| 1. |
- Marattukalam, Jithin James
(författare)
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Tailoring structure and morphology during additive manufacturing of metallic components
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work described in this thesis explores the use of laser process parameters to functionalize the material properties by the control of microstructure and optimization of morphology in components by selective laser melting. The microstructure in amorphous and crystalline metallic alloy systems is influenced by changing the laser power density and scanning strategies respectively. A combination of X-ray/neutron diffraction and optical/electron microscopy is used to evaluate the microstructure and phase formation in SLM components. The influence of the microstructure on the mechanical properties of as-printed samples was investigated using hardness and uniaxial tensile testing methods. To begin with, the process parameters for selective laser melting of a Zr-based bulk metallic glass Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMLOY-ZR01) are developed to obtain high density and crack-free bulk components. The influence of oxygen on the thermal stability and crystallization pathway in AMLOY-ZR01 was found to be significant in determining the formation of metastable crystalline phases within the amorphous matrix. It was also shown that the mechanical properties in AMLOY-ZR01 can be influenced by changing the amount of crystalline phases formed within the amorphous matrix. This was achieved by changing the laser power density during the SLM process. The alloy composition was also investigated for its biocompatibility, and the cell-material interactions under in-vitro test conditions showed no cytotoxic effect. These findings demonstrate that AMLOY-ZR01 is a promising candidate for orthopedic bio-implant applications. The latter half of this work demonstrates the influence of microstructure and crystallographic texture on the mechanical properties of 316L SS. This was achieved by changing the "laser scanning methodology" during the SLM process and a correlation between the applied scanning methodology and structure-property relation was identified. A single crystalline-like texture can be obtained using a bi-directional scanning methodology, whereas a fiber texture is achieved when rotating the laser scan vectors by 67° to melt consecutive powder bed layers. The mechanical properties of 316L SS are influenced by the type of laser scan used to fabricate the components, as it dictates the final grain orientation within the SLM samples. It is also shown that the scanning patterns can be altered during the SLM process to create position-specific crystallographic grain orientation within the component. This opens up the possibility to fabricate functionally graded components which contain a spatial variation in composition and/or microstructure for the specific purpose of controlling material properties. Finally, the functionalization of material properties through design of components by additive manufacturing was demonstrated by fabricating waveguides with the specific geometries.
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| 2. |
- Goetz, Inga Katharina, 1992-
(författare)
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Local structure and composition : in additively manufactured bulk metallic glasses and composites
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing enables the production of complex multi-material geometries and bulk metallic glass parts beyond their critical casting thickness. The local modification of structure, composition, and properties is explored in this thesis with the aim to design novel composite materials and functional gradients. The present work contributes to the process understanding required to produce bulk metallic glass composites in the laser powder bed fusion process. The investigated material systems include zirconium and iron based metallic glasses, metallic glass - nitride composites, as well as laser nitriding of titanium.The onset of devitrification of metallic glasses due to the processing or post-processing heat treatments induces the formation of nanoscale clusters. Features of > 1 nm can be detected and reliably distinguished from random fluctuations by atom probe tomography. The progression of crystallisation depends on the oxygen content of the samples and the applied heating or cooling rates. In-depth understanding of the crystallisation processes can be used to optimise both compositions and processing conditions. The process atmosphere contains reactive species such as oxygen or nitrogen, which can be incorporated during different stages of processing by surface oxidation of the powder or substrate pieces as well as by reactions with the gas during processing. While an inert gas atmosphere with a low residual oxygen content is sufficient to hinder reactions with residual oxygen, a nitrogen atmosphere can be used for local laser nitriding and, thus, the fabrication of metallic glass – ceramic composites. Due to the decompositions of nitrides formed in the preceding process steps and the tendency of a metallic glass matrix to crystallise, which limits the processing conditions, the incorporation of nitrogen is restricted to the first few hundred nm from the surface for a Zr-based amorphous alloy. In titanium, as a crystalline example, nitrogen is incorporated throughout the molten pool.The nitride composites exhibit increased hardness depending on the local nitride fraction, which can thus be used to fabricate specific property gradients within or on a printed piece. With optimised process parameters, the amorphous fraction of a printed Fe-based bulk metallic glass can be tailored for improved soft magnetic properties.
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| 3. |
- Haux, Caroline, 1960-
(författare)
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Framkallning : Skrift, konsumtion och sexualitet i Karin Boyes Astarte och Henry Parlands Sönder
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a comparative study in two novels which both try to understand modern life. The novels were both written between 1929 and 1930 as entries for the same major Nordic novel contest – Astarte in Sweden by Karin Boye, Sönder [To Pieces ]in Finland by the Finno-swedish author Henry Parland.This analysis tries to determine what is at stake in these novels by, apart from investigating them as novels in their own right, examining the complex interaction between a specific moment in history and the representation of that moment as literary text. The novels are examined as events in a specific historical situation; that is, how they write what is contemporary into themselves, trough answering the question about the historical moment that they themselves formulate. Four issues are fundamental to the study: The function of consumption and the commodity form in the novels. The way in which these novels render sexuality and desire. How other media forms function as aesthetic technologies for the novels. Lastly, aesthetics, writing and allegory: how the novels reflect on the possibility of representing this societal moment in history.A focus in the analysis is how woman is called forth as golden statue and photography, how she by letting herself be consumed as sexual symbol elicits male speech – thereby also making the speaker into author, and man. The economic side to this is the way in which men and women take on the form of commodities amongst themselves. It is in this sexual economy that they become men and women. The transformation of the sexed object into the systematic status of a sign, signifying its value, implies the simultaneous transformation of human relations into consumer relations: You consume or get consumed.
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| 4. |
- Karlsson, Dennis, 1991-
(författare)
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Additive Manufacturing of Ferritic Materials : A Journey from Stainless Steels to High-Entropy Alloys
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Design of new materials with complex geometries is an important part of new innovative solutions for technical applications. With the use of additive manufacturing (AM), the design possibilities are endless and geometries that are impossible to manufacture by conventional techniques are available. However, the number of alloys commercially available is limited and extensive research is needed to establish new materials with unique properties. An important group of materials is ferritic stainless steels which have a body centered cubic crystal structure. They are often used for their high strength, corrosion resistance or electrical properties at high temperatures. However, they are often less ductile than austenitic stainless steels and issues with cracking may arise during thermal cycling in the L-PBF process. In this thesis, two AM techniques, laser powder bed fusion (L-PBF) and binder jetting were used to produce components of two different ferritic stainless steels and of the AlCoCrFeNi high-entropy alloy (HEA). The main objective was to investigate the microstructural development, phase stabilities and mechanical properties in relation to conventional manufacturing routes. Furthermore, thermodynamic calculations were used to explain the phase stabilities and solidification. L-PBF enables manufacturing of the ferritic stainless steels SS441 and SS446 with excellent mechanical properties. It was shown that solid particles may form in the melt and act as heterogeneous nucleation points, resulting in effective grain refinement for SS441. Other secondary phases can form during the thermal cycling in the L-PBF process, enhancing the mechanical properties. An example is the formation of austenite in SS446. Furthermore, the formation of solid particles and segregated microstructure during solidification was predicted by thermodynamic calculations.The AlCoCrFeNi alloy could be produced with an intriguing hierarchical microstructure and excellent mechanical properties using binder jetting and post-treatments. The microstructure of the final component can also be controlled by pre-annealing of the feedstock powder. Thermodynamic calculations were used to design the phase composition of the alloy. A characteristic single-phase solid solution is only observed at very high temperatures close to the melting point. Hence, the AlCoCrFeNi alloy is not a thermodynamically true HEA, but is stabilized due to kinetic effects during manufacturing.
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| 5. |
- Srinath, Aishwarya
(författare)
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Investigations of the corrosion resistances of magnetron sputtered multicomponent materials : A study on high entropy alloys, high entropy sublattice ceramics, and metallic glasses
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The corrosion resistances of sputter deposited AlCrNbYZrN, CrNbTiTaWC and TaNiSiC alloys were assessed in acidic and alkaline environments. Compositionally complex alloying strategies, and whether they produced coatings with superior corrosion resistances, were explored. Another priority was to determine if the formation of a random solid solution was necessary for the design of corrosion resistant multicomponent alloys. The results showed that successful surface passivation/oxidation was the key factor determining corrosion resistance. The corrosion resistance was maximised when coating porosity was minimised, spontaneously passivating elements were used, and the oxide solubility in the electrolyte was low. The benefits of multicomponent alloying were demonstrated through the protective effect of oxides of more stable elements (Cr, Nb, Ta) reducing the loss of less corrosion resistant elements (Al, Ti). The supersaturation of nitrogen, carbon and/or silicon affected coating morphologies, surface repassivation rates, and even directly corrosion resistances, e.g. when silicon oxides were formed on TaNiSiC films.The N content (0-50 at.% N) in AlCrNbYZrN films affected the film porosity, the absence of which led to high corrosion resistances in 1.0 M HCl. The formation of a single phase did not govern the corrosion resistances. It was more important that the coatings were dense, as this permitted the formation of an impervious passive layer. The addition of 8 at.% C to nearequimolar CrNbTiTaW alloys affected surface repassivation rates of films whose oxide had been partially removed by chronoamperometry in a 1.0 M HCl electrolyte. However, the effect of carbon on the corrosion properties was minor. A comparison of the nearequimolar CrNbTiTaW, Nb and TaW alloys with their native oxides revealed comparable corrosion resistances in 0.01 M HCl + 0.1 M NaCl. The nearequimolar CrNbTiTaW, Nb, and TaW alloys also showed no evidence of pitting in 1.0 M HCl. The high corrosion resistances in CrNbTiTaW alloys was due to the presence of extremely stable surface oxides abundant in Cr, Nb, and Ta. TaNiSiC films with Si contents above 12 at.%, and C contents lower than 11 at.% showed high corrosion resistances in 10 mM sodium borate due to the formation of silicon oxides, even after three polarisations from −0.7 to +1.5 V vs. Ag/AgCl (3 M NaCl) and 50+ hours of immersion. Microscopy of the Si-rich TaNiSiC film showed negligible differences in surface condition before and after corrosion in sodium borate, while films with lower Si contents or higher Ta contents were more severely corroded.
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| 6. |
- Zendejas Medina, León, 1993-
(författare)
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Designing multicomponent alloy coatings for corrosion protection
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the design of metallic coatings for corrosion protection. The subject of the study was the new class of materials multicomponent alloys (MCAs, also known as high entropy alloys). They consist of near-equal concentrations of many (four or more) metals and are often reported to form a single phase with a simple crystal structure. Due to the complexity and range of possible MCA compositions, there is a need for design principles as guidelines for how the alloying elements can be chosen and combined. This work aimed at finding such principles through the systematic study of the synthesis and properties of three MCA systems. Their compositions were carefully chosen to answer fundamental questions about the materials class and the synthesis method and to generate conclusions that could be generalized to a larger group of MCAs. All three systems were based on the elements Cr, Fe, and Ni, and can therefore be considered an extension of stainless steels.The first alloy was CoCrFeMnNi, which is well-known as a single-phase bulk MCA. A systematic exploration of the synthesis parameters showed that there are fundamental differences in the phase formation of CoCrFeMnNi through magnetron sputtering compared to typical bulk synthesis. Literature studies revealed that this conclusion can be generalized; single-phase MCAs should not necessarily be expected from magnetron sputtering. It was also shown that the choice of substrate and even the crystal orientation of the individual substrate grains strongly influenced the outcomes of synthesis, including the phase formation, growth rate, morphology, and the formation of stacking fault structures. Two novel alloy systems were also explored: CrFeNiTa and CrFeNiW. Ta and W were added to achieve an alloy with higher corrosion resistance than stainless steels and more generally, to examine the interplay between passivating elements in MCAs during corrosion. Based on geometrical considerations, it was predicted that equal amounts of Ta and W would be needed to protect alloys from corroding (less than 20 at%). It was found that the prediction was only valid for the CrFeNiTa alloy system. The reason behind this was explored and a new criterion was then proposed: In an MCA, each passivating element should have similar electrochemical nobility.Further design possibilities were demonstrated by adding up to 50 at% carbon to the alloys. Thermodynamic calculations predicted decomposition into multiple metallic and carbide phases. However, the limited diffusion during magnetron sputtering suppressed the segregation. At lower carbon contents, the carbon-containing alloys were single-phase and amorphous. At higher carbon contents they formed alloy/amorphous carbon nanocomposites. The addition of carbon made the alloys stronger, more corrosion resistant, and more crack resistant.
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