| 1. |
|
|
| 2. |
- Fazi, Andrea, 1992, et al.
(författare)
-
Characterization of as-deposited cold sprayed Cr-coating on Optimized ZIRLO™ claddings
- 2021
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 549
-
Tidskriftsartikel (refereegranskat)abstract
- As-produced Cr-coated Optimized ZIRLO™ cladding material fabricated with the cold-spray (CS) deposition process is studied. Cross-sectional electron microscopy, nano-hardness profiling, transmission electron microscopy, transmission Kikuchi diffraction, and atom probe tomography (APT) were performed to investigate the nature of the CS Cr-coating/Optimized ZIRLO™ interface, the microstructure of the coating, and the effects of the deposition on the Zr-substrate microstructure. The former surface of the Zr-substrate was found to have a highly deformed nano-crystalline microstructure, the formation of which was attributed to dynamic recrystallization occurring during coating deposition. This microstructural change, evaluated with electron backscattered diffraction and nano-hardness profiling, appeared to be confined to a depth of a few microns. Through APT analysis, a 10–20 nm thick intermixed bonding region was observed at the interface between coating and substrate. The chemical composition of this region suggests that this layer originated from a highly localized shearing and heating of a thin volume of the outermost former surface of the substrate. The study of the intermixed bonding region's crystalline structure was performed with high resolution transmission electron microscopy and revealed a distorted hexagonal close-packed structure.
|
|
| 3. |
- Fazi, Andrea, 1992, et al.
(författare)
-
Cold sprayed Cr-coating on Optimized ZIRLO™ claddings: the Cr/Zr interface and its microstructural and chemical evolution after autoclave corrosion testing
- 2022
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 560
-
Tidskriftsartikel (refereegranskat)abstract
- Cr-coated Optimized ZIRLO™ cladding material fabricated with the cold-spray deposition process is studied. Microstructure and chemistry of this material are investigated before and after exposure to autoclave corrosion testing with scanning electron microscopy, energy dispersive spectroscopy analysis, electron backscattered diffraction, transmission electron microscopy and atom probe tomography. The results are used to assess what changes have occurred upon autoclave exposure. The formation of a compact, 80 – 100 nm thick Cr2O3 layer is observed on the surface of the exposed samples. Nucleation of ZrCr2 intermetallic phase is discovered at the Cr/Zr interface. This Laves phase nucleates inside the intermixed bonding layer that can be found in both pristine and exposed samples, and decorates the interface in the form of small particles (less than 50 nm in size). Using transmission electron microscopy and atom probe tomography the growth of a Zr-Cr-Fe phase was detected. This phase is found in the region of the Zr-substrate immediately adjacent to the coating, up to a few hundred nanometres distance from the Cr/Zr interface. A small degree of recrystallization occurs upon autoclave exposure in the 1-2 µm thick nanocrystalline layer produced on the Zr-substrate by the cold spray deposition method utilized for the fabrication of the Cr-coating.
|
|
| 4. |
- Fazi, Andrea, 1992, et al.
(författare)
-
Comparing CrN and TiN Coatings for Accident-Tolerant Fuels in PWR and BWR Autoclaves
- 2022
-
Ingår i: Journal of Nuclear Engineering. - : MDPI AG. - 2673-4362. ; 3:4, s. 321-332
-
Tidskriftsartikel (refereegranskat)abstract
- The development of coatings for accident-tolerant fuels (ATFs) for light water reactor (LWR) applications promises improved corrosion resistance under accident conditions and better performances during operation. CrN and TiN coatings are characterized by high wear resistance coupled with good corrosion resistance properties. They are generally used to protect materials in applications where extreme conditions are involved and represent promising candidates for ATF. Zr cladding tubes coated with 5 µm-thick CrN or TiN, exposed in an autoclave to simulated PWR chemistry and BWR chemistry, were characterized with SEM, EDS, and STEM. The investigation focused on the performance and oxidation mechanisms of the coated claddings under simulated reactor chemistry. Both coatings provided improved oxidation resistance in a simulated PWR environment, where passivating films of Cr2O3 and TiO2, less than 1 µm-thick, formed on the CrN and TiN outer surfaces, respectively. Under the more challenging BWR conditions, any formed Cr2O3 dissolved into the oxidizing water, resulting in the complete dissolution of the CrN coating. For the TiN coating, the formation of a stable TiO2 film was observed under BWR conditions, but the developed oxide film was unable to stop the flux of oxygen to the substrate, causing the oxidation of the substrate.
|
|
| 5. |
- Fazi, Andrea, 1992, et al.
(författare)
-
CrN–NbN nanolayered coatings for enhanced accident tolerant fuels in BWR
- 2023
-
Ingår i: Journal of Nuclear Materials. - 0022-3115. ; 586
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 6. |
- Fazi, Andrea, 1992, et al.
(författare)
-
Performance and evolution of cold spray Cr-coated optimized ZIRLO™ claddings under simulated loss-of-coolant accident conditions
- 2023
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 576
-
Tidskriftsartikel (refereegranskat)abstract
- The performance of Cr-coated Optimized ZIRLO™ as accident tolerant fuel cladding material for pressurized water reactors (PWRs) is assessed. The coating oxidation mechanisms, oxide stability, and the transformation of the Cr-coating/Optimized ZIRLO™ interface are among the studied phenomena. For this purpose, samples were exposed at 1200°C in steam for 3 min, 20 min and 40 min. As-fabricated coated claddings, plus specimens tested in autoclave at 415°C for 90 days in simulated PWR water chemistry were employed for comparison. Characterization techniques such as scanning electron microscopy, energy dispersive x-ray spectroscopy, electron backscattered diffraction, and transmission electron microscopy were used to determine the chemistry and crystalline structure of the various phases formed during the different exposures. When exposed to loss-of-coolant accident (LOCA) conditions for 40 min, a layer of Cr2O3 up to 8 µm thick was measured on the outer surface of the Cr-coating. No significant oxidation of the underlaying Optimized ZIRLO™ alloy occurred, and the applied coating appears to be very effective at delaying the cladding degradation under accident conditions. At the coating-substrate interface, a 1–2 µm thick layer of (Cr,Fe)2Zr Laves phase was found. The presence of this phase appears to have no detrimental effects on the coating performance, and it might play a role in slowing down the dissolution of the coating into the substrate. ZrO2 particles were frequently found at grain boundaries in the coating after exposure to LOCA conditions. For longer exposure time, these particles are expected to grow into a ZrO2-network, creating a fast diffusion path for O, and compromising the oxidation protection offered by the coating.
|
|
| 7. |
|
|
| 8. |
- Slabanja, Mattias, 1976, et al.
(författare)
-
Early stages of phase separation using three-dimensional atom probe and atomistic modelling
- 2007
-
Ingår i: Surface and Interface Analysis. ; 39, s. 178-
-
Tidskriftsartikel (refereegranskat)abstract
- The early stages of clustering in Al-14.8 Mg alloy have been studied using atomistic modelling and 3DAP analysis. Atomistic modelling was based on first-principles quantum mechanical calculations and Monte Carlo (MC) based techniques. A good agreement between the experimental results and simulated data was obtained showing the appropriateness of the theoretical approach used. It has been shown that after 720 h at 35 °C (308 K) clustering of Mg was clearly detectable in the experimental alloy. The simulated data after 200 MC steps corresponded well to the experimental results obtained after ageing for 720 h.
|
|
| 9. |
- Stiller, Krystyna Marta, 1947, et al.
(författare)
-
Atom probe tomography of interfaces in ceramic films and oxide scales
- 2016
-
Ingår i: MRS Bulletin. - : Springer Science and Business Media LLC. - 0883-7694 .- 1938-1425. ; 41:1, s. 35-39
-
Tidskriftsartikel (refereegranskat)abstract
- Atomic-scale characterization of interfaces in ceramic materials is needed in order to fully understand their electronic, ionic, mechanical, magnetic, and optical properties. The latest development of laser-assisted atom probe tomography ( APT), as well as new specimen preparation methods, have opened the realm of ceramics for structural and chemical characterization with high sensitivity and nearly atomic spatial resolution. This article reviews recent APT investigations of interfaces in thin nitride films and thermally grown oxides: TiAlN layers and oxide scales on alumina- and chromia-formers and Zr alloys. The selected examples highlight the role of interfaces in the decomposition of films and in transport processes.
|
|
| 10. |
- Stiller, Krystyna Marta, 1947, et al.
(författare)
-
Atom probe tomography of oxide scales
- 2013
-
Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 79:3-4, s. 227-238
-
Tidskriftsartikel (refereegranskat)abstract
- Atom probe tomography, APT, is the only microstructural method that can routinely analyse and position individual atoms in a material with a spatial resolution of 0.1-0.5 nm. Recent implementation of pulsed-laser to APT made investigation of less conducting materials, such as oxides, feasible. In this paper a short description of the principle of the techniques is presented, followed by examples of recent APT studies of thermally grown oxide scales produced on alumina formers (Pt-modified NiAl diffusion coating and FeCrAl alloy), at the crack tips in a Ni-based alloy and on a Zr-alloy. Additionally, results from preliminary studies of ZnO and MgO bulk materials are shown. The obtained information on the atomic scale about the chemistry variations in the scales and at the metal oxide interfaces provides valuable insights into oxidation processes. © 2012 Springer Science+Business Media New York.
|
|
| 11. |
|
|
| 12. |
- Thuvander, Mattias, 1968, et al.
(författare)
-
Quantitative atom probe analysis of carbides
- 2011
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 111:6, s. 604-608
-
Tidskriftsartikel (refereegranskat)abstract
- Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti 2 AlC grains in different materials as well as on large M 2 3C 6 precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the 13 C isotope, and calculating the concentration of 12 C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) andTi 2 AlC an additional difficulty arises from the overlap between C 2 + ,C 2+ 4 and Ti 2+ at the mass-to-charge 24 Da. © 2010 Elsevier B.V.
|
|
| 13. |
- ANDERSSON, MARCUS, 1976, et al.
(författare)
-
Comparison of early stages of precipitation in Mo-rich and Mo-poor maraging stainless steels
- 2007
-
Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 39:2-3, s. 195-200
-
Tidskriftsartikel (refereegranskat)abstract
- The precipitation process during ageing in the Mo-poor alloy Carpenter 455 has been investigated by three-dimensional atom probe (3DAP) and energy-filtered transmission electron microscopy. The ageing treatment was performed at 475 degrees C for 5 min, 25 min, 1 h and 2 h. The precipitation of a mixed Ni-, Ti- and Cu-rich phase was observed after 25 min of ageing. A complete separation between Ni/Ti-rich and Cu-rich regions occurred within 1-2 h of ageing. After 2 h of ageing at 475 degrees C, coarsening of the eta-Ni3Ti precipitates was observed. Moreover, large Ni/Ti-rich and Cu-rich particles were observed in the lath boundaries after 2 h of ageing. The results were compared with the precipitation process in the Mo-rich alloy 1RK91. Copyright (C) 2007 John Wiley & Sons, Ltd.
|
|
| 14. |
|
|
| 15. |
|
|
| 16. |
|
|
| 17. |
|
|
| 18. |
|
|
| 19. |
|
|
| 20. |
|
|
| 21. |
|
|
| 22. |
|
|
| 23. |
|
|
| 24. |
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
- Boll, Torben, 1979, et al.
(författare)
-
An APT investigation of an amorphous Cr-B-C thin film
- 2015
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 159, s. 217-222
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 28. |
- Boll, Torben, 1979, et al.
(författare)
-
Grain Boundary Chemistry and Transport Through Alumina Scales on NiAl Alloys
- 2017
-
Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 88:3-4, s. 469-479
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 29. |
- Boll, Torben, 1979, et al.
(författare)
-
Interfaces in Oxides Formed on NiAlCr Doped with Y, Hf, Ti, and B
- 2017
-
Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23:2, s. 396-403
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 30. |
- Boll, Torben, 1979, et al.
(författare)
-
On aliovalent cations control of α-alumina growth on doped and undoped NiAl
- 2021
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 210
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 31. |
- Chyrkin, Anton, 1984, et al.
(författare)
-
High-temperature oxidation behavior of additively manufactured IN625: Effect of microstructure and grain size
- 2022
-
Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 205
-
Tidskriftsartikel (refereegranskat)abstract
- High-temperature oxidation of additively manufactured (AM) Ni-base alloy IN625 has been studied in air and Ar-5%H2-3%H2O at 900–1000 °C. AM material is found to oxidize faster than the conventionally manufactured (CM) IN625 due to severe intergranular oxidation observed in the former. The AM IN625 was heat treated at 1100–1250 °C and hot rolled at 980 °C in order to modify the AM microstructure, primarily grain size, and analyze its role in alloy oxidation behavior. Grain size is shown to affect overall oxidation kinetics but not the intergranular oxidation morphology.
|
|
| 32. |
- Chyrkin, Anton, 1984, et al.
(författare)
-
Intergranular oxidation of additively manufactured Ni-base alloy 625: The role of Si
- 2023
-
Ingår i: Corrosion Science. - 0010-938X. ; 219
-
Tidskriftsartikel (refereegranskat)abstract
- Additively manufactured (AM) Ni-base alloy 625 exposed in air and H2/H2O at 900 °C has been shown to suffer from intergranular oxidation attack unlike its conventionally manufactured (CM) variant. Modification of the AM microstructure via hot-rolling and heat-treatment did not alter the oxidation pattern. Detailed analysis of minor differences in chemical composition of AM and CM exposed the critical role of Si in oxidation protection of alloy 625. Experiments with model alloys as well as diffusion modeling confirmed the crucial role of a minimum Si concentration to form a SiO2 subscale preventing thereby intergranular oxidation attack.
|
|
| 33. |
- Frodelius, Jenny, et al.
(författare)
-
Ti2AlC coatings deposited by High Velocity Oxy-Fuel spraying
- 2008
-
Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 202:24, s. 5976-5981
-
Tidskriftsartikel (refereegranskat)abstract
- High Velocity Oxy-Fuel has been utilized to spray coatings from Ti2AlC (MAXTHAL 211®) powders. X-ray diffraction showed that the coatings consist predominantly of Ti2AlC with inclusions of the phases Ti3AlC2, TiC, and Al–Ti alloys. The fraction of Ti2AlC in coatings sprayed with a powder size of 38 μm was found to increase with decreasing power of the spraying flame as controlled by the total gas flow of H2 and O2. A more coarse powder (56 μm) is less sensitive to the total gas flow and retains higher volume fraction of MAX-phase in the coatings, however, at the expense of increasing porosity. X-ray pole figure measurements showed a preferred crystal orientation in the coatings with the Ti2AlC (000l) planes aligned to the substrate surface. Bending tests show a good adhesion to stainless steel substrates and indentation yields a hardness of 3–5 GPa for the coatings sprayed with a powder size of 38 μm.
|
|
| 34. |
- Gündüz, Kerem, 1985, et al.
(författare)
-
The effect of additive manufacturing on the initial High temperature oxidation properties of RE-containing FeCrAl alloys
- 2021
-
Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 188
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of additive manufacturing on the high temperature oxidation properties of FeCrAl materials was investigated. For this purpose, additively manufactured Kanthal AM100 cut parallel and perpendicular to the building direction and hot-rolled Kanthal AF as a reference were exposed to air at 900 and 1100 °C for 168 h. AM100 performed slightly better than AF in terms of mass gain. Nevertheless, an oxide scale with local differences in thickness formed on AM100 due to the bimodal grain structure of the underlying metal, which was composed of coarse-grained cuboidal repeating units (100 μm wide), separated by fine-grained rims.
|
|
| 35. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
-
Crack Growth Studies in a welded Ni-base superalloy
- 2017
-
Ingår i: Solid State Phenomena. - 1662-9779 .- 1012-0394. ; 258, s. 237-240
-
Konferensbidrag (refereegranskat)abstract
- It is well known that the introduction of sustained tensile loads during high-temperaturefatigue (dwell-fatigue) significantly increases the crack propagation rates in many superalloys. Onesuch superalloy is the Ni-Fe based Alloy 718, which is a high-strength corrosion resistant alloy usedin gas turbines and jet engines. As the problem is typically more pronounced in fine-grainedmaterials, the main body of existing literature is devoted to the characterization of sheets or forgingsof Alloy 718. However, as welded components are being used in increasingly demandingapplications, there is a need to understand the behavior. The present study is focused on theinteraction of the propagating crack with the complex microstructure in Alloy 718 weld metalduring cyclic and dwell-fatigue loading at 550 °C and 650 °C.
|
|
| 36. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
-
Fracture of Cr2O3 single crystals on the microscale
- 2021
-
Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 15
-
Tidskriftsartikel (refereegranskat)abstract
- Studying cleavage properties of protective oxide scales is imperative to understand their fracture behaviour, since transgranular fracture is observed in many cases. The small thickness and polycrystalline structure of such scales makes it difficult to identify active cleavage planes directly from mechanical testing. To resolve this issue for Cr2O3, we present an approach to experimentally identify cleavage planes through micro-cantilever bending. Single crystal wafers are used to prepare micro-cantilevers of pentagonal cross-section in different orientations, targeting possible cleavage planes. Fracture surface imaging showed rhombohedral and pyramidal fracture, though surface energy studies predict rhombohedral as the dominant plane. There does exist a preference for rhombohedral fracture over pyramidal, which is also revealed from the experiments.
|
|
| 37. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
-
Influence of dwell time on fatigue crack propagation in Alloy 718 laser welds
- 2017
-
Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 704, s. 440-447
-
Tidskriftsartikel (refereegranskat)abstract
- The introduction of welded assemblies in aerospace components aid in weight reduction, but also lead to an increased risk of defects. It is therefore important to analyze the high temperature crack growth resistance of such welds. The results from high temperature cyclic and dwell-fatigue testing of surface flawed Alloy 718 welds are presented here. An increasing temperature and application of a dwell time accelerate the crack growth and increase interaction with secondary phases. During cyclic loading at 550 °C, there is little interaction with the microstructure during transgranular propagation, but the application of dwell times results in a mixture of transgranular propagation and intergranular cracking of boundaries between different dendrites. At 650 °C, mixed intergranular and transgranular mode of crack growth is seen under both cyclic and dwell conditions. However, during dwell-fatigue the interfaces between the secondary arms of the same dendrite are also weakened, leading to an interfacial type of crack growth also in the intergranular parts.
|
|
| 38. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
-
Microscale fracture of chromia scales
- 2019
-
Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the mm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were obtained as well. The observed mixed trans- and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales.
|
|
| 39. |
- Harihara Subramonia Iyer, Anand, 1990, et al.
(författare)
-
Room temperature plasticity in thermally grown sub-micron oxide scales revealed by micro-cantilever bending
- 2018
-
Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462. ; 144, s. 9-12
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a new geometry for focused ion beam milled micro-cantilevers, which allows production of residual stress-free, isolated thin film specimens from film-substrate systems. This geometry was used to demonstrate the presence of permanent deformation in about 200 nm thick thermally grown oxide scales on a Ni-base superalloy, after applying large bending displacements in-situ in a scanning electron microscope. Stiffness measurements performed before and after the bending tests confirmed the absence of micro-cracks, leading to the conclusion that plastic deformation occurred in the oxide scale. The proposed method is extendable to other film-substrate systems and testing conditions, like non-ambient temperatures.
|
|
| 40. |
- Hörnqvist Colliander, Magnus, 1979, et al.
(författare)
-
High-temperature crack growth in a Ni-base superalloy during sustained load
- 2014
-
Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 609, s. 131-140
-
Tidskriftsartikel (refereegranskat)abstract
- he high-temperature sustained load crack growth behaviour of a Ni-base superalloy was investigated using a combination of mechanical testing in controlled atmosphere, fractographical and microanalytical investigations, and finite element modelling. The results show that the local crack front geometry is un- even on two scales – jaggedness on the scale of 100 μm was observed in all specimens, whereas mm- scale waviness could occasionally be observed. The jaggedness can be explained by a percolation-type crack growth along weaker grain boundaries, whereas the large-scale waviness is presumably due to larger regions of the material having specific grain texture with high crack growth resistance. The un-even crack front is shown to potentially have considerable effects on the loading conditions at the crack tip, whereas ligaments of un-cracked material in the crack wake are deemed to have less effect on the crack tip loading due to their low area fraction. The ligaments fail intergranularly in the wake as the crack grows in the present case, as opposed to by creep fracture as previously proposed. Finally, the plastically deformed regions about the crack and crack tip are shown not to exhibit any elevated oxygen levels, implying that the damage in these regions is purely mechanical.
|
|
| 41. |
|
|
| 42. |
- Hörnqvist, M., et al.
(författare)
-
Hold-time fatigue crack growth of Allvac 718Plus
- 2010
-
Ingår i: 7th International Symposium on Superalloy 718 and Derivatives 2010. Pittsburgh, 10-13 October 2010. - 9781617827709 ; 2, s. 705-717
-
Konferensbidrag (refereegranskat)abstract
- The hold-time fatigue behaviour of Allvac 718Plus was investigated both in the as-heat treated condition and after long-time exposure at temperature close to the maximum operating limit. At temperatures above 450°C the introduction of 90 s hold-time at maximum load significantly increased the fatigue crack growth rate. Both conditions showed purely cycle dependent behavior at 450°C and close to purely time dependent behavior at 700°C. At 600°C intermediate behavior was observed. The long-time exposure had little effect on the cyclic (0.5 Hz) crack growth rates, but the resistance to crack growth with 90 s hold-time decreased. No microstructural effects of long-time exposure (700°C/140 h + 675°C/460 h) could be observed by SEM, but there were indications that the hardness of the material increases somewhat after exposure.
|
|
| 43. |
- Jahns, Katrin, et al.
(författare)
-
On oxide formation on a single crystalline Ni-based superalloy at 900 °C in SO2 containing atmosphere: The effect of surface treatment
- 2021
-
Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 180
-
Tidskriftsartikel (refereegranskat)abstract
- The oxidation behavior of the single crystalline SCA425+ (17Cr-10Al-3Ta-Ni bal.) after two different surface finish procedures (grinding and polishing) was investigated after cyclic exposures at 900 degrees C in SO2 containing atmosphere up to 260 h. The microstructure and chemistry of the produced oxide scales were studied using highresolution methods, SEM/EDX, TEM, and STEM. The investigation revealed that grinding provides more effective diffusion paths, so that the material is able to form a protective continuous alumina layer, while an extensive internal oxidation takes place on the polished sample. In addition, a numerical model, based on the cellular automata approach, was developed and used to consider the effect of dislocation pipe diffusion in combination with diffusion processes and oxide formation. Good agreement between the model and experimental results was obtained.
|
|
| 44. |
- Johansson, S., et al.
(författare)
-
Fracture mechanisms during intergranular hold time fatigue crack growth in Inconel 718 superalloy
- 2013
-
Ingår i: 13th International Conference on Fracture 2013, ICF 2013; Beijing; China; 16 June 2013 through 21 June 2013. ; 3, s. 1833-1841
-
Konferensbidrag (refereegranskat)abstract
- Ni-base superalloy IN718 is known to display time-dependent intergranular crack growth under dwell time mechanical loading at high temperature under atmospheric conditions. Oxygen has been pointed out as a cause of the intergranular damage causing embrittled crack growth during both cyclic and hold time loading. Investigation of the mechanisms responsible for the embrittlement should not only focus on the effect of environment but also on the combined action of fatigue, creep, temperature and time. In this work material from experiments with fatigue crack growth in combination with hold times of different length at different temperatures has been investigated. Fractographic studies and metallographic cross sections of fatigued specimens has been subjected to careful analysis using ECCI-imaging in order to shed light on the fracture mechanisms. The results show that the damage is caused by the influence of a combination of environment and severe local damage manifested as a transformation of the microstructure into sub cells, micro twins and recrystallised areas close to the crack tip. The damage mechanism is thus influenced by a combination of oxidation and severe local plastic deformation.
|
|
| 45. |
- Johnson, Lars, et al.
(författare)
-
Blind deconvolution of time-of-flight mass spectra from atom probe tomography
- 2013
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 132, s. 60-64
-
Tidskriftsartikel (refereegranskat)abstract
- A major source of uncertainty in compositional measurements in atom probe tomography stems from the uncertainties of assigning peaks or parts of peaks in the mass spectrum to their correct identities. In particular, peak overlap is a limiting factor, whereas an ideal mass spectrum would have peaks at their correct positions with zero broadening. Here, we report a method to deconvolute the experimental mass spectrum into such an ideal spectrum and a system function describing the peak broadening introduced by the held evaporation and detection of each ion. By making the assumption of a linear and time-invariant behavior, a system of equations is derived that describes the peak shape and peak intensities. The model is fitted to the observed spectrum by minimizing the squared residuals, regularized by the maximum entropy method. For synthetic data perfectly obeying the assumptions, the method recovered peak intensities to within +/- 0.33 at%. The application of this model to experimental APT data is exemplified with Fe-Cr data. Knowledge of the peak shape opens up several new possibilities, not just for better overall compositional determination, but, e.g., for the estimation of errors of ranging due to peak overlap or peak separation constrained by isotope abundances. (C) 2013 Elsevier By. All rights reserved.
|
|
| 46. |
- Johnson, Lars, et al.
(författare)
-
Self-organized nanostructuring in Zr0.69Al0.31N thin films studied by atom probe tomography
- 2016
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 615, s. 233-238
-
Tidskriftsartikel (refereegranskat)abstract
- We have applied atom probe tomography (APT) to analyze self-organizing structures of wear-resistant Zr0.69Al0.31N thin films grown by magnetron sputtering. Transmission electron microscopy shows that these films grow as a three-dimensional nanocomposite, consisting of interleaved lamellae in a labyrinthine structure, with an in-plane size scale of ~ 5 nm. The structure was recovered in the Al APT signal, while the Zr and N data lacked structural information. The onset of the self-organized labyrinthine growth was observed to occur by surface nucleation, 5–8 nm above the MgO substrate, due to increasing Zr–Al compositional fluctuations during elemental segregation. At a final stage, the labyrinthine growth mode was observed to be interrupted by the formation of larger ZrN grains.
|
|
| 47. |
- Johnson, Lars, et al.
(författare)
-
Spinodal decomposition of Ti0.33Al0.67N thin films studied by atom probe tomography
- 2012
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 520:13, s. 4362-4368
-
Tidskriftsartikel (refereegranskat)abstract
- Details of the phase decomposition in NaCl-structure Ti0.33Al0.67N thin films deposited by cathodic arc evaporation are studied by atom probe tomography. We demonstrate that as-deposited films are in the earliest stage of decomposition for which electron microscopy and x-ray diffraction indicate a single-phase solid solution. Annealing at 900 degrees C further activates spinodal decomposition of the films, although pockets of unde-composed material remain after 2 h. N preferentially segregates to the AlN and TiN domains, causing the TiAlN matrix to be understoichiometric, by the energetics of N vacancies in TiAlN. The corresponding modulation in stoichiometry implies a Kirkendall effect, caused by different Al and Ti diffusivities.
|
|
| 48. |
- Kaczmarski, J., et al.
(författare)
-
Controlling In-Ga-Zn-O thin films transport properties through density changes
- 2016
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090. ; 608, s. 57-61
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 49. |
- Kaczmarski, J., et al.
(författare)
-
Transparent Ru–Si–O/In–Ga–Zn–O MESFETs on flexible polymer substrates
- 2018
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 65:1, s. 129-135
-
Tidskriftsartikel (refereegranskat)abstract
- With the development of novel device applications, e.g., in the field of Internet of Things or point-of-care personalized diagnostic systems, came an increased demand for MESFETs for fast and low-power consumption integrated circuits and active-matrix displays. In this paper, we present fabrication and characterization of transparent Ru–Si–O/In–Ga–Zn–O MESFETs on flexible substrates. The use of transparent conducting oxide, namely, Ru–Si–O, as Schottky gate electrode, allows for processing the devices at room temperature, enabling the utilization of such low-temperature substrates as polyethylene terephthalate foil and paper. It was shown that tuning the device geometry allows realization of transistors providing on-current up to 2 mA, while the highest on-to-off current ratio equals 2 × 105, with off-current below 1 nA, carrier mobility in the channel exceeds 9 cm2·V−1·s−1, and subthreshold swing is below 250 mV·decade−1
|
|
| 50. |
- Kadkhodazadeh, S., et al.
(författare)
-
Towards quantitative three-dimensional characterisation of buried InAs quantum dots
- 2011
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 326:1
-
Konferensbidrag (refereegranskat)abstract
- InAs quantum dots grown on InP or InGaAsP are used for optical communication applications operating in the 1.3 - 1.55 μm wavelength range. It is generally understood that the optical properties of such dots are highly dependent on their structural and chemical profiles. However, morphological and compositional measurements of quantum dots using transmission electron microscopy can be ambiguous because the recorded signal is usually a projection through the thickness of the specimen. Here, we discuss the application of scanning transmission electron microscopy tomography to the morphological and chemical characterisation of surface and buried quantum dots. We highlight some of the challenges involved and introduce a new specimen preparation method for creating needle-shaped specimens that each contain multiple dots and are suitable for both scanning transmission electron microscopy tomography and atom probe tomography.
|
|
