| 1. |
- Grolig, Jan Gustav, 1986
(författare)
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Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
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| 2. |
- Jaladurgam, Nitesh Raj, 1993
(författare)
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Heterogeneous deformation of multi-phase engineering materials - an in-situ neutron diffraction study
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas turbines are complex power generation systems used in aerospace or land-based-power stations. Materials such as Ni-base superalloys are involved in the combustion zone of these machines, which continuously experiences harsh environments with loading at high temperatures. Moreover, the continuous demand for increasing operating temperature to achieve higher efficiencies and reduced emission levels opens the scene to new heat resistant materials like the state-of-the-art high entropy alloys (HEAs), which require a thorough understanding of the structure-process-property relationships. The microstructures of these advanced multi-phase, multi-component alloys are complex, and the deformation is generally heterogeneous both with respect to the different phases and to the crystallographic orientation within each phase. Hence, it is important to understand their behavior and performance during processing and service. In-situ neutron diffraction is a unique technique to probe the deformation behaviour during service/processing-like conditions, including plastic deformation at various temperatures, in order to provide insights into the structure-property relations. In the first part of this work the deformation mechanisms of a newly developed Ni-base superalloy was investigated using in-situ neutron diffraction and electron microscopy at room temperature. In addition, elasto-plastic self-consistent (EPSC) crystal plasticity simulations are used to obtain insights into the operating deformation mechanisms. In the second part, the as-cast eutectic high entropy alloy AlCoCrFeNi2.1 was studied using in-situ neutron diffraction at temperatures from 77 to 673 K. These investigations provide unique insights into the complex heterogeneous deformation behavior of these high-performance multi-phase engineering materials.
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| 3. |
- Hanning, Fabian, et al.
(författare)
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The Effect of Grain Size on theSusceptibility Towards Strain Age Crackingof Wrought Haynes® 282®
- 2020
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Ingår i: SPS 2020. - Amsterdam : IOS Press. - 9781643681467 - 9781643681474 ; 13, s. 407-416
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effect of grain size on the suceptibility towards strain age cracking (SAC) has been investigated for Haynes® 282® in the tempeature range of 750 to 950°C after isothermal exposure up to 1800s. Grain growth was induced by heattreating the material at 1150°C for 2h, resulting in a fourfold increase in grain size. Hardness was significanlty reduced after heat treatment as compared to millannealed material. Large grain size resulted in intergranular fracture over a widertemperature range than small grain size material. Ductility was lowest at 850°C, while lower values were observed to be correlated to increased grain size. The rapid formation of grain boundary carbide networks in Haynes® 282® is found to be notable to compensate for higher local stresses on grain boundaries due to incresedgrain size.
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| 4. |
- Shoja, Siamak, 1980
(författare)
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Microstructure and plastic deformation of textured CVD alumina coatings
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is known that the wear performance of α-alumina coatings produced by chemical vapor deposition (CVD) is significantly influenced by the type and degree of texture. However, the main reasons behind this behavior are not fully understood. This thesis contains studies of two related topics for increasing the understanding of α-alumina coatings. The first topic concerns the microstructure and texture development of CVD α-Al2O3 coatings, and the second topic concerns calculations and analysis of the Schmid factors ( m ) for coatings with different textures. By combining different analysis methods (such as XRD, SEM, FIB/SEM, TKD, TEM, STEM, XEDS), and theoretical and experimental Schmid factor analysis by MATLAB and EBSD, the microstructure and plastic deformation of α-alumina coatings were investigated. The microstructures of three different CVD α-Al2O3 layers deposited onto a Ti(C,N,O) bonding layer were studied. Grain boundary diffusion of heavy elements from the substrate to the bonding layer/α-Al2O3 interface was observed. This may be the cause of a disturbance in the early growth of α-Al2O3. Additionally, it was found that the number of interfacial pores at the bonding layer/α-Al2O3 interface increased by introducing the H2S gas. The H2S gas also promoted an earlier development of the (0001) texture. The orientation of the grains was developed to the desired texture both as a gradual change over several grains and as an abrupt transformation from one grain to another. The probability of plastic deformation in different wear zones on the rake face of a cutting tool was investigated theoretically and experimentally by analyzing Schmid factors for textured α-Al2O3 coatings. Schmid factor diagrams were constructed using MATLAB/MTEX and used to extract frequency distributions for different slip systems and textures. The results were compared with lateral distribution maps of Schmid factors obtained from experimental coatings. It was observed that basal slip is most easily activated in the transition zone, followed by prismatic slip systems 1 and 2 in coatings with an (0001)-texture. The homogeneous plastic deformation behavior observed in this coating is also connected to mostly high Schmid factors in the m -value distribution. The differences between the m -value distributions for the three slip systems are not that pronounced in the (01-12) and (11-20) textures, and the distributions are relatively wide. The low wear rate and more homogeneous deformation of the coating with (0001) texture compared to the other coating textures may be the result of the high plasticity, offered by the easy activation of basal slip and prismatic 1 slip, and the low spread of Schmid factor values at the transition zone. In conclusion, the results presented in this thesis form a knowledge platform that can be used to understand the microstructure and wear mechanisms of textured CVD α-alumina coatings.
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| 5. |
- Fazi, Andrea, 1992
(författare)
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Cold spray Cr-coated Optimized ZIRLO claddings: an option for accident tolerant fuels
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accident tolerant fuel development has started with the aim of providing nuclear fuels able to endure severe accident conditions. Research in this field has also sparked a wave of material renewal in the nuclear industry that had being delayed for the last few decades. Climate change is an ever-growing public concern, and policies about greenhouse gas emissions are becoming more stringent both at the national and international level. Nuclear energy produces very low carbon emissions and the successful development of new accident tolerant materials might play a role in making this technology a viable solution to this global issue. Cr-coated zirconium claddings are one of the most promising candidates as near-term response to the need for accident tolerant materials. These coatings can be produced via a range of different technologies, but the two main designs being currently developed are obtained with physical vapour deposition or cold spray (CS) deposition. In cold spray, high pressure gas is fed through a nozzle together with Cr powder. The Cr particles are accelerated up to 1200 m/s and directed to the substrate, in this case cladding tubes made of Optimized ZIRLO™ alloy. The resulting Cr-coated zirconium cladding is the subject of this work. As-fabricated samples and autoclave tested material are characterized with atom probe tomography and a range of electron microscopy techniques. The scope of the investigation is to evaluate the performances of Cr-coated claddings under operating conditions. Additionally, the nature of the adhesion in cold spray coating and the effects of this deposition method on the substrate are explored. A 10-20 nm thick intermixed bonding region was observed at the Cr/Zr interface of the as-fabricated cladding. This region is deemed to play an important role in the strong adhesion of CS coatings. When exposed to operating conditions, ZrCr2-Laves phase was found to nucleate in the intermixed bonding region. CS deposition involves severe plastic deformation of both coating and substrate. As a result, a 1-2 μm thick nanocrystalline layer can be found in the substrate adjacent to the Cr/Zr interface. After autoclave exposure, a Zr-Cr-Fe phase was discovered precipitating in this nanocrystalline layer at the Zr grain boundaries.
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| 6. |
- Nikas, Dimitrios, 1984
(författare)
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Influence of combined thermal and mechanical loadings on pearlitic steel microstructure in railway wheels and rails
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the most important aspects in railway operation is the interaction between rail and wheel. The contact patch between these two components is around the size of a small coin, and since high loads act on this small area, stresses will give rise to wear and damage in both components. Frictional forces on the surface of wheels and rails caused by recurring train acceleration, braking, curving and occasional slippage can cause cyclic plastic deformation and heating, which in turn causes an aligned, anisotropic microstructure with altered mechanical behaviour. Control of material property degradation is an important topic for guiding maintenance, as well as ensuring safety of railways, since it will allow for a more accurate prediction of material wear and lifetime. The thesis focuses on the mechanical properties of railway wheel and rail steels after exposure to elevated temperatures and plastic deformation. Specifically examined are the carbon wheel steels, UIC ER7T and ER8T (~0.55 wt. %C) and rail steel R260 (~0.72 wt. %C). During their service life, the surface layers of rails and wheels are subjected to very high rolling contact loads. These lead to accumulation of large shear strains close to the running surface. Moreover the high thermal loads that wheels experience when block brakes are used can cause severe degradation of the material microstructure, more specifically spheroidisation of the pearlite, which combined with plastic deformation (that makes the material more prone to spheroidisation) can lead to severe deterioration of the material’s mechanical properties. Both un-deformed and pre-strained wheel materials were heat treated at various temperatures from 250°C to 600°C for various durations, and the change in room temperature hardness was analysed. Additionally, Electron Backscatter Diffraction Analysis (EBSD) was used to evaluate if orientation gradients in the pearlitic colonies affect the spheroidisation of the pearlitic microstructure, that is observed at higher temperatures. Uniaxial (tension-compression) and biaxial (including torsion) low cycle fatigue tests were performed to study the behaviour of R7T and R8T material at different temperatures. The influence of hold times as well as the ratchetting behaviour with mean stress effects were also studied. Virgin rail material was twisted using a biaxial machine to various shear strain levels to create a microstructure representative for the surface layer observed in field samples. The microstructure was characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness measurements. The results showed that wheel material hardening due to strain ageing takes place at around 300°C while microstructural degradation caused softening at higher temperatures. Spheroidisation of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained. The spheroidised areas appear to have lost their initial orientation gradients after spheroidisation and obtain a more uniform orientation. Cyclic tests at elevated temperature revealed cyclic hardening at around 300°C, as an effect of dynamic strain ageing. At higher temperatures, cyclic softening followed due to a combination of increasing thermal activation and spheroidisation. Biaxial testing showed a more severe effect of strain hardening and shorter fatigue life. For the rail material, the dislocation density was found to increase with increasing shear strain. The flow stresses calculated using microstructural parameters such as dislocation density and interlamellar spacing of the pearlite seem to agree well with those evaluated from hardness measurements.
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| 7. |
- Forsgren, Lilian, 1990
(författare)
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Processing and properties of thermoplastic composites containing cellulose nanocrystals or wood-based cellulose fibres
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cellulose nanocrystals (CNC) were surface modified with dialkylamines to increase the compatibility between the CNC and the polymeric matrix, and promising results were obtained, with a 300 % stiffness increase when the mixed dispersion was compression moulded on a laboratory scale. The manufacturing process was up-scaled using water-assisted mixing in a twin-screw extruder (TSE) followed by a second compounding step and injection moulding (IM). The composites were successfully produced using conventional melt-processing techniques but these did not show the same improvement in mechanical performance, probably due to the formation of CNC aggregates. There were indications of network formation when CNC was added, especially in the case of surface-modified CNC. Cellulose fibres and thermomechanical pulp were used as reinforcement in similar types of polymer matrices and the mixtures were similarly processed by TSE and IM. These materials were characterized with regard to appearance and durability. The discoloration of the composites due to excessive heat during processing did not significantly affect their mechanical properties, and the addition of the cellulose-based reinforcement to the polymer did not reduce its resistance to thermo-oxidative degradation compared to that of the pure matrix. In fact, the resistance to degradation was increased when lignin was present in the reinforcing element, showing a synergistic effect together with the added anti-oxidant. Superior properties were expected for the CNC composites compared to those of the larger cellulose fibre reinforcements, but in continuous production the stiffening effects were similar regardless of reinforcement type. These results confirm that the processing method and properties strongly affect the final properties of the composite.
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| 8. |
- Hosseini, Seyedehsan, 1994
(författare)
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Additive-Driven Improvements in Interfacial Properties and Processing of TMP-Polymer Composites
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Efforts to address environmental concerns have resulted in new regulations designed to plan the reduction of plastic and synthetic polymer usage, necessitating the search for sustainable natural alternatives with comparable cost-effectiveness and mechanical performance. Thermomechanical pulp (TMP) fibres are one of the most affordable natural fibres that have no chemical refining in production, production have a high yield of 90-98% and TMP fibres have been demonstrated to improve the mechanical characteristics (strength, stiffness and toughness) of wood-polymer composites (WPCs) compared to the pure polymer. The integration of TMP fibres with non-polar synthetic polymers remains a challenge due to surface polarity differences. This PhD thesis aims to ease the processing of TMP fibre composites through the incorporation of additives. The hypothesis posits that incorporating magnesium stearate (MgSt), molybdenum disulfide (MoS2) and alkyl ketene dimer (AKD) as additives in TMP composites will enhance interfacial properties, resulting in improved processability and flow behaviour at high temperatures. MoS2 is known for its interaction with lignin, which exists in TMP and MgSt is recognised for its ability to improve flow in pharmaceutical processing when combined with cellulose, also a component of TMP. AKD modifies the hydrophilic properties of lignocellulosic surfaces. The experimental work explores the effect of these additives on the properties of TMP composites of ethylene acrylic acid copolymer (EAA) and polypropylene (PP) matrices. The dynamic mechanical analysis (DMA) and mechanical analysis results reveal that MoS2 exhibits superior interaction with TMP fibres, yielding enhanced interfacial properties compared to MgSt in between EAA and TMP fibres. Rheological studies elucidate the transition from a fluid-like state to a network-like structure upon the incorporation of TMP into the PP matrix. The incorporation of AKD with C18 reduces the viscosity of TMP-PP composites and PP itself, and, as determined through theoretical Hansen solubility parameter (HSP) calculations, increases compatibility between cellulose in TMP fibres and PP. The addition of AKD influences both the colour (lighter) and shape (smoother surface) of the extrudate filaments in the TMP-PP composites, indicative of improved processing. In addition, frictional analysis demonstrates the reduction of the coefficient of friction (COF) between metal and TMP fibre by MgSt and AKD treatments.
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| 9. |
- Kahlin, Magnus
(författare)
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Fatigue Performance of Additive Manufactured Ti6Al4V in Aerospace Applications
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive Manufacturing (AM) for metals includes is a group of production methodst hat use a layer-by-layer approach to directly manufacture final parts. In recent years, the production rate and material quality of additive manufactured materials have improved rapidly which has gained increased interest from the industry to use AM not only for prototyping, but for serial production. AM offers a greater design freedom, compared to conventional production methods, which allows for parts with new innovative design. This is very attractive to the aerospace industry, in which parts could be designed to have reduced weight and improved performance contributing to reduced fuel consumption, increased payload and extended flight range. There are, however, challenges yet to solve before the potential of AM could be fully utilized in aerospace applications. One of the major challenges is how to deal with the poor fatigue behaviour of AM material with rough as-built surface.The aim of this thesis is to increase the knowledge of how AM can be used for high performance industrial parts by investigating the fatigue behaviour of the titanium alloy Ti6Al4V produced with different AM processes. Foremost, the intention is to improve the understanding of how rough as-built AM surfaces in combination with AM built geometrical notches affects the fatigue properties.This was done by performing constant amplitude fatigue testing to compare different combinations of AM material produced by Electron Beam Melting(EBM) and Laser Sintering (LS) with machined or rough as-built surfaces with or without geometrical notches and Hot Isostatic Pressing (HIP) treatment. Furthermore, the material response can be different between constant amplitude and variable amplitude fatigue loading due to effects of overloads and local plastic deformations. The results from constant amplitude testing were used to predict the fatigue life for variable amplitude loading by cumulative damage approach and these predictions were then verified by experimental variable amplitude testing.The constant amplitude fatigue strength of material with rough as-built surfaces was found to be 65-75 % lower, compared to conventional wrought bar, in which HIP treatments had neglectable influence on the fatigue strength. Furthermore, the fatigue life predictions with cumulative damage calculations showed good agreement with the experimental results which indicates that a cumulative damage approach can be used, at least for a tensile dominated load sequences, to predict the fatigue behaviour of additive manufactured Ti6Al4V.
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| 10. |
- Vega, Alberto, 1984, et al.
(författare)
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Degradation mechanisms in PUR foam of district heating pipes after accelerated ageing
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Modern societies demand the use of non-fossil and sustainable energy resources. In this sense, district heating (DH) systems have been playing an important role in the last years. However, some questions still remains unanswered such as technical life time prediction, heat losses challenge or status assessment. These questions are the driving force of DH system development, especially for the DH pipe manufacturers. Pre-insulated heating pipes include a HDPE jacket, rigid polyurethane (PUR) foam and steel service pipe, and are used to transport the heat from a central point out to the customers and then back again for reuse. These pipes must withstand axial mechanical loads and have good long-term thermal properties. Therefore, it is important to have reliable methods to evaluate the current status of a DH system at any time. The main aim of the project is to determine the degradation mechanisms during natural and accelerated aging of DH pipes. In this project, the pipes have been aged using an accelerated thermal ageing and both mechanical and thermal properties were investigated. Mechanical property was evaluated using the SP plug method, which determined the remaining adhesion force between the PUR and the steel service pipe [1]. This method has also been designed for application in field for a quick status check. At the same time, the thermal property has also been measured in real-time using a transient plane source (TPS) technique instead of a steady-state method [2]. Characterization of the PUR material has been performed using a three-point flexural test, Fourier transform infra-red spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) analysis [3]. Preliminary results suggest three different phases where the degradation of DH pipes is a combination of physical and chemical phenomena. In the early aging period, the changes in both mechanical and thermal properties are caused by physical effects. After that, an upturn is observed due to changes in the PUR chemical structure observed in the FTIR analysis. Finally, the chemical degradation takes over which is an effect of thermo-oxidation. At the end a better model comprising all three phases for the entire degradation process is needed and will be proposed in order to determine the real technical life time of DH pipes.
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