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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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Mechanical and physical properties of magnesium alloys prepared by rotary swaging
Škraban, Tomáš ; Drozd, Zdeněk (advisor) ; Mathis, Kristián (referee)
in english For their positive influence on mechanical and physical properties of the material, methods of severe plastic deformation are popular for quite some time today. Rotary swaging is one of them. With its simplicity and productivity, it has the potential for industrial use. It is a radial swaging of rods or tubes, which results in decreasing of their diameter. Influence of this method is researched on extruded rods made of magnesium alloy AZ31. Experiments are made on five samples of different degree of swaging (different diameter). This allows to research gradual evolution of properties during the swaging. Results show significant positive influence on grain size (and microstructure in general) of originally extruded rod. During swaging there is an evolution of material texture and increase in strength.
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Study of hydrogen interaction with defects in thin metallic films
Hruška, Petr ; Čížek, Jakub (advisor) ; Krsjak, Vladimír (referee) ; Mathis, Kristián (referee)
Thin metallic films are particularly interesting as potential hydrogen storage materials as well as hydrogen sensitive optical sensors. Thin films with various microstructure from nanocrystalline to epitaxial can be relatively easily prepared by varying the deposition parameters. Deposition of multi-layers enables preparation of thin films with arbitrary composition. Defects structure plays a key role in hydrogen absorption. Hydrogen atoms segregated at open volume defects reduce their formation energy leading to enhan- ced concentration of hydrogen-induced defects in the material. Moreover hydrogen diffusion along dislocations and grain boundaries facilitates hydrogen absorption in the metal lattice. Thin films clamped to the stiff substrate undergo anisotropic volume expansion during the hydrogen loading. As a consequence high stresses are induced in the film and can result in detachment of the film from the substrate. In this work hydrogen absorption in Gd and Pd films and Pd-Mg multi-layers was studied. Development of the defect structure of hydrogen-loaded films was investigated by means of variable energy positron annihilation spectroscopy com- bined with X-ray diffraction, atomic force microscopy and optical transmittance measurement. Complementary studies of interaction of hydrogen with defects in...
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Anisotropy of thermal expansion in magnesium materials exhibiting texture
Škraban, Tomáš ; Drozd, Zdeněk (advisor) ; Mathis, Kristián (referee)
Magnesium alloys are valued for their low density and good mechanical pro- perties. Their characteristic is the tendency to obtain a texture during processing, which together with their hcp crystal structure results in the anisotropy of some physical properties. In this work we examine the effect of the texture on the aniso- tropy of thermal expansion of the widely used AZ31 and ZE10 magnesium alloys. These alloys are in the form of thin rolled sheets and we examine them in an as-recieved state and in a condition after an annealing. The measured samples are cut out of the metal sheet in the direction of rolling and in the perpendicular direction. The measurement is carried out in the temperature range from room temperature to 400 ◦ C. Measuring of the thermal expansion of sheets is problema- tic because they can bend and devalue the measurement results. The result of this work is a confirmation of the existence of this anisotropy and its description. 1
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Investigation of the dynamics of the deformation processes in Mg-RE alloys using in-situ experimental methods
Szabóová, Andrea ; Mathis, Kristián (advisor) ; Drozd, Zdeněk (referee)
In this present work, the dynamics of deformation mechanisms activated in binary magnesium-gadolinium alloys with respect to amount of Gd were investigated with in-situ experimental methods. Cast alloys are characterized by random texture. Compression test were done at room temperature with simultaneous record of acoustic emission response. The acoustic emission signal was subsequently analysed using advanced clustering method providing information about the dominant deformation mechanisms. High speed camera was used to study the dynamics of twinning, including estimation of the velocity of twin propagation with respect to Gd concentration. The deformation tests were repeated in a chamber of scanning electron microscope (in-situ SEM) with concurrent following the microstructure development using secondary electrons and electron back-scattered diffraction (EBSD) in different stages of the deformation. Main goal of this measurements was to identify active slip systems and the progress of twin volume fraction during deformation. Keywords: magnesium alloy, deformation tests, acoustic emission, high-speed camera, electron microscopy, twinning
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Study of the selected properties of aluminium alloys with Yb and Zr addition
Zikmund, Sebastien ; Vlach, Martin (advisor) ; Mathis, Kristián (referee)
The presented thesis focuses on the effect of addition of ytterbium, manganese and zir- con in alloys AlYb and AlMnYbZr in the state as casted, cold rolled with the deformation degree of 40 % and 70 % and after high-temperature annealing. The materials were exa- mined with the methods of electrical rezistometry, differential scanning calorimetry and microhardness measurements. The characterization of the examined alloys was completed by a parallel research of the microstructure with the methods of transmission and scanning electron microscopy, electron diffraction and electron backscatter diffraction. A positive influence of the addition Mn and Zr and cold rolling was proven at hardening of the mate- rials. In the context of messurement error a different influence in the microhardness of the materials was not observed among the different degrees of cold roling deformation. The si- multaneous addition of Mn and Zr has a positive effect on the size of the grain, whereas the size of the grain at the as prepered state was determined in millimetres (in AlYb) and in hundres of mictometers (AlMnYbZr). On the border of (sub)grains an eutetic phase rich in Yb, Fe (or more precisely Yb, Mn, Cu, Fe for the alloy AlMnYbZR) was observed for both alloys. At the annealing temperature around 400 řC, an additional...
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Investigation of titanium alloys using neutron diffraction
Németh, Gergely ; Mathis, Kristián (advisor) ; Nový, František (referee) ; Seiner, Hanuš (referee)
Title: Investigation of titanium alloys using neutron diffraction Author: Gergely Németh Department / Institute: Department of Physics of Materials Supervisor of the master thesis: prof. RNDr. Kristián Mathis, Ph.D., DrSc., Department of Physics of Materials Abstract: Titanium grade 2 was treated by multiple passes of the continuous equal- channel angular pressing technique (CONFORM ECAP) and, after each pass, additionally by rotary swaging. The residual strain field in samples processed by only CONFORM ECAP was studied by neutron diffraction strain scanning. In order to elucidate the microscopic background and calculate the related residual stress field, the local microstructure was thoroughly investigated by various experimental techniques. The microstructure and the deformation behavior of the rotary swaged samples was studied by transmission electron microscopy and by in-situ neutron diffraction during compression. The results of the analyses indicated that microstructural gradients were present in the material as the result of the inhomogeneous deformation during the CONFORM ECAP treatment. These gradients were identified as the main reason of the presence of residual stress fields. The distributions of stress fields calculated based on microstructural parameters were in correlation with simulation...
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Mechanical and physical properties of magnesium alloys prepared by rotary swaging
Škraban, Tomáš ; Drozd, Zdeněk (advisor) ; Mathis, Kristián (referee)
in english For their positive influence on mechanical and physical properties of the material, methods of severe plastic deformation are popular for quite some time today. Rotary swaging is one of them. With its simplicity and productivity, it has the potential for industrial use. It is a radial swaging of rods or tubes, which results in decreasing of their diameter. Influence of this method is researched on extruded rods made of magnesium alloy AZ31. Experiments are made on five samples of different degree of swaging (different diameter). This allows to research gradual evolution of properties during the swaging. Results show significant positive influence on grain size (and microstructure in general) of originally extruded rod. During swaging there is an evolution of material texture and increase in strength.
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Investigation of dislocation structure evolution in advanced magnesium alloys
Šimko, Pavol ; Mathis, Kristián (advisor) ; Drozd, Zdeněk (referee)
: This thesis deals with evolution of dislocation structures in mag- nesium alloys containing long-period stacking ordered phase (LPSO). Before de- formation the samples were studied by optical light microscopy, in the process of deformation by neutron diffraction. Obtained diffraction profiles were of two types, axial and radial diffraction profiles (created by planes oriented in the di- rection of deformation and orthogonally to the direction of deformation). Profiles were evaluated by Convolutional Multiple Whole Profile method and by Hexbur- ger program. This work is focused on the role of LPSO phase in the evolution of dislocation density and population of dislocations in slip systems depending on the composition of alloys and on the volume fraction of LPSO phase. 1
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Investigation of deformation mechanisms in Mg-Gd alloys
Szabóová, Andrea ; Mathis, Kristián (advisor) ; Drozdenko, Daria (referee)
Title: Investigation of deformation mechanisms in Mg-Gd alloys Author: Andrea Szabóová Department: Department of Physics of Materials Supervisor: doc. RNDr. Kristián Mathis, DrSc. Abstract: In the present work, the deformation behavior of magnesium-gadolinium binary alloys was investigated. Dependence on the concentration of Gd and deformation temperatures was studied. Extruded samples had relatively strong initial texture. Compression tests were done at room temperature and 200řC. Simultaneously with deformation acoustic emission was recorded. Data from acoustic emission was analyzed with advanced statistical methods. Results of the combination of these two experimental methods indicated that at the beginning of the deformation twinning is the dominant mechanism. In the following stage of plastic deformation non-basal slip systems became the governing deformation mechanism. With higher content of Gd the size of twins decreases as a result of the decreased mobility of twin boundaries caused by solute atoms. At higher temperatures twinning activity was increasing. In addition, results were confirmed by optical light and scanning electron microscopy. Keywords: magnesium alloy, deformation tests, acoustic emission, microscopy
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