| 1. |
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| 2. |
- Huang, Liangzhao, et al.
(författare)
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Impact of the local microstructure fluctuations on radiation-induced segregation in dilute Fe-Ni and Ni-Ti model alloys : A combined modeling and experimental analysis
- 2022
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 225
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Tidskriftsartikel (refereegranskat)abstract
- From a systematic atom probe tomography (APT) characterization of the radiation-induced segregation (RIS) in dilute Fe-Ni and Ni-Ti model alloys, we highlight fluctuations of the solute local concentration up to the scale of the APT specimens. We deduce the RIS at dislocation loops from a solute diffusion equation, that is solved at steady state, within the Voronoi's volume occupied by a single loop. From a statistical sampling of the Voronoi's volume and the dislocation loop radius modeled after the characterization of the microstructure by transmission electron microscopy, we provide the full RIS distribution. The present statistical approach of RIS demonstrates that the fluctuation of local solute concentrations in Fe-Ni and Ni-Ti mainly results from the dispersion in size and density of the dislocation loop population. Besides, we highlight the impact of the post-treatment parameters used in the APT protocol on the extracted RIS profiles. In Ni-Ti alloys, the simulated Ti-depletion profiles are in very good agreement with the measured ones. Furthermore, the dispersion of the loop radius and density is shown to play a critical role on the fluctuations of the Ti local concentration. In Fe-Ni, the identification of discrepancies between the simulated Ni-enrichment profiles and the measured ones provides a signature of additional operating mechanisms of the solute redistribution, such as radiation-induced precipitation.
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| 3. |
- Toijer, Elin, et al.
(författare)
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Ab initio investigation of effects of solute segregation onintergranular fracture in nickel: Importance of fracture path andstructural modification
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the present work, the impact of chromium and silicon impurities on thegrain boundary strength of nickel has been investigated by means of densityfunctional theory (DFT) modelling. The excess-energy approach has beenused to model the impact of the solutes, and a quantitative comparison of their corresponding impact in combination with the impact of phosphorus, self-interstitial Ni is here presented. The results of main importance are inreference to the Cr: It is shown that the common assumption that interstitial Cr segregates to the center of a GB gives decisively erroneous resultsregarding the impact of the solute in this context. The solute will instead remain at the GB surface, and consequently self interstitial Ni will migrateto the GB center. This configuration will give an important weakening tothe structure, in clear contrast to previous studies were Cr has been shown to promote strengthening of the same structure. However, the results in the current work further show that this weakening is in fact not due to the presence of Cr, but to the enrichment of self-interstitial atoms in the GB center.
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| 4. |
- Toijer, Elin, et al.
(författare)
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Ab initio modelling of intergranular fracture of nickel containing phosphorus : Interfacial excess properties
- 2021
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 28
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, the impact of phosphorus impurities on the grain boundary strength of nickel has been investigated by means of density functional theory (DFT) modelling. Owing to different outcomes and trends previously reported in the literature, it is unclear whether P is strengthening or weakening the Ni grain boundary. To address this issue, we utilize three different DFT based methods: the excess-energy approach, rigid grain separation, and Rice-Wang's thermodynamic approach. The results show that the commonly used rigid model predicts P to have an increasing effect on the peak stress of Ni of up to 14%, as opposed to a reduction, which is indicated by the excess-energy approach. Employment of the Rice-Wang approach, on the other hand, displays a slight reduction in work of separation. The results show that the discrepancies between previous works can be attributed not so much to the physics of the system, but to the applied model, the partition scheme and the interpretation of the outcomes. This underlines the importance of a proper description of the fracture process, and shows that common simplifications can have a decisive impact on the observed trends.
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| 5. |
- Toijer, Elin, et al.
(författare)
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Ab initio modelling of intergranular fracture of nickel containing phosphorus: Interfacial excess proper-ties
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the present work, the impact of phosphorus impurities on the grain bound-ary strength of nickel has been investigated by means of density functionaltheory (DFT) modelling. Owing to different outcomes and trends previouslyreported in the literature, it is unclear whether P is strengthening or weakening the Ni grain boundary. To address this issue, we utilize three different DFT based methods: the excess-energy approach, rigid separation, and Rice-Wang’s thermodynamic approach. The results show that the commonly used rigid model predicts P to have a strengthening effect in Ni of up to 14% forthe peak stress, as opposed to a weakening effect, which is indicated by themore rigorous excess-energy approach. Employment of the Rice-Wang approach, on the other hand, displays a slight reduction in work of separation. The result show that the discrepancies between previous works can be attributed not so much to the physics of the system, but to the applied model. This underlines the importance of a proper description of the fracture pro-cess, and shows that common simplifications can have a decisive impact onthe observed trends.
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| 6. |
- Toijer, Elin, et al.
(författare)
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Anion effects on the catalytic decomposition of H2O2 on ZrO2(s) in aqueous systems
- 2020
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Ingår i: CHEMISTRYSELECT. - : Wiley. - 2365-6549. ; 5:43, s. 13754-13760
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Tidskriftsartikel (refereegranskat)abstract
- Various oxide surfaces have been shown to catalyze H2O2 decomposition, resulting in the intermediate formation of hydroxyl and hydroperoxyl radicals. In this work, the impact of bromide, chloride, and perchlorate separately and in combination with bicarbonate on the reaction between H2O2 and ZrO2 was investigated. The reaction was studied by monitoring the concentration of H2O2 as a function of time, and by probing the hydroxyl radical production using tris as a scavenger and monitoring the product formaldehyde. The most significant impact was seen in the presence of Br- and Br-/HCO3-. Br- significantly increases the H2O2 decomposition rate and the yield of formaldehyde, whereas the combination of Br-/HCO3- was shown to decrease the H2O2 decomposition rate as well as the yield of formaldehyde. The increased formaldehyde yield is explained by its formation in reaction with Br-2(center dot-). This contrasts the previous assumption that formaldehyde is only formed upon hydrogen abstraction from tris.
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| 7. |
- Toijer, Elin, et al.
(författare)
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H 2 O 2 and γ-radiation induced corrosion of 304L stainless steel in aqueous systems
- 2019
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Ingår i: Radiation Physics and Chemistry. - : Elsevier Ltd. - 0969-806X .- 1879-0895. ; 159, s. 159-165
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Tidskriftsartikel (refereegranskat)abstract
- In light-water reactors the water used as neutron moderator and coolant is subjected to a constant radiation field which leads to the formation of a number of oxidizing and reducing species. In this work, the reactivity of the radiolysis product H 2 O 2 towards the 304L alloy, commonly used for structural materials in nuclear power plants, was investigated as well as oxidative dissolution of steel components as a consequence of γ-radiation and chemically added H 2 O 2 . The concentration of hydrogen peroxide as a function of time was monitored in the presence of different amounts of steel powder, and the second order reaction rate constant was determined to k 2 = (1.8 ± 0.2) × 10 −10 m s −1 . In the case of catalytic decomposition of hydrogen peroxide, hydroxyl radicals are formed which can be scavenged by methanol. In this reaction formaldehyde is formed, which can be detected spectroscopically. A high yield of formaldehyde was observed, indicating that catalytic decomposition is the main reaction path of H 2 O 2 in the current system. A significant contribution of the homogeneous Fenton reaction to both the reaction rate constant and the formaldehyde formation must however be considered, as Fe(II) will be released from the oxide layer in solution. In the case of γ-irradiation, an increased nickel content in solution compared to background experiments is seen. When the steel is subjected to chemically added hydrogen peroxide on the other hand, the chromium content in solution is increased. This indicates that the different types of exposure will impact different parts of the oxide layer characterized by different composition.
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| 8. |
- Toijer, Elin
(författare)
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Radiation-Induced Damage Processes in Nuclear Reactor Environments
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is essential that the materials used in nuclear reactor applications maintain their structural integrity. However, consequences of the strong radiation field in the reactor core include radiolysis of the water surrounding the materials, as well as direct defect generation in the material itself. This may result in the combined corrosive attacks from the environment, and the internal weakening due to processesin the bulk. The goal of this thesis is to extend the current knowledge of the many radiation-induced processes in the materials used in nuclear reactor applications, and to assess how such processes in turn can influence the materials’ stress tolerance. Radiation-induced surface effects have here been evaluated from experiments, whereas bulk processes are investigated in a theoretical perspective. This providesa broad picture of the many phenomena which lead to material degradation in an irradiated environment.In the first part of this thesis, the reaction mechanisms of H2O2 , which is an important product of water radiolysis, and oxide surfaces of interest in nuclear reactorapplications, have been evaluated. As a first step, the impact of Br– , Cl– and HCO3– on the reaction mechanisms between ZrO2 and H2O2 are determined. This provides an increased understanding for how competing reactions between the anions and H2O2 for the oxide surface can influence the kinetics of the system. As a second step, reaction mechanisms and reaction rate constants between H2O2 and the 304L steel in solution have been determined. Oxidative dissolution of steel components are here assessed, and the reaction paths of H2O2 towards the steel are discussed.In the second part of this work, the objective has been to evaluate how direct effects of radiation can influence the stress tolerance of the material. Radiation-induced segregation of chemical components in the model material fcc Ni have been quantified, and focus has been on the driving forces behind this segregation. Based on the segregation trends, the impact of solute enrichment on grain boundaries have been assessed. In this context, focus has been not only to quantify the effects of the solute segregation, but also to evaluate how common modeling techniques can influencethe results. Particularly, it is here shown that the applied model can have a decisive impact on observed trends. Moreover, it is shown that the common assumption that all solutes that are driven to sinks such as grain boundaries will end up in the grain boundary center, can also give very misleading results. The concentration-dependent effect of Si, the slightly weakening effect of P, and the particular effects of Cr are all quantified using state of the art atomistic modeling. As a final step, the impact of magnetic disorder on thermal expansion and defect formation energies have here been calculated in Ni. The model used in this part of the work very accurately predict the thermal expansion of the material, and shows that this is a majorcomponent in the temperature dependence of the defect formation enthalpies.
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| 9. |
- Toijer, Elin, et al.
(författare)
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Solute-point defect interactions, coupled diffusion, and radiation-induced segregation in fcc nickel
- 2021
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- Radiation-induced segregation (RIS) of solutes in materials exposed to irradiation is a well-known problem. It affects the lifetime of nuclear reactor core components by favoring radiation-induced degradation phenomena such as hardening and embrittlement. In this work, RIS tendencies in face centered cubic (fcc) Ni-X (X = Cr, Fe, Ti, Mn, Si, P) dilute binary alloys are examined. The goal is to investigate the driving forces and kinetic mechanisms behind the experimentally observed segregation. By means of ab initio calculations, point-defect stabilities and interactions with solutes are determined, together with migration energies and attempt frequencies. Transport and diffusion coefficients are then calculated in a mean-field framework, to get a full picture of solute-defect kinetic coupling in the alloys. Results show that all solutes considered, with the exception of Cr, prefer vacancy-mediated over interstitial-mediated diffusion during both thermal and radiation-induced migration. Cr, on the other hand, preferentially migrates in a mixed-dumbbell configuration. P and Si are here shown to be enriched, and Fe and Mn to be depleted at sinks during irradiation of the material. Ti and Cr, on the other hand, display a crossover between enrichment at lower temperatures, and depletion in the higher temperature range. Results in this work are compared with previous studies in body centered cubic (bcc) Fe, and discussed in the context of RIS in austenitic alloys.
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| 10. |
- Toijer, Elin, et al.
(författare)
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The impact of magnetic disorder on defectformation energies in fcc nickel
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this work, the impact of magnetic disorder of material properties in fccNi has been investigated based on the disordered local moment - moleculardynamics approach. The thermal expansion of the material was calculated, together with temperature dependence of the defect formation enthalpies of common point defects. Results show that the effect of the thermal expansion is an important component for both vacancy and interstitial formation enthalpies, but that magnetic disordering and vibration dynamics is also of clear significance for the interstitial. Based on the results, and models for entropy available in the literature, the Gibbs free energies of defect formation were estimated as a function of temperature. The results in this work providea good basis for a further study of defect properties the paramagnetic regionof the system.
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