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- Yildiz, Ahmet Bahadir, 1989-
(författare)
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Neutron scattering studies of hard metals
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since their discovery about 100 years ago, tools made of hard metals have been the enablers of development in various areas: from drilling subway lines for more sustainable cities to the machining of complex next-generation airplane engine parts for reduced CO2 emissions. An increase in properties of hard metals thus leads to efficient operations with shorter lead-time, less environmental footprint, and reduced cost. To tailor the as-sintered hard metal structure with desired properties and develop reliable modelling tools, it is critical to have statistically representative experimental data acquired in conditions mimicking the real processes for hard metals. However, to date, nano- and microstructural investigations of hard metals and related systems have been mainly focused on the lab-scale techniques, limiting the bulk-scale in-situ investigations. This thesis focuses on neutron scattering techniques and demonstrates how the utilization of small angle neutron scattering (SANS) techniques and neutron diffraction (ND) in a complementary manner with lab-scale techniques and computational tools can enhance the mechanistic understanding during processing of hard metals and related systems. Some of the presented information within the thesis is unique to neutron scattering experiments.SANS experiments are used for the quantification of nano- and microstructural features including Co-rich binder pocket size, WC grain size, and size and volume fraction of (V,W)Cx interfacial layers. The quantitative data enables us to draw conclusions regarding the mechanism of grain coarsening inhibition in V-doped hard metals at different V additions. The findings indicate that the grain coarsening inhibition in V-doped hard metals originates from reduced WC/Co interface mobility and total driving force for coarsening. The complex nature of structural evolution at sintering temperatures is further investigated by in-situ SANS up to 1500 oC. Our results show that the size and volume fraction of interfacial layers strongly depend on the presence of bulk (V,W)Cx precipitates and V activity in the binder phase.In-situ ND experiments during aging of (Ti,Zr)C-based systems provide time-resolved insights into the kinetics and structural evolution during phase separation at 1600 oC for 10 h. The results reveal that the decomposition of (Ti,Zr)C into TiC- and ZrC-rich phases can be significantly retarded by minor HfC or NbC additions. During decomposition, in line with the nucleation and growth process, no change is observed in the lattice parameter of ZrC-rich phase. In contrast, the lattice parameter of TiC-rich phase reduces with decomposition, resulting from TiC enrichment, i.e. it reaches equilibrium composition in the course of time. Furthermore, a novel multi-principal element carbide system (Ti,Zr,Hf,W)C with exceptional hardness is designed. Although the system has a miscibility gap, only minor decomposition is observed after 100 h aging at 1350 oC, where the formation of (Ti,W)C- and (Zr,Hf)C-rich decomposition products and WC precipitates occur. Such hindered decomposition enables the carbide system to preserve its high hardness.In summary, by using neutron scattering techniques, this thesis contributes to a better understanding of nano- and microstructural evolution in hard metals and related systems during their processing at elevated temperatures. The thesis and appended papers also guide readers regarding the planning, e.g. sample preparation and sample environment selection, and data analysis of neutron scattering experiments. The thesis can thus serve as a starting point for the more widespread utilization of neutron scattering techniques by the hard metal industry.
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| 2. |
- Kolmskog, Peter, 1983-
(författare)
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Does Bainite form with or without diffusion? : The experimental and theoretical evidence
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys.
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| 3. |
- Zhao, Guo-Hua
(författare)
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High-performance Load-bearing Alloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis is to study advanced metallic alloys as load-bearing componentsfor engineering and biomedical applications. The investigations have been focusing onthe improvement of the properties and performance of existing materials as well assynthesizing and developing completely new materials. This thesis covers alloy design,microstructure characterizations, mechanical and electrochemical tests, together withevaluating the tribocorrosion performances under the combined action of wear andcorrosion. The thesis consists of three parts:In part one (paper I and II), two types of multicomponent Ti-Fe-Sn-Nb alloys weredeveloped showing superior mechanical properties with distinct microstructures. Firstlythe hypoeutectic alloy consisting of ductile dendrites and ultrafine eutectic presentedsuperior yield strength and enhanced ductility compared to those of bulk metallicglasses (BMGs). Secondly, the β-type alloys completely retaining of β-Ti phasedisplayed outstanding plasticity without sacrificing the high mechanical strength.In part two (paper III), a series of Ti-based glassy alloys containing no biological toxicelements e.g. Ni, Cu and Al, were designed by a novel method coupling thermodynamiccalculations and topological instability criterion. A self-consistent thermodynamicdatabase was constructed based on the CALPHAD (Calculation of Phase Diagrams)approach. The experimental verifications matched reasonably well with the theoreticalcalculation, suggesting this method provides an effective approach for glass formingpredictions.In part three (paper IV and V), the degradation mechanisms of the load-bearing Zrbased BMGs were evaluated under the effects of tribological wear and electrochemicalcorrosion. Comparatively the tribo-electrochemical performances of the surfacemodified (plasma nitriding or diamond-like carbon) conventional metallic biomaterialsi.e. medical grade pure Ti, stainless steel and CoCrMo alloys, were investigated in linewith that of the BMGs.
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