Vacancies and carbon impurities in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>- iron: Electron irradiation

Vehanen, A.; Hautojärvi, P.; Johansson, J.; Yli‐Kauppila, J.; Moser, P.

doi:10.1103/physrevb.25.762

Cited by 530 publications

(254 citation statements)

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“…Binding energies at 0 K and 433 K (160 C) for dominant PDCs (15 out of 38 PDCs studied) are summarized in Table I. At 0 K, the formation energy for a C octahedral is 0.86 eV lower than that for a C tetrahedral interstitial, consistent with experimental observations [12]. In contrast, hydrogen is more stable in a tetrahedral site, with a calculated formation energy 0.12 eV lower than that for an octahedral hydrogen site, also consistent with empirical data [14].…”

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confidence: 76%

Hydrogen-Vacancy Interactions in Fe-C Alloys

et al. 2009

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Energetics and concentrations of hydrogen-containing point defect clusters (PDCs) in Fe-C alloys are calculated and cast into a PDC dominance diagram. Because of the strong binding effects of iron vacancies on the stability of the clusters, hydrogen accumulation requires the total hydrogen and vacancy concentrations to be comparable. As a result of the interplay between repulsive and attractive binding processes, PDC populations in Fe-C-H effectively decouple into the binary systems Fe-C and Fe-H. This results in significant vacancy-hydrogen PDC populations even for low total hydrogen concentrations. DOI: 10.1103/PhysRevLett.103.085501 PACS numbers: 61.72.JÀ, 61.72.SÀ, 71.15.Mb, 71.15.Nc In the past few decades, a variety of fundamentally new phenomena has been observed in metals and their alloys under hydrogen-rich conditions, such as large volume contractions in body-centered cubic (bcc) -Fe [1,2], and the enhancement of diffusion at metal-metal junctions [3]. Furthermore, the well-known problem of hydrogeninduced degradation of the mechanical properties of metals has had significant impact in Fe-rich alloys such as hardened steels that suffer severe embrittlement [4,5]. Experimental and theoretical evidence [1][2][3][4][5][6][7] suggests that one unifying theme behind these hydrogen-mediated effects is the strong interaction between hydrogen impurities and other point defects in the material, and the subsequent microstructural changes that occur as a result of that interaction. Although a rich experimental literature detailing macroscopic effects exists, fundamental understanding of the interactions of hydrogen with point defect clusters (PDCs) in metals is limited. In particular, a solid theoretical description of PDC stability, accumulation, and the changes in the PDC concentrations is not yet available.In this Letter, we study the formation, binding, and concentrations of PDCs in bcc Fe-C alloys containing hydrogen. We use density functional theory (DFT) calculations for the energetics, coupled with a free-energy functional to determine the PDC concentrations as a function of total hydrogen and vacancy concentrations. The processes by which hydrogen binds to PDCs in Fe-C alloys are found to be dominated by the interaction of hydrogen (H) and carbon (C) with bcc iron vacancies (Va). By graphically presenting our results on the PDC concentrations in a dominance diagram, the important role of vacancies in determining the PDC populations is readily appreciated. The PDC dominance representation directly indicates the essential changes in the point defect population as a function of point defect composition in the alloy bulk, analogous in spirit to a phase diagram. An effective decoupling between the Va x H z and Va x C y populations is found, simplifying the full Fe-C-H system into a superposition of the PDC populations of the Fe-C and Fe-H binary systems. The strong Va-H interaction implies that the accumulation of hydrogen into PDCs comprising molecular hydrogen (H 2 ) is favored over PDCs containing atom...

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Hydrogen-Vacancy Interactions in Fe-C Alloys

et al. 2009

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“…Simulation temperature was set to 800 K which is higher than that of RPV operation (∼ 560 K). Calculated migration energy for Fe is higher than the experimental value (0.55 eV 17) ); the tendency is similar to previous calculations (0.67 eV and 0.65 eV). 3,5) Therefore we used the temperature in order to accelerate simulation progress.…”

Section: Kinetic Lattice Monte Carlo Calculationssupporting

confidence: 75%

Kinetic Monte Carlo Simulations of Initial Process of Solute Atom Cluster Formations Based on ab initio Data Base

Betsuyaku

Ohnuma

Soneda

2011

Progress in Nuclear Science and Technology

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An accurate prediction of embrittlement of neutron-irradiated reactor pressure vessel steel is very important for the long term operation of light water reactors. Although solute atom clusters formed in the steel have been observed by using three dimensional atom probe technique, understanding of the elementary mechanisms of the formation of the clusters have not been established. We have carried out ab initio calculations to determine the interactions among solvent, solute atoms and vacancies, and migration barriers in Fe based dilute alloys and built a database used to parameterize pair energies for kinetic lattice Monte Carlo simulations. The result of calculation shows surplus vacancies act as nucleus of small Cu clusters and are trapped by the clusters, and the number of clusters depends on the number of vacancies in the system. This indicates the importance of vacancy concentrations in the alloys caused by irradiations for the initial process of the solute atom cluster formation.

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“…PAS has already demonstrated its major role in the study of vacancy defects and material related properties. For example the results obtained by Vehanen et al [6] have had an outmost impact on the knowledge of the role of carbon on vacancy behavior in Fe. They have demonstrated the trapping of vacancy in Carbon Vacancy complexes and the non-localization of C atoms in the center of the vacancy.…”

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Positron Annihilation Spectroscopy to Characterize Irradiation Induced Vacancy Type Defects in Materials for Nuclear Fission and Fusion

Barthe

2016

EPJ Web of Conferences

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In nuclear reactors, materials are submitted to irradiations and modification of their macroscopic properties such as swelling or hardening is observed… It is of first importance to understand the origin of these evolutions and how damage at the atomic scale such as vacancy and interstitials type defects can interact with each other or with solutes and impurities to make the microstructure evolved. So the properties of defects have to be determined.Positron annihilation spectroscopy (PAS) is a well-established technique to characterize materials [1]. Due to its positive charge, positron is sensitive to the local variations of the Coulomb potential in solids. As the anti-particle of the electron, both particles can annihilate leading to the emission of gamma rays with energy depending on the momentum of the electron positron annihilated pair. The detection of these gamma rays arises original information on the defects because positron can be trapped and annihilate in localized state such as vacancies. Two different and complementary annihilation characteristics can be measured. Firstly Positron Annihilation Lifetime Spectrometer (PALS) allows to measure the positron lifetime which depends on the local electron density at the annihilation site. The second characteristic is the momentum distribution of the electron positron annihilated pairs and is obtained by measuring the energy spectrum of the gamma annihilation rays using a Doppler Broadening Spectrometer (DBS). Both characteristics give the signature of the defects and allow determining some of their properties such as their nature, concentration, chemical environment... By using slow positron accelerators which produce monokinetic positrons beams with energy varying from 0.1 to 30 keV it is possible to study the depth profile of defects in thin layers from 0.1 to about 5 µm depending on the density of materials with a resolution of the order of 0.1xdepth. 22Na source based beams [2] or user facilities taking advantages of high positrons flux available around nuclear reactor (FRMII, Garching [3]) or LINAC( as in AIST in Japan [4]) are now available to study defects in materials. PAS has numerous advantages among them, it is non-destructive and can be used for conducting as well as insulating materials crystalline or amorphous. This technique is especially useful to probe vacancy defects in metals for which it is one of the only direct characterization. The annihilation characteristics can be predicted by first principle calculations and these data when available can help to identify defects. Sensitive to the single vacancy, PAS can allow to determine the size of the vacancy clusters up to a maximum of 1 nm approximately in the concentration range from 10 15 to a few 10 18 cm -3 .In nuclear science this technique is very powerful to characterize not only damage induced by irradiation but also to determine some fundamental properties of defects that are required for modeling of the microstructural evolution of materials such as formation, migration, agglomeratio...

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Vacancies and carbon impurities inα- iron: Electron irradiation

Cited by 530 publications

References 41 publications

Hydrogen-Vacancy Interactions in Fe-C Alloys

Hydrogen-Vacancy Interactions in Fe-C Alloys

Kinetic Monte Carlo Simulations of Initial Process of Solute Atom Cluster Formations Based on ab initio Data Base

Positron Annihilation Spectroscopy to Characterize Irradiation Induced Vacancy Type Defects in Materials for Nuclear Fission and Fusion

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