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FATIGUE CHARACTERISTICS OF MODIFIED MAGNESIUM ALLOYS AFTER CORROSION DEGRADATION
Němcová, Aneta ; Očenášek, Vladivoj (referee) ; Hadzima, Branislav (referee) ; Pacal, Bohumil (advisor)
This doctoral thesis deals with the determination of the influence of plasma electrolytic oxidation (PEO) on fatigue behaviour of extruded AZ61 magnesium alloy in air and in the 3.5% NaCl solution. The coatings were formed in the silicate-phosphate electrolyte under pulsed current conditions at a frequency of 50 Hz. The influence of current density on coating formation was examined under current densities of 70, 130 and 200 mA cm-2 for different durations up to a maximum of 1800 s. 8 g dm-3 of KF were added to the electrolyte to study the influence of fluoride ions in plasma electrolytic oxidation. It is shown that fluoride ions inhibit localised oxidation in the initial stage of the process, associated with the secondary particles based on Al–Mn. The presence of fluoride also modified the sparking characteristics, decreased the rate of coating growth and changed the morphologies of the coatings. The influence of fluoride on the coating hardness, and the corrosion resistance of the alloy during exposure to salt spray, was negligible. Based on previous optimised PEO conditions, coatings formed under a current density of 130 mA cm-2 for 300 s in the electrolyte containing KF were chosen for fatigue testing. The high-cycle fatigue tests were carried out on cylindrical samples under a force controlled sinusoidal tension-tension cycle with asymmetry parameter R=0. The experimental data were fitted with Kohout & Věchet function. The fatigue limit of uncoated alloy in air for 107 cycles was determined at 145.4 MPa and the combination of PEO coating with chloride ions caused a reduction of ~55 %. Attention was paid to the fatigue crack initiation in different conditions of cyclic loading. The fracture surfaces underwent detailed fractography analysis including secondary crack observation on the gauge length. The contribution of Al–Mn particles were confirmed on the uncoated alloy in air and the presence of chloride ions were observed as another influential contributor to local corrosion attack. The cyclic loading caused spalling of the outer layer, and the multiple initiation was observed on PEO coated alloy, caused by cracks in the coating and stress transferring to the alloy.
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ELECTROCHEMICAL CHARACTERISTICS OF ADVANCED MAGNESIUM ALLOYS PROCESSED BY POWDER METALURGY
Minda, Jozef ; Nový, František (referee) ; Stoulil, Jan (referee) ; Hadzima,, Branislav (advisor)
The subject of the present dissertation is basic research in the field of advanced magnesium-based materials with high potential for medical applications prepared by powder metallurgy. These materials are evaluated in terms of corrosion behaviour in corrosive media by chemical composition and set conditions simulating the physiological environment of the human body (Hank's Balanced Salt Solution was used in this work). The aim is to analyze the influence of the processing parameters, chemical composition and structure on electrochemic characteristics of theprepared materials. For this purpose, a deeper of the the corrosion behavior, to obtain electrochemical characteristics and to investigate the degradation properties of the studied materials by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), with elemental analysis by energy-dispersive spectroscopy (EDS) and other complementary methods. The materials for the experiments are prepared using powder metallurgy, which allows the formation of composites with compositions that do not match conventional manufacturing technologies. Thus, pure Mg samples were prepared at process parameters with pressing pressures of 100 and 500 MPa at 400 °C and also samples pressed at 500 MPa and room temperature were tested. Furthermore, MgZn composites prepared at a pressing pressure of 500 MPa and a temperature of 300 °C with a Zn content of 1, 5, 10 and 15 wt.% or MgZn composites pressed at a pressure of 500 MPa and a temperature of 400 °C with a Zn content of 1, 5 and 10 wt.%. Composites of the Mg-hydroxyapatite binary system pressed at 500 MPa and 400 °C with hydroxyapatite contents of 10, 20, 30 and 40 wt.% were also studied. The degradation mechanisms of these materials were investigated using the above mentioned methods and the basic electrochemical parameters and their evolution during the time of exposure were determined. From the results, the influence of process parameters, chemical composition and the corrosive environment itself on the studied materials is evaluated. Several materials that appear to be most suitable for the medical application are described and the causes of degradation behaviour at the level of the structure of the materials and their electrochemical interaction with the corrosive environment are also discussed in the dissertation thesis. Compaction and densification positively influenced pure Mg samples, where samples pressed at 500 MPa and 400 °C are further selected as reference material. Increasing Zn content positively electrochemically affects the corrosion resistance of MgZn series of samples pressed at 500 MPa and 300 °C, on the contrary, in the combination of electrochemical - microgalvanic and structural effects, low Zn content is optimal for MgZn composites pressed at 500 MPa and 400 °C. For Mg-hydroxyapatite materials, in agreement with the literature, it was shown that the most stable corrosion product layers with the best corrosion properties and evolution in time are formed for samples with hydroxyapatite content up to 10 wt.%.
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Effect of microstructure on mechanisms of plastic deformation in fine-grained magnesium alloys
Minárik, Peter ; Král, Robert (advisor) ; Hadzima, Branislav (referee)
In this work were studied effects of the grain refinement on the mechanical properties and the corrosion resistance of magnesium alloys potential for medical applications - AE21 and AE42. Mechanical properties changed after ECAP pressing in consistency with literature data. Enhanced corrosion resistance in AE42 alloy was achieved by the grain refinement. Corrosion layer created on the surface was observed by light and electron microscopy. Effect of the grain size on the character of the corrosion layer was investigated. AE42 alloy is suitable for further investigation as potential biodegradable material. AE21 alloy was excluded from further investigation due to low corrosion resistance even after ECAP treatment.
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Complex investigation of fine-grained polycrystals of Cu and CuZr alloy processed by equal channel angular pressing a high pressure torsion
Srba, Ondřej ; Janeček, Miloš (advisor) ; Hadzima, Branislav (referee) ; Karlík, Miroslav (referee)
Title: Complex investigation of fine-grained polycrystals of Cu and CuZr alloy processed by equal channel angular pressing and high pressure torsion Author: RNDr.Ondřej Srba Department: Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University Prague Supervisor: Doc. RNDr. Miloš Janeček, CSc. Abstract: In the thesis the microstructure development, mechanical, elastic and corrosion properties of deformed specimens of pure Cu and binary alloy CuZr processed by equal channel angular pressing (ECAP) are investigated. Several properties of pure Cu processed by ECAP are compared with properties of the same material processed by high pressure torsion (HPT). The microstructure development is characterized in detail by several experimental techniques (light and electron microscopy, electron back scatter diffraction, positron annihilation spectroscopy, etc.). The microstructure development in specimens processed by ECAP is characterized by the continuous fragmentation of the initial coarse grain structure and the formation of new grains having the sizes in the submicrocrystalline range (of 460 nm and 260 nm in Cu and CuZr alloy, respectively). During the deformation by ECAP the fraction of high-angle grain boundaries, the dislocation density and the concentration of vacancies are...
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Study of ultrafine-grained materials prepared with different methods of severe plastic deformation
Krajňák, Tomáš ; Mathis, Kristián (advisor) ; Hadzima, Branislav (referee) ; Kalvoda, Ladislav (referee)
Title: Study of ultrafine-grained materials prepared with different methods of severe plastic deformation Author: RNDr. Tomáš Krajňák Department: Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague Supervisor: Doc. RNDr. Kristián Máthis, PhD., Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague Abstract: Interstitial free steel with ultrafine-grained (UFG) structure was prepared by high-pressure torsion (HPT). The development of the microstructure as a function of the number of HPT turns was studied at the centre, half-radius and periphery of the HPT-processed disks by X-ray line profile analysis (XLPA), positron annihilation spectroscopy (PAS) and electron microscopy. The dislocation densities and the dislocation cell sizes determined by XLPA were found to be in good agreement with those obtained by PAS. The evolution of the dislocation density, the dislocation cell and grain sizes, the vacancy cluster size, as well as the high-angle grain boundary (HAGB) fraction was determined as a function of the equivalent strain. It was found that first the dislocation density saturated, then the dislocation cell size reached its minimum value and finally the grain size got saturated. For very high strains after the...
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Study of mechanical properties and microstructure development of fine-grained Cu and Cu alloy polycrystals
Srba, Ondřej ; Janeček, Miloš (advisor) ; Hadzima, Branislav (referee)
The objective of the thesis is the study of mechanical properties, microstructure development and corrosion resistance of technical purity copper and Cu-0.18 wt.% Zr polycrystals prepared by ECAP. The tensile tests showed that the characteristic stresses 0,2 and max increase with increasing strain imposed by ECAP. The improvement of mechanical properties is compensated by the drop of ductility in the deformed state. The microstructure after 8 passes of ECAP is almost homogeneous consisting of equiaxed grains of the average size of 200 nm. The severe plastic deformation resulted in the grain size reduction of the factor of 1000. The specimen after 8 passes contains almost 50% of high angle grain boundaries. From the thermodynamic viewpoint the deformation did not change significantly the corrosion properties. However, the corrosion in the deformed material is slower and much more homogeneous.
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Microstructure and mechanical properties study of the finegrained magnesium alloys processed by severe plastic deformation
Šašek, Stanislav ; Stráská, Jitka (advisor) ; Hadzima, Branislav (referee)
Two magnesium alloys (Mg-4Y-4Gd-2Ca and Mg-2Y-2Gd-1Ca) with high ignition tem- perature were successfully processed by extrusion. Mg-2Y-2Gd-1Ca alloy was addition- ally processed by equal channel angular pressing (ECAP) to achieve ultrafine-grained microstructure. The effect of extrusion parameters on fraction of recrystallized grains, grain size, and texture was revealed by EBSD analysis. The presence of Mg2Ca, REH2 and Mg5RE secondary phases was proven by SEM and TEM. Microstructural condition including distribution and morphology of secondary phase par- ticles directly affected the mechanical properties. Yield tensile stress exceeding 200 MPa was achieved in each condition. Large non-recrystallized grains with strong {10̄10} tex- ture resulted in a significant anisotropy in mechanical properties. Processing by ECAP led to a homogeneous microstructure with a mean grain size below 1 µm. ECAP condition showed superior mechanical properties with a low anisotropy. The developed and analysed microstructural condition resulted in favourable mechanical properties. The studied alloys are therefore promising for the application in aerospace industry. 1
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Preparation and Characterization of Advanced Thermally-sprayed Coatings on Magnesium Alloys
Buchtík, Martin ; Hadzima, Branislav (referee) ; Fintová, Stanislava (referee) ; Ptáček, Petr (advisor)
The proposed dissertation thesis deals with the characterization of HVOF and APS-thermally sprayed coatings prepared on the AZ31 and AZ91 magnesium substrates. The theoretical part of the thesis describes in-detail Mg substrates used in the experimental part of the thesis. There are also characterized materials and coatings based on NiCrAlY and FeCrNiMoSiC metals, WC-CoCr cermets, and YSZ ceramic materials. At the end of the theoretical part, the literary research summarizing the characterization and analysis performed on thermally sprayed coatings on Mg alloys. Based on the theoretical knowledge, the characterization of Mg substrates and deposited coatings was performed in terms of the surface morphology, microstructure, and the chemical composition using the light microscopy (LM) and scanning electron microscopy with energy-dispersive spectroscopy (SEM+EDS). The phase composition of the coatings was analyzed using the X-ray diffraction (XRD). The diffractions corresponding to the sprayed coatings were compared with the feedstock powders, i.e. materials used for the spraying of the coatings. The characterization of the prepared coatings in terms of the mechanical and tribological properties was performed. The hardness and microhardness of the coatings as well as the coefficient of friction, and the wear rate were measured. The last chapter of the experimental part deals with the evaluation of the electrochemical corrosion properties by the potentiodynamic measurements in a 3.5% NaCl solution. In the case of exposed samples, the evaluation of the surface and coating/substrate interface was performed using LM and SEM with EDS. The mechanism of the corrosion attack and degradation was determined from the acquired knowledge and base on the results of the short-term measurements. Based on the measured results, it can be stated that the deposited coatings were successfully applied on the surface of both Mg alloys. All the coatings increase the surface hardness of the Mg alloys and significantly improve their tribological properties. However, except for FeCrNiMoSiC coatings, the corrosion properties of Mg alloys deteriorate due to the fact that the corrosion environment can pass through the coating to the less noble Mg substrate and the corrosion microcells are created.
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Preparation and Characterization of Fluoride Conversion Coatings on Biodegradable Magnesium Alloys
Drábiková, Juliána ; Hadzima, Branislav (referee) ; Brezinová,, Janette (referee) ; Ptáček, Petr (advisor)
The submitted work is aimed at the unconventional fluoride conversation coating preparation on the AZ31, AZ61, ZE10 and ZE41 magnesium alloys by their immersion in Na[BF4] molten salt. The influence of the preparation parameters (such as temperature and time) on the quality of the fluoride conversion coating is investigated. Methods of light and scanning electron microscopy were used for the evaluation of morphology, chemical composition and thickness of the coating. Short and long-term corrosion tests were executed to analyze the corrosion performance in simulated body fluid solution at 37 ± 2 °C with and without the fluoride conversion coating. The short-term behavior was evaluated by potentiodynamic tests, namely by the linear polarization. Long-term performance was assessed by electrochemical impedance spectroscopy or immersion tests. The coating preparation parameters influence on the character of the formed fluoride conversion coating was defined based on the obtained results. The next part of the thesis deals with the description of the possible mechanism of formation and kinetics of growth of the unconventional fluoride conversion coating on the selected AZ61 magnesium alloy. In this part, further detailed analyses were carried out to investigate the microstructure and chemical composition of the fluoride conversion coating using focused ion beam, transmission electron microscopy and X-ray photoelectron spectroscopy.
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Electrochemical corrosion characteristics of Mg-Zn systems prepared by powder metallurgy
Kotek, Jakub ; Hadzima, Branislav (referee) ; Fintová, Stanislava (advisor)
The diploma thesis deals with evaluation of electrochemical corrosion characteristics of Mg-Zn systems prepared by powder metallurgy in SBF solution. The main aim of the thesis is to analyze the influence of chemical composition, achieved structure and parameters of the process of production of Mg-Zn systems on their electrochemical corrosion characteristics. The basic electrochemical properties of the prepared materials will be evaluated by electrochemical impedance spectroscopy. In order to clarify the mechanism of corrosion of materials, the immersion tests will be used, accompanied by metallographic observations.
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