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High-temperature embrittlement of 14%Cr ODS ferritic steel in liquid lead environment
Dohnalová, Eva ; Čelko, Ladislav (referee) ; Hadraba, Hynek (advisor)
The thesis deals with the high-temperature embrittlement of 14%Cr ODS ferritic steel in liquid lead environment. The 14%Cr ferritic ODS steel ODM401 manufactured by powder metallurgy technology was used as an experimental material. The effect of the long-term annealing, surface corrosion attack in the melt Pb and Pb-Bi on the microstructure and mechanical properties was described. The subsequent microstructural changes were evaluated by means of scanning electron microscopy and transmission electron microscopy The mechanical behaviour of the experimental material was proven by means of microhardness test, impact test and static tensile test. The fractographical evaluation of fracture surfaces were performed on all samples. The surface attack and outstanding microstructural ganges were found after the long-time exposition of the steel in Pb/Pb-Bi melts at temperature 550°C/1000h and 500°C/1000h respectively. The embrittlement of the steel after exposition in Pb/Pb-Bi melts was comparable to the embrittlement after high-temperature annealing of the steel at temperature 650°C/1000h.
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Determination of optimum sintering schedule of oxide ceramics
Průdek, Miloš ; Hadraba, Hynek (referee) ; Maca, Karel (advisor)
The sintering of cubic ZrO2 (stabilized with 8mol% Y2O3) by Single-Step sintering was studied in this bacherol‘s thesis. Two types of commercial powders with different particle size of 80nm and 140nm were used. It was studied whether the grain size of the sample sintered for shorter time at higher density is higher or smaller than after sintering for longer time at lower temperature. It was found taht for cubic zirconia the sintering for a longer time at lower temperature is advantageous from the point of view of decreasing the grain size fo cintered ceramics.
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Core-Shell Ceramic Structures Prepared by Thermoplastic Co-Extrusion Method
Kaštyl, Jaroslav ; Pabst, Willi (referee) ; Hadraba, Hynek (referee) ; Trunec, Martin (advisor)
In the doctoral thesis, the bi-layer ceramic bodies with core-shell geometry were prepared by thermoplastic co-extrusion method and for these composite bodies the mechanical properties were studied. For study of co-extrusion and mechanical properties were designed two composite systems. First system ZTA-A combined the dense core ZTA (zirconia-toughened alumina) and the dense shell Al2O3. Second system ZST-Z consisted of porous core and dense shell made from ZrO2 for both cases. In the thesis, the rheology of ceramic thermoplastic suspensions and their mutual influence during co-extrusion was studied. Subsequently, the debinding process and sintering were studied, and based on the optimization of all process steps were obtained defect-free bodies with core-shell geometry. The mechanical properties (elastic modulus, hardness and bending strength) were determined for sintered bodies. To estimate the stress path in the core shell bodies loaded in bending, the relationship considering different elastic moduli of the core and the shell was used. For bodies of ZTA-A system was increased the strength in comparison with monolithic bodies of the individual components. Thus, bodies with high surface hardness of shell from Al2O3 and moreover having high fracture strength in bending were obtained. The effective elastic modulus was decreased for bodies of ZST-Z system up to 25 % in comparison with the elastic modulus of dense monolithic samples. The same effective modulus of elasticity was possible to achieve with core-shell bodies while maintaining significantly higher fracture strength than monolithic porous bodies or pipes.
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Mechanical alloying and compactization of metallic composite powders
Husák, Roman ; Čelko, Ladislav (referee) ; Hadraba, Hynek (advisor)
Master´s thesis is focus on the proces of mechanical alloying. It is the proces of modifying a hetegeneous mixture of powder materials into a homogeneous composite powder. Experiments are focus on three types of composite materials. A magnetic soft alloy Permalloy, ODS steel based on commercially available powder steel 434 LHC and low-activation high-chrome ODS steel 14Cr-2W. On composite powders are made a series of mechanical tests and chemicel analysis. Based on this tests and analysis it was possible to confirm the milling time needed to create fully homogeneous composite powder. Next step is compaction of composite powder into compact volume and another mechanical tests ana analysis of microstructure. In these analyzes to determine whether i tis necessary to use protective atmosphere during mechanical alloying. All three type of materials succesfull prepared by mechanical alloying. It was found that for created of a fully homogeneous composite powder is necessary to perform mechanical alloying for 24 hours. When processing of corrosion resistant materials, i tis possible to perform mechanical alloying in an air atmosphere. During mechanical alloying materials which are subject to oxidation, i tis necessary to use protective atmosphere.
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Influence of parameters of electrophoretic deposition on properties of ceramics
Husák, Roman ; Čelko, Ladislav (referee) ; Hadraba, Hynek (advisor)
Electrophoretic deposition is experimentally undemanding shaping method enabling preparation of ceramic material from stable suspension of ceramic particles by means of direct electric current. The aim of the work was to describe effect of electric current magnitude on velocity and final microstructural and mechanical properties of the ceramics. The alumina and zirconia layers were prepared by electrophoretic deposition from stable suspensions of ceramic particles in the isopropanol stabilised with monochloracetic acid. It was found that the real time dependance of particle deposition differs from the theoretical predisction for given electrical conditions. By precise measurement of kinetics of the electrophoretic deposition the actual electrophoretic mobility of the particles and the actual amount of particles taking part in the deposition process were found. It was found that with increasing the electrical current the actual electrophoretic mobility was decreased and actual amount of particles taking part in the deposition process was increased. The increasing velocity of particles under higher electrical currents led to the detorioration of particle arrangement in the elctrode and thus to the increasing of the pore sizes and final densities of the deposits. These microstructural changes reflected in the lowering of the hardness of the deposited ceramics of about 300HV5 in the case of alumina.
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High entropy alloys fabricated via SPS compaction of high energy milled feedstock powders
Gubán, Ivan ; Hadraba, Hynek (referee) ; Čížek, Jan (advisor)
The subject of this thesis is preparation of CoCrFeMnNiNx high entropy mixtures via the methods of mechanical alloying and spark plasma sintering (SPS). Three series of specimens were fabricated in this thesis: samples milled in argon (benchmark materials), samples milled in nitrogen atmosphere (to observe their ability of nitrogen absorption) and samples microalloyed with CrN, FeN nitrides (to observe their dissociation into the solid solution potential). The fabricated powders and SPS compacts were subsequently observed by electron microscopy and their phase content by X-Ray diffraction (XRD) and elemental composition by EDS analysis were carried out. A method of reduction melting in inert atmosphere was used to determine the exact oxygen and nitrogen content in powders, while the respective particle size distribution measured by laser diffraction method. The influence of nitrogen content on the hardness of the samples was studied via the microhardness measured. After completing the process of mechanical alloying under the Nitrogen atmosphere was the maximal concentration of nitrogen in the structure 0,208% after 24 hours of milling (dependency on time was linear), which means, the method of milling under the Nitrogen atmosphere was successful. XRD of milled samples showed the existence of the only FCC single solid solution phase, while samples milled under the Nitrogen atmosphere showed the trend of the growth of the lattice parameter with the increasing nitrogen content. There was observed the presence of the chromium nitrides precipitates on the grain boundaries of the FCC phase in microalloyed samples. All specimen were contaminated by a mixture of metallic oxides and manganeese sulphides, which were present in the default manganeese powder. The greatest value of microhardness showed the duplex sample. The increase in values of microhardness (344 HV 0,3) in comparison with the standard sample (262,9 HV 0,3) was recorded on the samples milled under the nitrogen atmosphere, which conforms the positive influence of the nitrogen content on strength characteristics of this alloy.
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Metal Matrix Composites Prepared by Powder Metallurgy Route
Moravčíková de Almeida Gouvea, Larissa ; Novák, Pavel (referee) ; Hadraba, Hynek (referee) ; Dlouhý, Ivo (advisor)
Vývoj nových materiálů pro součásti v moderních technologiích vystavené extrémním podmínkám má v současné době rostoucí význam. Děje se tak v důsledku neustále se zvyšujících požadavků průmyslových odvětví na lepší konstrukční vlastnosti nosných materiálů. Ve světle těchto faktů si tato studie klade za cíl posoudit nové složení slitin s vysokou entropií, které se vyznačují vysokým aplikačním potenciálem pro kritické aplikace. Slitiny jsou připravovány práškovou metalurgií, t.j. kombinací mechanického legování a slinování v pevné fázi. Pro účely srovnávaní vlastností jsou vybrané kompozice vyrobeny také tradičními metalurgickými metodami v roztaveném stavu, jako je vakuové indukční tavení a následné lití nebo vakuové obloukové tavení. Prášková metalurgie umožňuje postupný vývoj kompozitů s kovovou matricí (MMC) prostřednictvím přípravy oxidicky zpevněných HEA slitin. To je možné díky inherentním in-situ reakcím během procesu výroby. Když se naopak zvolí výrobní postup z taveniny, připravený kovový materiál vykazuje velké rozdíly v mikrostrukturách a souvisejících vlastnostech, v porovnání se stejným materiálem vyrobeným práškovou cestou (PM). Vyrobené práškové a tavené materiály jsou detailně charakterizovány s ohledem na komplexní vyhodnocení vlivu různých metod zpracování. Práce se zejména orientuje na mikrostrukturní charakteristiky materiálů a jejich mechanické vlastnosti, včetně vlivu tepelného zpracování na fázové transformaci a mikrostrukturní stabilitu připravených materiálů.
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Glass-ceramic composite biomaterials
Žaludek, Jakub ; Hadraba, Hynek (referee) ; Drdlík, Daniel (advisor)
The bachelor thesis is focused on the preparation of bioglass 45S5 and composites containing bioglass 45S5 and alumina or zirconia fibres in amount of 1, 3 and 5 wt% by slip-casting method from aqueous and 2-propanolic suspensions. The materials prepared were characterised in term their porosity, relative density, high temperature dilatometry, X-ray diffraction analysis, microstructure and hardness. The slip-casting method produced porous samples which is promising result for a possible biomedical application. The high temperature dilatometry identified three stages of the sintering process and a shrinkage of ~40% of the bioglass-based materials was measured. The crystalline phase of Na2Ca2Si3O9 was created at the sintering temperature of 1050°C but the dissolution of Al2O3 or ZrO2 fibres into the bioglass matrix was not confirmed using the X-ray diffraction analysis. It was found that the hardness of the samples decreased with the amount of the fibres in the materials which was caused by their increasing porosity and large clusters of fibres.
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New compositions of advanced oxide dispersion steels based on rare earth elements
Pech, Filip ; Kuběna, Ivo (referee) ; Hadraba, Hynek (advisor)
The main objective of present diploma thesis is to prepare three different classes of steels, differing by their content of chromium: 9Cr, 14Cr, 17Cr steels and their oxide dispersion strengthened variants. Steels were prepared from atomic and pre-alloyed powders by the mechanical alloying and compacted by the spark plasma sintering method. Used strengthening elements were yttrium, which is most commonly used, and aluminium. Preparation of oxide dispersion was done in two ways: direct adding of yttria and alumina and inner oxidation of aluminium and yttrium. In the experimental part has been found, that it is possible to make oxide dispersion by both ways, but aluminium strengthened steel has to be prepared by inner oxidation to ensure fine oxide dispersion.
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Manufacturing of calcium phosphates and silica based scaffolds for bioapllications
Virágová, Eliška ; Hadraba, Hynek (referee) ; Částková, Klára (advisor)
The aim of this diploma thesis was to prepare porous bioceramic scaffolds based on calcium phosphates and calcium phosphates doped with silica. Scaffolds are intended to be used in bone tissue engineering. Two main preparation methods were used for the creation of scaffolds – replica method and direct foaming method. Theoretical part of the diploma thesis is focused on a general description of the skeletal system, biomaterials and methods of preparation of highly porous calcium phosphate ceramics. Experimental part contains a description and the results of prepared scaffolds by above mentioned methods. The preparation process by the direct foaming method was optimized to obtain a defined structure. Calcium phosphate scaffolds containing 0–20 wt.% SiO2 were sintered and studied in terms of material characteristics (phase composition, pore size and porosity, microstructural study by scanning electron microscopy (SEM)), bioactive properties (simulated body fluid (SBF) interaction tests and tests of simulated degradation) and mechanical properties in order to evaluate the effect of silica doping. Scaffolds prepared by both methods were composed of a mixture of hydroxyapatite and/or tricalcium phosphate and cristobalite and wollastonite with comparable porosity in the range of 80–88 %. The pore size of the scaffolds prepared by the direct foaming method reached the interval of 5–250 µm opposite to template method reached the pore size up to 430 µm. The SBF interaction tests and tests of the simulated degradation confirmed the bioactive behavior of the prepared scaffolds and their ability to degrade under the simulated conditions. The scaffolds prepared by the direct foaming method showed better mechanical properties (compressive strength up to 1,8 MPa) than the scaffolds prepared by the template method. The results showed that the prepared scaffolds are suitable and promising for potential applications in bone tissue engineering.
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