into the damage in structural materials for large-scale engineering applications.

Interface-enabled defect reduction in He ion irradiated metallic multilayers

Metallic multilayers are good model systems to explore the effects of heterophase interfaces in reducing radiation damage in structural materials. We summarize recent studies on radiation damage in immiscible face-centered cubic/body-centered cubic metallic multilayers, in particular Cu/V and Cu/Nb. These multilayers have shown unique characteristics compared to bulk metals under irradiation, including several orders of magnitude higher He solid solubility, dramatic reduction of bubble density, interface confined growth of He bubbles, and much lower radiation hardening. The mechanisms for interface enhanced radiation tolerance are briefly discussed.