| 1. |
- Boll, Torben, 1979, et al.
(author)
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An APT investigation of an amorphous Cr-B-C thin film
- 2015
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In: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 159, s. 217-222
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Journal article (peer-reviewed)abstract
- A magnetron sputtered amorphous Cr-B-C thin film was investigated by means of atom probe tomography (APT). The film is constituted of two phases; a Cr-rich phase present as a few nanometer large regions embedded in a Cr-poor phase (tissue phase). The Cr-rich regions form columnar chains oriented parallel to the growth direction of the film. It was found that the Cr-rich regions have a higher B:C ratio than the Cr-poor regions. The composition of the phases was determined as approximately 35Cr-33B-30C and 15Cr-40B-42C (at%), respectively. The results suggest that this type of nanocomposite films has a more complex structure than previously anticipated, which may have an importance for the mechanical and electrical properties.
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| 2. |
- Boll, Torben, 1979, et al.
(author)
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Grain Boundary Chemistry and Transport Through Alumina Scales on NiAl Alloys
- 2017
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In: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 88:3-4, s. 469-479
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Journal article (peer-reviewed)abstract
- It is widely accepted that the growth of protective ?-Al2O3 scales on Ni-based alloys is governed by the inward diffusion of oxygen through the oxide grain boundaries (GB). However, there is also some outward diffusion of metal ions to the surface, but it is difficult to quantify. In this work we apply atomic force microscopy, scanning electron microscopy and transmission electron microscopy to investigate the outward flux of Al, which manifests as the growth of small ridges along the alumina GBs after the removal of the outermost oxide layer by mechanical polishing or focused ion beam techniques followed by additional oxidation. As a model alumina-former, NiAl with Hf and Zr additions was investigated. In comparison to Zr, Hf was found to reduce the outward Al diffusion. This outward diffusion was six orders of magnitude smaller than the O inward diffusion.
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| 3. |
- Boll, Torben, 1979, et al.
(author)
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Interfaces in Oxides Formed on NiAlCr Doped with Y, Hf, Ti, and B
- 2017
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In: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23:2, s. 396-403
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Journal article (peer-reviewed)abstract
- This study applies atom probe tomography (APT) to analyze the oxide scales formed on model NiAlCr alloys doped with Hf, Y, Ti, and B. Due to its ability to measure small amounts of alloying elements in the oxide matrix and its ability to quantify segregation, the technique offers a possibility for detailed studies of the dopant's fate during high-temperature oxidation. Three model NiAlCr alloys with different additions of Hf, Y, Ti, and B were prepared and oxidized in O2 at 1,100°C for 100 h. All specimens showed an outer region consisting of different spinel oxides with relatively small grains and the protective Al2O3-oxide layer below. APT analyses focused mainly on this protective oxide layer. In all the investigated samples segregation of both Hf and Y to the oxide grain boundaries was observed and quantified. Neither B nor Ti were observed in the alumina grains or at the analyzed interfaces. The processes of formation of oxide scales and segregation of the alloying elements are discussed. The experimental challenges of the oxide analyses by APT are also addressed.
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| 4. |
- Boll, Torben, 1979, et al.
(author)
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On aliovalent cations control of α-alumina growth on doped and undoped NiAl
- 2021
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In: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 210
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Journal article (peer-reviewed)abstract
- Alumina forming Ni-base superalloys are essential due to their oxidation resistance at elevated temperatures. A two-step procedure allows to assess the outward growth of the oxide scale from the resulting oxide ridges that form at 1100°C and cap the alumina grain boundaries. Employing undoped 50Ni50Al (at%) as reference, the impact of reactive elements on the diffusion processes, here Zr and Hf, is quantified using atom probe tomography. Unexpectedly, we find that up to one monolayer of Ni may co-decorate the alumina grain boundaries. Additionally, a decrease in Al-, and Ni-diffusivity of two orders of magnitude is observed owing to the reactive element effect. We employ density functional theory calculations to better understand the role of aliovalent cations, here Ni(II), Zr(IV), and Hf(IV) in the α−alumina scale. The calculations show that Ni may not only decorate the alumina grain boundaries but also facilitates transport of electrons as well as oxygen vacancies. Thereby oxide scale growth becomes enhanced. In turn, the dual impact of reactive elements, i.e. to annihilate oxygen vacancies and to remove impurity states in the band gap, explains the reduced scale growth rate.
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| 5. |
- Fazi, Andrea, 1992, et al.
(author)
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CrN–NbN nanolayered coatings for enhanced accident tolerant fuels in BWR
- 2023
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In: Journal of Nuclear Materials. - 0022-3115. ; 586
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Journal article (peer-reviewed)abstract
- The accident tolerant fuel (ATF) concept has emerged in the years after the 2011 Fukushima accident as part of a renewed effort in research for light water reactors. The primary focus is to further improve safety measures under and beyond design basis accident conditions, and to improve fuel cladding performance in normal operation. The application of a coating on zirconium claddings can achieve both these aims without extensive changes to the reactor design. Metallic chromium coatings have been profusely studied as solution for pressurized water reactors, but the search for an effective ATF coating able to withstand the environment inside boiling water reactors (BWRs) is still ongoing. In this work, two different versions of a novel nitride coating composition were studied. Zirconium claddings coated with 8 µm thick layers of superlattice CrN–NbN and a nanolayered CrN–NbN were tested in autoclave under BWR operating conditions for 60 days. Scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron back-scattered diffraction, x-ray diffraction, and atom probe tomography were employed to characterize as-deposited and autoclaved samples of these two materials. During exposure, both coating versions formed a stable, dense and passivating oxide scale (200–300 nm thick) on the surface, demonstrating improved oxidation protection under operating conditions. Some differences in the oxide growth mechanism were observed between the superlattice and the nanolayered CrN–NbN coatings, which allowed to glimpse at the effect of the layer thickness on the oxidation protection provided by these coatings. The nano-structured morphology of both coatings remained unaffected by the autoclave test, but a 35 nm thick Zr-Cr-N phase was found at the coating-substrate interface of the superlattice CrN–NbN coated cladding.
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| 6. |
- Glezer, A. M., et al.
(author)
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Pinning of nanocrystals growth at Fe-Ni-B amorphous alloy crystallization: Atom probe investigations
- 2015
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In: Materials Letters. - : Elsevier BV. - 1873-4979 .- 0167-577X. ; 160, s. 339-342
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Journal article (peer-reviewed)abstract
- The crystallization processes in Fe50Ni33B17 amorphous alloy have been investigated by transmission electron microscopy (TEM) and a three dimensional atom probe (3DAP). The γ-Fe,Ni FCC crystals, 25 nm in size, were found in the amorphous alloy annealed at temperatures ranging 360-400 °C for 1 h. TEM demonstrates that the nanocrystals growth stops as the temperature rises. The formation of an amorphous layer enriched in boron at the border of γ-Fe,Ni nanocrystals responsible for the pinning of nanocrystals size was observed by 3DAP. The composition of an amorphous barrier by 3DAP was found to be Fe50Ni29B21. It was proposed that this composition reveals higher crystallization temperature, which is the reason of pinning nanocrystals growth.
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| 7. |
- Kaczmarski, J., et al.
(author)
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Controlling In-Ga-Zn-O thin films transport properties through density changes
- 2016
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090. ; 608, s. 57-61
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Journal article (peer-reviewed)abstract
- In the following study we investigate the effect of the magnetron cathode current (I-c) during reactive sputtering of In-Ga-Zn-O (a-IGZO) on thin-films nanostructure and transport properties. All fabricated films are amorphous, according to X-ray diffraction measurements. However, High Resolution Transmission Electron Microscopy revealed the a-IGZO fabricated at I-C = 70 mA to contain randomly-oriented nanocrystals dispersed in amorphous matrix, which disappear in films deposited at higher cathode current. These nanocrystals have the same composition as the amorphous matrix. One can observe that, while I-C is increased from 70 to 150 mA, the carrier mobility improves from mu(Hall) = 6.9 cm(2)/Vs to mu(Hall) = 9.1 cm(2)/Vs. Additionally, the increase of I-C caused a reduction of the depletion region trap states density of the Ru-Si-O/In-Ga-Zn-O Schottky barrier. This enhancement in transport properties is attributed to the greater overlapping of s-orbitals of the film-forming cations caused by increased density, evidenced by X-ray reflectivity, at a fixed chemical composition, regardless nanostructure of thin films. (C) 2016 Elsevier B.V. All rights reserved.
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| 8. |
- Khushaim, M., et al.
(author)
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Characterization of Precipitation in Al-Li Alloy AA2195 by means of Atom Probe Tomography and Transmission Electron Microscopy
- 2015
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In: Advances in Condensed Matter Physics. - : Hindawi Limited. - 1687-8108 .- 1687-8124. ; 2015, s. Art. no. 647468-
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Journal article (peer-reviewed)abstract
- The microstructure of the commercial alloy AA2195 was investigated on the nanoscale after conducting T8 tempering. This particular thermomechanical treatment of the specimen resulted in the formation of platelet-shaped T1Al2CuLi/θ′Al2Cu precipitates within the Al matrix. The electrochemically prepared samples were analyzed by scanning transmission electron microscopy and atom probe tomography for chemical mapping. The θ′ platelets, which are less than 2 nm thick, have the stoichiometric composition consistent with the expected Alu equilibrium composition. Additionally, the Li distribution inside the θ′ platelets was found to equal the same value as in the matrix. The equally thin T1 platelet deviates from the formula (AluLi) in its stoichiometry and shows Mg enrichment inside the platelet without any indication of a higher segregation level at the precipitate/matrix interface. The deviation from the (AluLi) stoichiometry cannot be simply interpreted as a consequence of artifacts when measuring the Cu and Li concentrations inside the T1 platelet. The results show rather a strong hint for a true lower Li and Cu contents, hence supporting reasonably the hypothesis that the real chemical composition for the thin T1 platelet in the T8 tempering condition differs from the equilibrium composition of the thermodynamic stable bulk phase.
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| 9. |
- Korchuganova, O. A., et al.
(author)
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Microstructural evolution of Fe-22%Cr model alloy under thermal ageing and ion irradiation conditions studied by atom probe tomography
- 2016
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 477, s. 172-177
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Journal article (peer-reviewed)abstract
- Nanostructure evolution during ion irradiation of two thermally aged binary Fee22Cr alloys has been investigated using atom probe tomography. Specimens aged at 500 °C for 50 and 200 h were irradiated by 5.6 MeV Fe ions at room temperature up to fluences of 0.3 × 1015 ions/cm2 and 1 × 1015 ions/cm2. The effect of irradiation on the material nanostructure was examined at a depth of 1 ?m from the irradiated surface. The analysis of Cr radial concentration functions reveals that dense ??-phase precipitates in the 200 h aged alloy become diffuse and thereby larger when subjected to irradiation. On the other hand, less Cr-enriched precipitates in the alloy aged for 50 h are less affected. The CreCr pair correlation function analysis shows that matrix inhomogeneity decreases under irradiation. Irradiation leads to a decrease in the number density of diffuse clusters, whereas in the case of well-developed precipitates it remains unchanged.
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| 10. |
- Krupp, Ulrich, 1968, et al.
(author)
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The potential of spinodal ferrite decomposition for increasing the very high cycle fatigue strength of duplex stainless steel
- 2016
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In: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 93, s. 363-371
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Journal article (peer-reviewed)abstract
- Duplex stainless steels (DSS) have become candidate materials for structural applications, where conventional austenitic stainless steels fail due to very high cycle fatigue (VHCF) in combination with corrosive attack. It seems that DSS exhibit a fatigue limit, which can be attributed to the two-phase austenitic-ferritic microstructure. Ultrasonic VHCF testing revealed that the phase boundaries are efficient obstacles for the transmission of slip bands and microstructural fatigue cracks up to 10(9) cycles and even beyond. The barrier strength is determined by the misorientation relationship between neighbouring grains but also by the strength of the individual phases. By thermal treatment at 475 degrees C, spinodal decomposition of the ferrite phase results in the formation of Cr-rich alpha' precipitates. While during static loading these precipitates give rise to a loss in ductility (475 degrees C embrittlement), it was shown that the HCF strength can be increased and that there is also a tendency towards a beneficial effect on the VHCF behaviour. A more detailed analysis of the local plasticity sites by means of atom probe tomography (APT) revealed a dissolution of the a' precipitates within operated slip bands. The dissolution might be an indication for a local softening mechanism that limits the VHCF strengthening effect of spinodal decomposition.
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