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Investigation of microstructure and phase transformations in zirconium alloys
Strnad, Adam ; Šmilauerová, Jana (advisor) ; Šíma, Vladimír (referee)
Title: Investigation of microstructure and phase transformations in zirconium alloys Author: Adam Strnad Department: Department of physics of materials Supervisor: RNDr. Jana Šmilauerová, Ph.D., Department of physics of materials Abstract: In this work, we studied microstructure and phase transformations in three α+β zirconium alloys Zr-4Nb-1Cr, Zr-4Nb-2Al and Zr-6Sn-4Nb. We used methods such as light microscopy and scanning electron microscopy to examine the microstructure of α and β phase and its changes due to homogenization annealing. Differential scanning calorimetry and resistometry were employed to investigate the phase transitions occurring in studied alloys. We determined using these complementary methods the temperature dependency of α → β phase transition. As a result, the beta transus temperature was determined for each alloy. By quenching the samples from temperatures below and above the beta transus, we verified the measured value of beta transus temperature. 1
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Diffusion of native defects and impurities in CdTe/CdZnTe
Šedivý, Lukáš ; Belas, Eduard (advisor) ; Šíma, Vladimír (referee)
Title: Diffusion of native defects and impurities in CdTe/CdZnTe. Author: Lukáš Šedivý Author's e-mail address: luky.sedivy@seznam.cz Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc. Supervisor's e-mail address: belas@karlov.mff.cuni.cz Abstract: In this thesis, the influence of structural defects on the electrical and de- tection characteristics of CdTe material was investigated. The performed research fo- cused on the reduction of structural defects in the material by annealing in Cd or Te vapor, while preserving acceptable features for X-ray and γ-ray detection. The mate- rial was characterized by measurement of the electrical resistivity and concentration and mobility of free carriers. Tellurium and cadmium inclusions were studied using infrared microscope. The static and dynamic properties of defect structures at high temperatures and de- fined Cd pressures was investigated, as well, and chemical diffusion coeficients describing the dynamic properties of these defects were experimentally determined. Keywords: monocrystal CdTe, structural defects in semiconductors, annealing in Cd or Te, chemical diffusion coefficient, γ-ray detectors. 1
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Distribution of dispersoids in rapidly solidified aluminium alloys.
Bajtošová, Lucia ; Cieslar, Miroslav (advisor) ; Šíma, Vladimír (referee)
Dispersoids formed during rapid solidification of a quaternary Al-Fe-Mn-Si alloy prepared by twin-roll casting and their transformation after following heat treatment were studied. A conventional transmission electron microscopy and modern automated phase identification methods such as electron backscatter diffraction in scanning electron microscope and automated orientation crystal phase and orientation mapping in transmission microscopy were applied. Energy dispersive X-ray spectroscopy was used to determine the chemical composition of the dispersoids. The phase identification methods were tested on as cast samples and samples annealed for 4 hours at 580 žC. Strategies for statistical evaluation of particle distribution and their crystallographic structure were discussed.
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Microstructure and mechanical properties of ultra-fine grained titanium alloys
Václavová, Kristína ; Stráský, Josef (advisor) ; Šíma, Vladimír (referee)
Title: Microstructure and mechanical properties of ultra-fine grained titanium alloys Author: Bc. Kristína Václavová Department / Institute: Department of Physics of Materials Supervisor of the master thesis: PhDr. RNDr. Josef Stráský, Ph.D. Abstract: Metastable β-Ti alloys Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) were subjected to severe plastic deformation by high pressure torsion. Microhardness of Ti-15Mo and TIMETAL LCB alloys increases with increasing inserted deformation, i.e. with increasing number of HPT rotations and also with increasing distance from the centre of the sample. The highest microhardness after HPT exceeds significantly the microhardness of two- phase α + β heat-treated material. Increasingly deformed microstructure was also demonstrated by scanning electron microscopy and by electron back-scatter diffraction. Significant twinning was observed in both studied alloys. Mechanism of multiple twinning contributes notably to the fragmentation of grains and thus to the refinement of the microstructure. Defect structure in Ti-15Mo alloy was studied by positron annihilation spectroscopy. It was proved that dislocations are the only detectable defects in the material by positron annihilation spectroscopy and that dislocation density increases with the number of HPT revolution and with...
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Microstructure and mechanical properties of Ti15Mo alloy prepared by ECAP
Terynková, Anna ; Stráský, Josef (advisor) ; Šíma, Vladimír (referee)
In the bachelor thesis, mechanical properties and microstructure of Ti-15Mo alloy prepared by equal channel angular pressing were studied along with their dependence on the number of passes. Microhardness was studied by Vickers method, microstructure was studied by scanning electron microscopy and by electron back-scattered diffraction (EBSD). Finally, the elasticity modulus was measured by resonant ultra-sound spectroscopy. Mircohardness significantly increased after the first ECAP pass. After the second and the third pass it was almost constant and it again distinctly rose after the fourth pass. Grains with the size of hundreds of micrometres were observed in all samples. The volume fraction of twinned material increased with the number of passes. Deformation bands were also observed, namely in samples after two and three passes. Elastic modulus significantly increased with increasing deformation by ECAP and the evolution of elastic modulus is qualitatively similar to the evolution of microhardness. Considering that the material was processed by ECAP at 250řC, the omega phase may have formed during the processing. The increase of microhardness and elastic modulus can be explained by the increasing volume fraction of the omega phase.
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Materiály pro fúzní aplikace a jejich interakce s tokamakovým plazmatem
Klevarová, Veronika ; Janeček, Miloš (advisor) ; Šíma, Vladimír (referee)
Title: Materials for fusion applications and their interaction with tokamak plasma Author: Veronika Klevarová Department: Department of Physics of Materials Supervisor: doc. RNDr. Miloš Janeček, CSc., Department of Physics of Materials Abstract: Tungsten represents a perspective option in the context of fusion devices first-wall materials. In the first part of this work, set of tungsten samples with variable grain size was prepared by spark plasma sintering. Specimens were exposed to steady state deuterium plasma beam and high energy heat pulses, simulating thus the normal operation in the tokamak. As a consequence of the exposure, samples surfaces were roughened, as-prepared grains were recovered and in some cases cracks were formed. Moreover, post-irradiation analysis of the damaged samples revealed activation of in-grain slip systems within the loaded surfaces. Threshold grain diameter for this mechanism was determined to be between 5.5 - 6.6 μm at the particular loading conditions. However, damaged features showed to depend more on the fabrication parameters than on the grain diameter. Synergistic effects of simultaneous loading were proven to be important since those reduced the heat propagation within the volume of the tested samples. In the second part of this thesis, introduction to plasma-surface...
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Structural changes in aluminium alloys studied by in-situ transmission electron microscopy.
Bajer, Jan ; Cieslar, Miroslav (advisor) ; Šíma, Vladimír (referee)
Aluminium alloys of the 6### series are widely used in the industry. To achieve the optimal properties, the as-cast alloy must by treated by several steps. The first treating step is the homogenization annealing of the as-cast alloy. During homogenization dissolution of eutectic phases occurs and distribution of impurity elements in the alloy is more uniform. It requires a high annealing temperature near the melting point of the alloy. One of the method that allows direct observation of the microstructure during homogenization is in-situ electron microscopy. Specimens from EN AW 6082 alloy were annealed in several model homogenization regimes and in-situ monitored by electron microscopy. Observed processes were compared with the results of resistivity measurements. EDS measurements were taken at the beginning and at the end of the homogenization process.
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