|
|
Processing of Nb-containing TiAl intermetallics and its Physical and Chemical Aspects
Barták, Tomáš ; Dlouhý, Ivo (referee) ; Zemčík, Ladislav (referee) ; Dlouhý, Antonín (advisor)
Prezentovaná práce se zabývá vakuovým indukčním tavením intermetalické slitiny Ti-46Al-7Nb (at. %) v žáruvzdorných kelímcích na bázi Y2O3. Byla provedena série taveb pro teploty přehřátí taveniny 1630, 1680 and 1730 C a při různých dobách výdrže na této teplotě v rozmezí 5 až 30 minut. Ze slitin ztuhlých v tavících kelímcích byly připraveny metalografické výbrusy, které sloužily k hodnocení mikrostruktury a vyhodnocení složení fází. Pro získání těchto dat byly použity metosy elektronové mirkoskopie SEM a EDS. Kvantitativní hodnocení mikrostruktury, zejména obsahu oxidické faze ve slitině, bylo provedeno pomocí software Adaptive Contrast Control (ACC). Analýza obsahu kyslíku ve ztuhlé slitině byla provedena metodou IGF (fúze v inertním plynu). V této práci jsou pochody na rozhraní slitina/oxidický kelímek posuzovány také z termodynamického hlediska a to s použitím aktivit jednotlivých složek v systému. Data prezentovaná v této práci mohou být použita pro nastavení a optimalizaci procesů tavení intermetalik TiAlNb.
|
|
|
Finding suitable institutional arrangements for governing policy of science and research from the perspective of stakeholders involved
Barták, Tomáš ; Witz, Petr (advisor) ; Potůček, Martin (referee)
The thesis focuses on discussing the appropriate institutional arrangements for science and research policy at the national level from the perspective of stakeholders and experts in the governance of this field. The aim is to find out what model of institutional arrangement and governance of this public policy is considered appropriate by the addressed representatives of actors and experts and what arguments they use to legitimise this view and the chosen governance model. In terms of theoretical anchoring, the thesis relies on the theory of legitimacy focusing on the public policy environment and the concept of institutional field defining in the field of public policy governance the relations and interactions of a higher number of actors involved in decision-making on a certain issue. The analytical part of the thesis is based mainly on the implementation of an expert investigation using the Delphi method, preceded by an expert double interview to identify the main issues to be addressed in the current system of science and research policy in the Czech Republic. From the conducted research, it is possible to identify the main advantages and disadvantages of the Czech system of management of this policy, the diversity of opinions on the position of some actors (the Government Council for Research,...
|
|
|
30 years later - analysis of metaphors of development and transformation of Czech society in the period between 1989-2019
Barták, Tomáš ; Císař, Ondřej (advisor) ; Škvrňák, Michael (referee)
This bachelor's thesis deals with the analysis of metaphors constructed by important actors of the Velvet Revolution and the subsequent period of political and social transformation between 1989-2019 from interviews with two Czech Television programs (Interview ČT24 and Události, komentáře). The aim of the bachelor thesis is to describe the existing variability of discourse and arguments to the followed period from the point of view of important actors in relation to metaphors as a discursive means and to find out what significant events the speakers refer to. The bachelor thesis has an interpretive character and therefore does not test the hypothesis. The theoretical part of the thesis deals with the interpretation of the theory of social transformation and the concept of the so-called catching-up revolutions according to Jürgen Habermas and also with the interpretation of social and political divisions in a democratic society. The thesis then provides an overview of current research knowledge on the interpretation of the discourse on the events of 1989 and the transformation period and their interpretation. In the analytical part of the text, 10 categories of metaphors are explained through the analysis of metaphors and thematic analysis, which frame the observed statements of speakers into...
|
|
|
METALLURGY AND PROPERTIES OF ADVANCED NiAl-Mo EUTECTICS
Barták, Tomáš ; Kuchařová, Květa ; Záležák, Tomáš ; Dlouhý, Antonín
A NiAl-Mo eutectic alloy was melt from 99,99% purity components and cast by the drop casting technique. The drop-cast ternary alloy (nominal composition of Ni-45Al-9Mo at. %), was re-melted and directionally solidified using a high temperature optical floating zone furnace. A resulting in-situ composite consists of Ni-45,2Al matrix and Mo-10Al-4Ni fibers, all in at. %. The volume fraction of 14% Mo-fibers stems from the eutectic composition. Spacing and a diameter of Mo-fibers can be controlled within certain limits using different growth rates of the crystals. Microstructural parameters of the as-cast crystals were assessed by light microscopy, scanning and transmission electron microscopy. Backscatter diffraction shows that the NiAl-matrix and the Mo-fibers are both < 001 >-oriented with respect to the axis of the cylindrical rods. Preliminary creep experiments confirmed an immense improvement of high temperature strength due to the fine distribution of Mo-fibres. The amount of strengthening in terms of minimum creep rate can be as high as 7 orders of magnitude. Post-mortem transmission electron microscopy experiments provided evidence that creep in the temperature range of 800-900 degrees C results in an extensive formation of subgrain boundaries. The strengthening effect is very likely associated with the reactions between subgrain boundaries and fine Mo-fibres.
|
|
|
Processing of Nb-containing TiAl intermetallics and its Physical and Chemical Aspects
Barták, Tomáš ; Dlouhý, Ivo (referee) ; Zemčík, Ladislav (referee) ; Dlouhý, Antonín (advisor)
Prezentovaná práce se zabývá vakuovým indukčním tavením intermetalické slitiny Ti-46Al-7Nb (at. %) v žáruvzdorných kelímcích na bázi Y2O3. Byla provedena série taveb pro teploty přehřátí taveniny 1630, 1680 and 1730 C a při různých dobách výdrže na této teplotě v rozmezí 5 až 30 minut. Ze slitin ztuhlých v tavících kelímcích byly připraveny metalografické výbrusy, které sloužily k hodnocení mikrostruktury a vyhodnocení složení fází. Pro získání těchto dat byly použity metosy elektronové mirkoskopie SEM a EDS. Kvantitativní hodnocení mikrostruktury, zejména obsahu oxidické faze ve slitině, bylo provedeno pomocí software Adaptive Contrast Control (ACC). Analýza obsahu kyslíku ve ztuhlé slitině byla provedena metodou IGF (fúze v inertním plynu). V této práci jsou pochody na rozhraní slitina/oxidický kelímek posuzovány také z termodynamického hlediska a to s použitím aktivit jednotlivých složek v systému. Data prezentovaná v této práci mohou být použita pro nastavení a optimalizaci procesů tavení intermetalik TiAlNb.
|
|
|
INFLUENCE OF MELTING TEMPERATURE AND TIME ON THE OXYGEN CONTENT IN TiAlNb ALLOYS
Barták, Tomáš ; Zemčík, L. ; Dlouhý, Antonín
The present study investigates a cost-effective melting of TiAlNb intermetallics in a stable yttrium oxide refractory. We identify mechanisms that mainly contribute to the oxygen pick-up during melting. Systematic microstructural and chemical analysis provided evidence that erosion and dissolution of crucible wall are key factors of the process. Experiments were performed using two Ti46Al7Nb alloys with a different initial content of oxygen. The alloys were re-melted in vacuum induction furnace inside U-shaped yttria crucibles. The experiments involved three different temperatures in combination with melting times spanning from 300 to 1800 s. The oxygen content of re-melted and solidified alloys was assessed by means of inert gas fusion technique. Results of the chemical analysis provided clear evidence that the stability of the refractory wall in contact with the molten alloy and its rate of dissolution depend mainly on melting temperature. These results are further supported by microstructural observations that confirmed ever growing content of ceramic phases in the solidified intermetallic with increasing temperature of the melt. The work also contributed to the improved experimental methodology, in particular to temperature measurements of reactive melts.
|
|
|
METALLURGY AND PROPERTIES OF ADVANCED NiAl-Mo EUTECTICS
Barták, Tomáš ; Kuchařová, Květa ; Záležák, Tomáš ; Dlouhý, Antonín
A NiAl-Mo eutectic alloy was melt from 99,99% purity components and cast by the drop casting technique. The drop-cast ternary alloy (nominal composition of Ni-45Al-9Mo at. %), was re-melted and directionally solidified using a high temperature optical floating zone furnace. A resulting in-situ composite consists of Ni-45,2Al matrix and Mo-10Al-4Ni fibers, all in at. %. The volume fraction of 14% Mo-fibers stems from the eutectic composition. Backscatter diffraction shows that the NiAl-matrix and the Mo-fibers are both < 001 >-oriented with respect to the axis of the cylindrical rods. Preliminary creep experiments confirmed an immense improvement of high temperature strength due to the fine distribution of Mo-fibres. The amount of strengthening in terms of minimum creep rate can be as high as 7 orders of magnitude. Post-mortem transmission electron microscopy experiments provided evidence that creep in the temperature range of 800 - 900 degrees C results in an extensive formation of subgrain boundaries. The strengthening effect is very likely associated with the reactions between subgrain boundaries and fine Mo-fibres.
|
|
|
Impact of Heat Treatment Environment of Microstructure and Transformation Path in NiTi Shape Memory Alloy
Kuběnová, Monika ; Zálešák, Jakub ; Čermák, Jiří ; Barták, Tomáš ; Dlouhý, Antonín
We report results of differential scanning calorimetry (DSC) experiments in which heat flow (T) from and to Ti-50.9at%Ni shape memory samples was recorded during the temperature scan through a B2 B19’ transformation range. Prior to the DSC experiments, the samples were separately annealed in evacuated quartz capsules containing different hydrogen and helium mixtures with an overall filling pressure of 900 mbar. The quartz tubes containing the annealed samples were subsequently quenched into cold water. After quenching, the capsules were opened, martensitic transformations were investigated by DSC and the microstructure of the samples was assessed by transmission electron microscopy (TEM). Annealing in the mixtures with an increasing partial pressure of hydrogen led to a considerable drop in the latent heat associated with the B2 B19’ martensitic transformation. Results obtained using TEM suggest that hydrogen may function as a catalytic substance that accelerates the long range ordering of Ni atoms in early stages of Ni4Ti3-phase precipitation. The selected area diffraction study focused on patterns in <100>B2 and <110>B2 zones and provided evidence for diffuse scattering due to spatial modulations of the lattice constant. These variations in the background electron intensities might be related to a precursor of Ni4Ti3 phase in its early state of formation.
|
|
|
Creep in NiAl-Mo Eutectics
Dudová, Marie ; Barták, Tomáš ; Kuchařová, Květa ; Dlouhý, Antonín
We report on creep in NiAl-Mo ternary eutectic and NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%). These alloys were directionally solidified in a high-temperature optical floating zone furnace. The eutectic alloy exhibited a well-aligned rod-like microstructure, consisting of NiAl matrix and 14% (by volume) continuous Mo-fibres oriented along a [001]-crystal growth direction. Cylindrical [001]-oriented specimens were loaded in compression at temperatures in a range 1073 – 1173 K. Formidable strengthening by regularly distributed fine fibres (typical diameter, 400 nm) was observed which resulted in minimum creep rates of the NiAl-Mo eutectic that were seven orders of magnitude lower than the corresponding minimum creep rates of the NiAl matrix. Preliminary microstructural investigations suggested that the strengthening effect is due to an interaction between Mo fibres and subgrain boundaries that form in the course of creep. This interaction leads to an increase of dislocation density in the vicinity of fibres and to an efficient redistribution of stresses in the microstructure.
|
|
|
Interakce mezi Ti-Al-Nb taveninou a Y2O3 kelímky
Barták, Tomáš ; Dlouhý, Antonín ; Dočekalová, Kateřina ; Zemčík, L.
The present paper focuses on interactions between intermetallic melts (Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni alloy (at%)) and ceramic crucibles based on the Y2O3 refractory. Different levels of superheat and exposure time are taken into account. Vacuum induction melting (VIM) experiments were performed using the vacuum furnace BALZERS VSG 02. The controlled argon atmosphere inside the furnace volume was kept at the pressure level of 0.133 bar during the melting experiments. The reactions at the melt - crucible interface were studied by means of scanning electron microscopy (SEM) equipped with the energy dispersive x-ray analyzer (EDX). There has been clear benefit when a common refractory like alumina has been replaced by yttria. However, the results obtained in this study show, that the long - term exposure at melting temperatures above 1650°C lead to notable reactions at the interface. We discuss the stability of the melt - ceramic system using principles of thermodynamic equilibrium.
|
guest ::
RSS feed.