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Strukturní parametry martenzitu v Ti slitinách, ovlivněné přítomností atomů kyslíku
Šalata, Kristián ; Preisler, Dalibor (vedoucí práce) ; Lukáč, František (oponent)
Štruktúrne parametre martenzitu v titánových zliatinách modifikované prítomnosťou atómov kyslíka Táto štúdia pojednáva o vplyve β-stabilizačných prvkov a kyslíka na martenzitickú štruktúru v metastabilných β zliatinách titánu. Úvod popisuje kryštalografickú povahu metastabilnej martenzitickej fáze a jej vzťah ku kubickej priestorovo centrovanej a hexagonálnej tesne usporiadanej štruktúre. Štúdia navyše prezentuje výsledky merania transmisnej rentgenovej difrakcie, vďaka ktorej boli určené dve zložky podieľajúce sa na tvorbe martenzitickej fáze - šmyk a periodický posun atómových rovín (shuffle). Analýza prvej zložky, šmyku, bola uskutočnená kvantitatívne pomocou mriežkových parametrov. Druhá zložka, shuffle, bola analyzovaná stanovením kryštálovej štruktúry martenzitickej fáze na základe atómových pozícií. Na konci tejto práce je demonštrované meranie pomocou transmisnej elektrónovej mikroskopie s vysokým rozlíšením s následným spracovaním obrazu, ktoré priamo potvrdilo prítomnosť nanodomén O' fáze.
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Composite coatings containing nanometric constituents prepared by plasma spraying with liquid feedstocks
Mušálek, Radek ; Tesař, Tomáš ; Medřický, Jan ; Čížek, Jan ; Lukáč, František
Plasma spraying of liquid feedstocks allows deposition of materials with novel microstructures. Due to the combination of a liquid carrier and ultrafine powders (in case of suspensions) or in-situ formation of the final material in the plasma jet (in case of solutions), preparation of ultrafine composites is possible, typically in a form of coatings on the substrates. Moreover, both suspension and solution spraying routes can be easily combined, either together or even with conventional plasma spraying of coarse dry powders, thus merging benefits of “nanometric” and “micrometric” constituents in the final composite microstructure. As a consequence, liquid feedstock plasma spraying represents an industrially-relevant deposition process with wide variability, providing coatings with properties tailored for various applications ranging from compact wear-resistant layers, porous thermal barrier coatings, bio-compatible medical coatings, catalyst carriers, fuel-cells, etc. The paper introduces several examples of such ultrafine coatings recently deposited at IPP CAS using hybrid water-stabilized plasma torch.
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High temperature corrosion behaviour of nicr-based coatings applied by twin wire arc spray technology
Lencová, K. ; Vostřák, M. ; Naďová, J. ; Lukáč, František ; Mušálek, Radek
High temperature corrosion is a serious problem related to the combustion of heavy oils with high vanadium contents. Protective overlay coatings are used to allow functioning of engineering components under extreme conditions and provide corrosion resistance to extend the component life. This article is concerned with the high temperature corrosion behaviour of two protective NiCr-based coatings and bare structural steel 1.4959 (W.nr.) in a molten salt environment of 40% Na2SO4 and 60% V2O5 at 750 °C under cyclic condition. NiCr and NiCrMoNbTa coating was deposited on steel 1.4903 (W.nr.) by Twin wire arc spray technology (TWAS). To establish the kinetics of corrosion, the thermogravimetric technique was used. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the corrosion products and determine the corrosion mechanism. The hot corrosion resistance of both coatings was better than bare steel samples. The NiCrMoNbTa coating showed better corrosion behaviour than the NiCr coating. However, also the NiCr coating provides sufficient protection to the substrate material.
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Microstructure and microhardness evaluation for NiCrAlY materials manufactured by spark plasma sintering and plasma spraying
Hulka, Iosif ; Mušálek, Radek ; Lukáč, František ; Klečka, Jakub ; Chráska, Tomáš
NiCrAlY deposited by different thermal spraying methods is commonly used as the bond coat material in thermal barrier coatings (TBCs). In the present study, two experimental coatings were deposited by hybrid water stabilized plasma (WSP-H) and radio frequency inductively coupled plasma (RF-ICP) using the same feedstock powder. Spark plasma sintering (SPS) was used to manufacture a compact NiCrAlY from the same feedstock powder as a reference material. Microstructure, internal oxidation, phase characterization and quantification of the mechanical behaviour in terms of microhardness were studied. The investigations clearly showed microstructural and mechanical differences between the NiCrAlY samples manufactured by different plasma technologies. The results confirmed that SPS and RF-ICP provide dense structures with no oxides due to the fabrication under protective atmosphere and similar mechanical properties. Thus, RF-ICP may be used for deposition of very dense coatings with microstructure and hardness comparable to compacted materials prepared by SPS.
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High temperature corrosion studies of HVOF sprayed coatings in molten sulphate salts environment
Lencová, K. ; Česánek, Z. ; Houdková, Š. ; Lukáč, František ; Mušálek, Radek
The generally applied requirements on lowering of emission production in the field of power industry have led, among others, into the application of low-emission combustion process of coal fired power plants. Besides the emissions reduction, this process has led to an increase of boilers parts corrosion degradation due to the reducing atmosphere in low-emission boilers, responsible for sulphide corrosion and molten salt attack. In such environment, the protection of the exposed surfaces gains importance. Among others, the thermal spraying technology can offer a suitable way of surface corrosion protection. In the present work, the high temperature corrosion behaviour of CoCrAlYTaCSi and Cr3C2-25%NiCr High Pressure/High Velocity Oxygen Fuel (HP/HVOF) sprayed coatings is evaluated and compared to the behaviour of bare structural steel (SPT360) substrate. The specimens were exposed to a molten salt environment, i. e., 12% Na2SO4 + 82% Fe2(SO4)3 at 690 °C under cyclic condition, simulating the working environment in low-emission boilers of coal-fired power plants. The thermogravimetric method was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the corrosion products and determine the corrosion mechanism. The thermogravimetric measurement shows high oxidation rate of SPT360 steel. Cr3C2-25%NiCr coating provides slightly better performance compared to CoCrAlYTaCSi coating. Based on the obtained results, both coatings were found to have higher corrosion resistance than the SPT360 steel substrate, offering a sufficient surface protection in a given environment.
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Precipitační procesy ve slitině Mg-Nd
Drábek, Stanislav ; Čížek, Jakub (vedoucí práce) ; Lukáč, František (oponent)
Cílem této bakalářské práce bylo zjistit, zda dochází k precipitačním procesům v systému Mg-Nd s 3% hmotnostním zastoupením atomů příměsi Nd a proměřit, jaká je kinetika tohoto procesu a jaký má vliv na následné vytvrzení slitiny při zvýšených tep- lotách. Vývoj mechanických vlastností slitiny byl sledován Vickersovou metodou měření tvrdosti. Současně byla provedena charakterizace poruch krystalické mřížky vzorků zaka- lených slitin pomocí pozitronové anihilační spektroskopie. Bylo změřeno, že při zvýšených teplotách dochází ve zkoumaném systému k výraznému vytvrzování způsobenému tvor- bou precipitátů GP zón a β ′ fáze atomy Nd a Mg. 1
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Investigation of defects in Fe3Al - based alloys by positron annihilation spectroscopy
Lukáč, František
V zliatinách železa a hliníku bola študovaná korelácia koncentrácie vakancií s mikrotvrdosťou vzorkov zakalených z 1000 řC a následne žíhaných na nižších teplotách. Pomocou difrakcie röntgenovho žiarenia bola zistená mriežková konštanta a kryštalografická niektorých vzorkov Fe-Al zliatin. Meraním doby života pozitronov bola zistená vysoká koncentrácia vakancií v zakalených vzorkoch a následné žíhanie vzorkov spôsobilo značný pokles koncentrácie vakancií, pričom vo vzorkoch s obsahom hliníka nad 39% bol nameraný pokles mikrotvrdosti. Koincidenčné meranie Dopplerovského rozšírenia anihilačného píku vzorku pomohlo rozlíšiť anihilácie pochádzajúce od zachyteného a delokalizovaného pozitronu anihilovaného elektrónmi od oboch typov atómov, Fe a Al. Porovnaním týchto výsledkov s teoretickými kvantovo-mechanickými výpočtami prevedenými v tejto dilpomovej práci bol určený najpravdepodobnejší typ defektu ako vakancia v A-podmriežke B2 štruktúry.
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Effect of sintering temperature on microstructure and mechanical properties of AE42 magnesium alloy prepared by spark plasma sintering
Minárik, P. ; Lukáč, František ; Cinert, Jakub ; Šašek, S. ; Král, R.
Magnesium alloy AE42 was prepared by powder metallurgy technique of spark plasma sintering. The effect of sintering parameters, particularly sintering temperature, on the microstructure and mechanical strength was investigated. The gas-atomized powder was sintered at four temperatures in the temperature range of 400-550 °C. It was found that mechanical strength of the sintered samples was significantly affected by several microstructural features. Application of relatively high load during sintering caused deformation of the individual particles and consequent recrystallization depending on the processing temperature resulted in the release of internal strain and in grain growth. As a result, the evolution of the mechanical strength as a function of the sintering temperature was significantly affected by residual stress, grain size and coarsening of secondary phase particles.
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Characterization of flame sprayed abradable seal NiCrAl-bentonite coatings
Houdková, Š. ; Česánek, Z. ; Schubert, J. ; Lukáč, František ; Bystrianský, M.
The NiCrAl-21% Bentonite abradable coating, deposited by flame spraying, was exposed to 720°C / 1 hour air-annealing to evaluate its resistance against heat-induced changes. The SEM and XRD microstructure evaluation was done. After the air-annealing, the increase of HR15Y surface hardness was recorded and accompanied with decrease of abradability evaluated by Progressive Readability Hardness (PAH) test, although no demonstrable microstructural changes was recorded by SEM. XRD analyses evidenced the formation of nickel oxides and ordered Ni3Al phase during the air-annealing. Besides the abradable coating evaluation, this work brings the verification of the potential of PAH for scratch hardness testing. The measurement provides the results with low scatter, compared to usual surface hardness testing, and enables to distinguish between the depth of indentation in loaded and unloaded state, pointing out to the elastic/plastic deformation ratio.
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The investigation of an Al-Zr-Ti alloy prepared by spark plasma sintering of atomized powder
Molnárová, O. ; Málek, P. ; Lukáč, František ; Chráska, Tomáš ; Cinert, Jakub
The microstructure and mechanical properties of a powder metallurgical Al-Zr-Ti alloy was studied. Fine powder with a typical size below 50 ?m was prepared by gas atomization. The smallest powder particles with a diameter below 10 ?m exhibited a segregation free microstructure. Larger droplets were found to contain intermetallic particles rich in Zr and Ti. The gas atomized powder was consolidated by spark plasma sintering (SPS) at various temperatures ranging from 450 to 550 °C. During SPS the materials microstructure remained nearly unchanged and a fine grain size between 2 and 3 ?m was observed. All SPS samples exhibited a microhardness of around 90 HV. The annealing (1 h, 500 °C) of the sample sintered at the highest temperature (550 °C) resulted in a decrease of microhardness to 75 HV as a result of changes in the phase composition, the fine grain size was retained. Natural aging at room temperature was not observed.
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