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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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Tungsten-steel and tungsten-chromium composites prepared by RF plasma spraying
Matějíček, Jiří ; Klečka, Jakub ; Čížek, Jan ; Veverka, Jakub ; Vilémová, Monika ; Chráska, Tomáš ; Ganesh, V.
For future fusion reactors, materials able to withstand harsh environments are needed. In particular, this concerns the plasma-facing components which are foreseen to consist of tungsten based plasma-facing armor and structural and cooling part made of steel. Currently, joining of these materials presents a significant challenge. The stress concentration at their interface, which arises due to thermal exposure and the difference in thermal and mechanical properties, can be reduced by composite/graded interlayers. Plasma spraying is among the prospective technologies for their preparation. In this work, tungsten-steel and tungsten-chromium composites were prepared by radio-frequency inductively coupled plasma (RF-ICP) spraying. Initial optimization of the spraying process for pure materials as well as their mixtures was carried out. Basic characterization of the layers for their structure, porosity and composition is presented.
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Metastable alloy Ti-15Mo prepared by powder metallurgy
Veverková, Anna ; Bartha, Kristina (vedoucí práce) ; Chráska, Tomáš (oponent)
Práce se zabývá charakterizací metastabilní beta slitiny Ti-15Mo připravené kryogenním mletím a následným sintrováním elektrickým proudem. Výchozí prášek byl připraven plynovou atomizací a následně deformován kryogenním mletím (mletý prášek). Výchozí prášek a mletý prášek byly kompaktizovány metodou sintrování elektrickým proudem při teplotách v intervalu 750 řC až 850 řC. Během kryogenního mletí práškové částice výrazně změnily tvar z kulatých na placaté. Velikost částic se nezmenšila, ale částice byly intenzivně plasticky deformovány. Pomocí skenovacího elektronového mikroskopu bylo zjištěno, že všechny připravené vzorky obsahují dvojfázovou alfa + beta mikrostrukturu. Objemový podíl alfa fáze je výrazně větší po sintrování mletého prášku kvůli zvýšené teplotě beta přechodu způsobené kontaminací mletého prášku kyslíkem, ale také kvůli snazší precipitaci fáze alfa díky zjemněné mikrostruktuře. Maximální dosažená mikrotvrdost je asi 350 HV pro oba sintrované prášky. Vysoká mirkotvrdost výchozího prášku může být vysvětlena vznikem omega fáze, zatímco mletý prášek je vytvrzen jemnou mikrostrukturou a malými precipitáty fáze alfa. Kryogenní mletí umožňuje získat materiál s vysokou mikrotvrdostí při nižších teplotách sintrování.
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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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The effect of spark plasma sintering on the porosity and mechanical properties of Ti-15Mo alloy
Terynková, A. ; Kozlík, J. ; Bartha, K. ; Chráska, Tomáš ; Dlabáček, Zdeněk ; Stráský, J.
Metastable β-titanium alloys are receiving much interest for various applications such as aircraft industry and medicine thanks to their excellent mechanical properties and biocompatibility. The common way of preparing the titanium alloys is hindered by its production costs. Powder metallurgy (PM) approach is a promising route for cost-effective fabrication of titanium alloys due to possibility of near net shaping. In this study, binary biomedical Ti-15Mo alloy was prepared by PM. Gas atomized powder was sintered by spark plasma sintering (SPS) above the β-transus temperature of the studied alloy. The compaction of the powders was accomplished by short-time sintering. The effect of the time of sintering on the porosity and the microhardness in centre part as well as in periphery part of the sample was investigated. The samples revealed significant inhomogeneity the porosity increases with the distance from the centre of the specimen. With increasing sintering times the porosity decreases and simultaneously the microhardness increases.
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Microstructure of commercially pure titanium after cryogenic milling and spark Plasma sintering
Kozlík, J. ; Harcuba, P. ; Stráský, J. ; Chráska, Tomáš ; Janeček, M.
Commercially pure titanium was prepared by advanced powder metallurgy methods with the aim to produce the ultra-fine grained material. Cryogenic attritor milling was used as a first step to refine the microstructure at liquid argon temperatures to suppress recovery and dynamic recrystallization. Spark plasma sintering was subsequently employed to produce bulk material, exploiting its ability to achieve fully dense structure in short time and thus to reduce the grain growth. In order to understand the undergoing microstructural changes during the process, detailed investigation was performed after each preparation step. Powder morphology was changed significantly after milling, while particle fragmentation was only limited. Grain size after sintering was in micrometer scale, relatively independent of sintering conditions.
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Microstructure and Texture of Titanium Prepared by Powder Metallurgy
Kozlík, Jiří ; Stráský, Josef (vedoucí práce) ; Chráska, Tomáš (oponent)
Komerčně čistý titan byl připraven jako objemový materiál pomocí práškové metalurgie, konkrétně pomocí kryogenního mletí a sintrování elektrickým prou- dem, s cílem připravit ultrajemnozrnný materiál se zvýšenou pevností. Mikro- struktura prášků po mletí byla detailně zkoumána pomocí transmisního EBSD - nové metody, která umožnila první přímé pozorování textury uvnitř částic. Tato textura je podobná textuře po válcování díky podobnosti deformačních mecha- nismů. V prášku byla pozorována zrna s velikostí pod 100 nm. Vliv parametrů sintrování na vlastnosti materiálu byl studován pomocí skeno- vací elektronové mikroskopie včetně EBSD, rentgenové difrakce a pomocí měření mikrotvrdosti. Byl odhalen kompromisní vztah mezi porozitou a velikostí zrna - zcela neporézní materiál s ultrajemnozrnnou strukturou není možné připravit. Zvýšený obsah kyslíku je hlavním faktorem přispívajícím ke zpevnění materiálu, zatímco porozita má značně nepříznivý vliv na mechanické vlastnosti. Textura prášku byla za- chována i v sintrovaném materiálu. Možnost stabilizace ultrajemnozrnné struktury mechanickým legováním Ti s oxi- dem ytritým byla studována se smíšenými výsledky. Stabilizace byla úspěšná,...
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SPARK PLASMA SINTERING OF BALL MILLED AND ATOMIZED POWDER BASED ON Fe-Al
Šíma, V. ; Minárik, P. ; Chráska, Tomáš
High-quality compacts were prepared using the spark plasma sintering (SPS) method from powders of similar composition Fe-Al-Zr-B. The properties of the sintered compacts are strongly dependent on the morphology and properties of the feedstock powder. The first powder was obtained by ball milling of the as cast alloy and the second was prepared by atomization under argon atmosphere. The morphology and structure of the two powders are compared and mechanical properties and microstructure of compacts prepared under the same conditions of the SPS procedure are discussed. The milled powder has an irregular morphology and shape of the polycrystalline particles, which have a completely disordered BCC structure with considerable internal stresses, high concentration of structural defects and the presence of aluminum oxide. The atomized powder particles are nearly spherical, polycrystalline with ordered B2 structure, with no significant signs of internal stresses and oxidation. Microhardness of particles of both powders was measured and compared with the microhardness of compact materials, the results of compression tests of compacts at room temperature were compared and discussed.
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THE INVESTIGATION OF AN Al7075 ALLOY PREPARED BY SPARK PLASMA SINTERING OF MILLED POWDERS
Molnárová, O. ; Málek, P. ; Nemeth, G. ; Kozlík, J. ; Lukáč, František ; Chráska, Tomáš ; Cinert, Jakub
Atomized powder of an Al7075 alloy was high energy ball milled at room and cryogenic temperatures and compacted by spark plasma sintering (SPS) method. The influence of processing parameters on phase composition and microstructure was studied by X-ray diffraction, light and scanning electron microscopy. The mechanical properties were characterized by microhardness measurements. The atomized powder contained a large volume fraction of intermetallic phases located predominantly in continuous layers separating cells or dendrites in the interior of individual powder particles. Consolidation by SPS destroyed partially this morphology and replaced it by individual particles located at boundaries of original powder particles, at cell boundaries or arranged in chains in previous dendritic regions. High energy milling destroyed most intermetallic particles and enriched the matrix by solute atoms. The high deformation energy introduced into the powder during milling enhanced microhardness up to 220 HV. Consolidation of milled powders by SPS led to the formation of very fine-grained structure with the grain size even below 1 μm and with the fraction of high-angle boundaries about 0.9. Two main types of heterogeneously distributed precipitates were found. The irregularly shaped precipitates with a size about 1 μm seemed to encompass areas with rod like nano-precipitates in most samples. A drop in microhardness to 118HV was observed after SPS, predominantly due to a release of introduced deformation energy.
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Influence of isothermal exposure on microstructural changes resulting in delamination of eutectic Al2O3+ZrO2+SiO2 thermal barrier coatings
Jech, D. ; Čelko, L. ; Chráska, Tomáš ; Slámečka, K. ; Klakurková, L. ; Dubský, Jiří ; Švejcar, J.
Eutectic ceramic Al2O3-ZrO2-SiO2 also termed Eucor (as a top coat) and CoNiCrAlY (as a bond coat) coatings were sprayed onto the surface of recently developed fine-grained cast polycrystalline nickel-based superalloy Inconel 713 LC by means of water stabilized plasma (WSP) and atmospheric plasma spraying (APS) techniques, respectively.Specimens were subjected to isothermal oxidation at 1050 °C for 10, 50, 100, 200 and 500 hours in ambient atmosphere,prior to which a half of the as-sprayed specimens was annealed at the temperature of 950 °C for 10 hours.Influence of short-time and long-time isothermal exposure on interactions at the bond-coat/top-coat interface was studied.The uniform and continuous oxide layer,also termed the thermally grown oxide (TGO),was formed and grew at the bond-coat/top-coat interface in all samples, both with and without the heat pre-treatment. Relationship between the dwell time, heat treatment and TGO growth kinetics was quantified.Microstructural changes
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