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Microstructure and mechanical properties of lightweight structural AZ31 alloy prepared by twin-roll casting method.
Zimina, Mariia ; Cieslar, Miroslav (advisor)
Title: Microstructure and mechanical properties of lightweight structural AZ31 alloy prepared by twin-roll casting method Author: Mariia Zimina Abstract: Microstructure of AZ31 twin-roll cast magnesium alloy was studied using light optical, electron and atomic force microscopy. The effect of annealing temperature on the microstructure was tested. Mechanical properties of a thin magnesium strip were investigated by means of microhardness tests and tensile tests at a relatively low strain rate 10-3 s-1 . Results show that the ductility of the twin roll cast strip increases with increasing deformation temperature, however, a remarkable decrease was observed at about 200 řC. This effect appears also in a conventionally cast ingot of a master alloy and is caused by a change of a deformation mode occurring at this temperature. Moreover, the effect of severe plastic deformation on the microstructure and mechanical properties was studied. Equal channel angular pressing was applied on magnesium strip samples and lead to a significant grain refinement accompanied by an unfavorable strengthening of the initial basal texture, which is effectively suppressed by a subsequent annealing. A constrained groove pressing was for the first time successfully applied on AZ31 twin-roll cast strip leading to an increase of...
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Mechanical properties of magnesium alloy AZ91E prepared by ECAP method
Darida, Jiří ; Válka, Libor (referee) ; Pantělejev, Libor (advisor)
This thesis deals with mechanical and fatigue properties of magnesium alloy AZ91 processed by EX-ECAP method. This method involves the application of extrusion followed by equal-channel angular pressing. To obtain basic mechanical characteristics, the tensile test were carried out at room and elevated temperatures. Further the fatigue tests were performed and obtained data were used to compile S-N curve. The work also includes metallographics analysis of microstructure and fractographic analysis of fracture surfaces of tensile and fatigue test specimens.
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Ni-P based coatings preparation on wrought magnesium alloys
Buchtík, Martin ; Fintová, Stanislava (referee) ; Wasserbauer, Jaromír (advisor)
The aim of this diploma thesis was summary of all steps and knowledge necessary to deposition of quality Ni-P coatings deposited on wrought magnesium alloys AZ31 and AZ61. There is the treatise about wrought magnesium alloys AZ31 and AZ61. Thesis includes its phase composition in the theoretical part. There are given its possible processing methods too. Next, there is desribed the mechanism of deposition of Ni-P coatings, components required to electroless deposition and factors affecting the quality and properties of these coatings. The theoretical part is ended by serie of reviews. Authors of these reviews deal with pretreatment of substrates, preparation, characterization and measuring of mechanical, structure and corrosion properties of deposited coatings. The optimalization of pretreatment, parametres and composition of nickel bath suitable for magnesium alloys is described in experimental part. The microstructure, present interlayer between substrate and Ni-P coating and chemical composition of deposited coatings was observed and measured by optical and electron microscopy. The mechanical characterization of Ni-P coatings was performed by microhardness tester.
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Fatigue Behaviour of AZ31 and AZ61 Magnesium Alloys
Gejdoš, Pavel ; Lukáč, Pavel (referee) ; Kohout, Jan (referee) ; Podrábský, Tomáš (advisor)
This doctoral thesis deals with the fatigue behaviour of AZ31 and AZ61 magnesium alloy casted by advanced methods of casting squeeze casting. Based on the regression functions and measured fatigue data were determined fatigue characteristics of these alloys. The work also extends into the area of fatigue cracks in the AZ31 alloy. Were measured fatigue crack propagation rate on modified cylindrical specimens. The paper also outlines the possibilities for describing the fatigue behaviour of AZ61 alloy in extremely low-cycle fatigue.
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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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Evaluation of Corrosion on Selected Magnesium Alloys
Pořický, Vladislav ; Tulka,, Jaromír (referee) ; Pacal, Bohumil (advisor)
This master´s thesis is focused on classification of corrosion resistivity magnesium alloy type AZ91 in environment of salt vapour testing in corrosion chambers. For tests were used samples of magnesium alloy manufactured by three different methods of casting: cast-iron mold, die vacuum casting, die casting without vacuum and method of die casting with additional pressure (squeeze casting). In this work was accomplished metallographic evaluation of corrosion attack and detailed analysis of corrosion products. Conclusions of exposits tests are assembled of analysis of influence of corrosion environment on structure of alloy. On the basis of these conclusions of corrosion tests were evaluated influences of corrosion speed [mm/year] on time [hours] for individual states of magnesium alloy.
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Corrosion degradation of magnesium alloys in SBF solutions
Ročňáková, Ivana ; Horynová, Miroslava (referee) ; Doležalová Weissmannová, Helena (advisor)
Nowadays, magnesium alloys are used in many sectors of human activity, particularly in the aerospace and automotive industries, due to a combination of good mechanical properties and low density of magnesium. Recently, however, the attention is paid to the possibility of using magnesium alloys in the medical field, mainly due to the chemical properties of magnesium. The diploma thesis is focused on evaluating of the corrosion behavior of AZ31alloy and AZ61 alloys in SBF solutions that are solutions simulating body fluids. The alloys were subjected to immersion test in Hank’s solutions for time span from 24 to 1008 hours. The corrosion process was evaluated based on several factors: alloy composition, time and pH changes. The thesis also describes in detail the methodology for establishing magnesium and zinc by method of flame absorption spektrometry (F AAS). The determination of aluminum was carried out by atomic absorption spectrometry with electrothermal atomization (ET AAS).
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Reactivity and controll of properties of metallic materials
Ševčíková, Barbora ; Opravil, Tomáš (referee) ; Zmrzlý, Martin (advisor)
The main objective of the bachelor thesis is to investigate the reactivity and corrosion of materials, namely of magnesium alloys. The theoretical part comprises a brief summary of the existing knowledge on magnesium alloys in the field of metal corrosion and a description of essential properties of the examined and utilized material. These are followed by the protection technology and a material resistance analysis including the specification of the processes applied in the experimental part. The thesis is focused on a comparison of the microstructure and corrosion resistance of magnesium alloys AZ31, AZ61 and AZ91. The applied methodology lied on metallography preparation of light optical micrographs of the surface-region cross-sections and on corrosion tests. These were carried out in 3% sodium chloride solution where the samples were immersed for the period of 8 hours.
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Heterodifúze 65Zn v binárních slitinách Mg - x Al av komerční slitině AZ91
Čermák, Jiří ; Stloukal, Ivo
Bulk diffusion coefficient of Zn, D, in Mg – x Al model alloys (x = 0, 1.8, 3.9 and 9 at. %), in commercial alloy AZ91 and in intermetallic compound Mg17Al12 intermetallic was measured by residual activity and serial sectioning methods. Composition dependence of D was obtained, that shows a slight increase of D with increasing Al concentration. Measurements were done at several diffusion temperatures, which enabled to evaluate the pre-exponential factor and the activation enthalpy of Zn diffusion.
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Difúze 65Zn ve dvoufázové eutektické slitině Mg - 33 wt.% Al
Čermák, Jiří ; Stloukal, Ivo
Diffusion of zinc in a two-phase eutectic alloy Mg – 33 wt.% Al and in its components, Mg – Al solid solution and in the intermetallic compound Mg17Al12, was studied. The measurement of Zn diffusion coefficient was carried out by radiotracer technique in temperature interval 498 – 848 K. Zinc is believed to simulate the migration behavior of magnesium. It was observed that the interphase boundaries do not act as short-circuit paths. The study of the stability of the eutectic structure enabled to estimate the mean specific energy of the (Mg)/ Mg17A12 phase interface.
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