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Structure analysis of some transition metal silicides using X-ray diffraction and dynamical refinement against electron diffraction data
Antunes Corrêa, Cinthia ; Janeček, Miloš (advisor) ; Kalvoda, Ladislav (referee) ; Kužel, Radomír (referee)
Title: Structure analysis of some transition metal silicides using X-ray diffraction and dynamical refinement against electron diffraction data Author: Cinthia Antunes Corrˆea Department: Physics of Materials Supervisor: prof. RNDr. Miloš Janeček, CSc., Department of Physics of Materials Abstract: This thesis presents the crystal structure analysis of several transition metal silicides. The crystal structures were studied primarily by precession electron diffraction tomography (PEDT) employing the dynamical refinement, a method recently developed for accurate crystal structure refinement of electron diffraction data. The optimal values of the parameters of the method were proposed based on the comparison between the dynamical refinement of PEDT data and a high- quality reference structure. We present the results of the comparison using a Ni2Si nanowire with the diameter of 15 nm. The average atomic distance between the model obtained by the dynamical refinement on PEDT data and the one by single crystal X-ray diffraction was 0.006 ˚A. Knowing the accuracy and limitations of the method, the crystal structure of Ni3Si2 was redetermined on a nanowire with 35 nm of diameter. The model obtained had an average error in the atomic posi- tions of 0.006 ˚A. These results show that the accuracy achieved by the dynamical...
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Phase transformations in modern titanium alloys
Šmilauerová, Jana ; Janeček, Miloš (advisor) ; Kalvoda, Ladislav (referee) ; Strunz, Pavel (referee)
This study deals with phase transformations in metastable β titanium alloys, focusing on the investigation of evolution of nanoparticles of thermodynamically metastable ω phase. For the purpose of this research, single crystals of two metastable β titanium alloys - LCB (Ti-6.8Mo-4.5Fe-1.5Al) and Ti-15Mo (in wt. %) - were grown in an optical floating zone furnace. It was established by differential scanning calorimetry that the phase transformations occurring in the material did not change significantly as a result of the single crystal growth process. Using single crystal X-ray diffraction, the shape and the size of ω particles were determined in a series of aged samples. The lattice parameters of ω particles and the β matrix, as well as the misfit between the two structures were calculated. The β phase was found to be locally deformed in compression around ω particles. Small-angle X-ray scattering (SAXS) experiments revealed a spatial ordering of ω particles in the β matrix in a disordered cubic array with the basis vectors along 100 β directions. The SAXS data also allowed the evaluation of the mean ω particle sizes and distances and confirmed that the ω particle growth obeys the t1/3 law following from the Lifshitz-Slyozov-Wagner theory. In situ SAXS performed during isothermal ageing at selected...
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Effect of molybdenum content on phase transformations in binary Ti-Mo alloys
Valešová, Veronika ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
Title: Effect of molybdenum content on phase transformations in binary Ti-Mo alloys Author: Veronika Valešová Department: Department of Physics of Materials Supervisor: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Mate- rials Abstract: β titanium alloys are highly appreciated group of structural materials. They provide an extraordinary combination of strength, toughness and fatigue resistance. These alloys are used predominantly in aerospace sector, chemical industry and medical engineering. We have studied three binary titanium alloys: Ti-12Mo, Ti-15Mo and Ti-18Mo (wt %). Apart from low temperature hexagonal close packed α phase and high temperature base centered cubic β phase, a metastable ω phase can be also found in this type of alloys. The ω phase is created in a diffusionless process by shifting the planes in the β phase and affects both mechanical properties and the α phase precipitation. We have examined the phase transformations occurring in the studied alloys using both in-situ and ex-situ methods. Resistometry, dilatometry and differential scanning calorimetry were used as the indirect in-situ methods. For the ex-situ methods, scanning electron microscopy and microhardness measurement were used. Complex characterization of β metastable titanium alloys showed the formation of the...
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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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Interaction of tokamak plasma with selected fusion materials
Klevarová, Veronika ; Janeček, Miloš (advisor) ; Horáček, Jan (referee)
Title: Interaction of tokamak plasma with selected fusion materials Author: Veronika Klevarová Department: Department of Physics of Materials Supervisor: Doc. RNDr. Miloš Janeček, CSc. Supervisor's e-mail address: Milos.Janecek@mff.cuni.cz Abstract: The following work is devoted to structural analysis and study of relevant physical properties of materials with perspective use for the first wall and as construction materials of the tokamak device, prior to their interaction with the high-temperature plasma in Compass-D tokamak. The theoretical part contains an introduction to the interaction of tokamak plasma with fusion materials, including a description of the processes taking place during the plasma discharge (such as implantation, erosion, particles traveling in the tokamak vacuum vessel, redeposition, co-deposition with tritium etc.) and selection of appropriate fusion materials. The experimental part consists of structural analysis of selected materials by optical and scanning electron microscopy. Analysis of the structure revealed the presence of oxide layers in some samples prepared by the technology of plasma spraying; which was probably caused by sample preparation in the ambient air. Qualitative elemental analysis of the samples was performed by electron dispersive X-ray spectroscopy (EDS) in SEM....
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Metastable beta titanium alloys for biomedical use
Preisler, Dalibor ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
In this work the effect of die-forging and annealing on the mechanical properties and microstructure of biocompatible metastable alloy Ti-35.3Nb-7.3Zr -5.7Ta-0.7O (wt. %) was studied. Light and scanning electron microscopy, microhardness tests and tensile tests were used to study all prepared conditions. As-cast and as-annealed conditions showed chemical inhomogeneities exhibited as dendritic structure, grains with size of hundreds of micrometers and numerous pores having size of units of micrometers. Precipitation of alpha phase particles was negligible in as-annelaed conditions and only grain boundary alpha paticles formed. Die-forged condition showed grain sizes in the wide range of hundreds of micrometers down to micrometers. Both as-annealed and die-forged conditions showed increased microhardness compared to as-cast condition, but there were no significant differences between different annealing conditions. Yield stress of 870 MPa was found for as-cast condition while as-forged condition exhibited even higher yield stress of 1120 MPa. In both conditions, good room-temperature ductility was demonstrated. Due to its high strength, this alloy is perspective for manufacturing of joint implants.
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Effect of substantial grain refinement on microstructure and mechanical properties of precipitation hardenable magnesium alloys.
Hofman, Daniel ; Minárik, Peter (advisor) ; Janeček, Miloš (referee)
Properties of magnesium alloy WE43 and the effect of substantial grain refinement by ECAP were studied. The compressive strength test revealed a significant increase of yield strength after ECAP, resulting in yield strength of over 400 MPa after 8 ECAP passes. The material remained surprisingly ductile in both compression and tension. Thermal stability of the material was investigated via Vickers hardness test. The ultrafine-grained structure shows good thermal stability up to 300 řC, where the precipitates start to dissolve and grain size starts to increase, resulting in sharp fall of hardness. Annealing of the initial state showed best results for 210 řC where the precipitation hardening led to an increase in hardness up to ∼101 HV.
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