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Struktura a vlastnosti tepelných bariér typu YSZ nanesených na krycí vrstvy CoNiCrAlY přetavené elektronovým paprskem
Slavíková, Barbora ; Jan, Vít (oponent) ; Dlouhý, Ivo (vedoucí práce)
Diplomová práce se zabývá charakterizací struktury a vlastností tepelných bariér YSZ nanesených pomocí technologie hybridního plazmatického systému na vazebné povlaky CoNiCrAlY modifikované pomocí elektronového paprsku a vakuového žíhání. Depozice vazebných povlaků byla provedena za použití technologie vysokorychlostního nástřiku plamenem a studené kinetické depozice. V rámci experimentálního vyhodnocení byla analyzována mikrostruktura a chemické složení krycích keramických nástřiků nanesených ve formě prášku a suspenze. Stejně tak byly vyhodnoceny i vazebné povlaky ve stavu po přetavení za použití dvou parametrů elektronového paprsku. Dále byly sledovány změny v mikrostruktuře a chemickém složení přetavených nástřiků po žíhání. Následně byly vyhodnoceny mikromechanické vlastnosti keramických a vazebných povlaků. Keramické povlaky nanesené v prášku vykazovaly strukturu tvořenou splaty, zatímco povlaky nanesené ve formě suspenze měly jemnou strukturu tvořenou kolumnárními zrny. Struktura přetavených vazebných nástřiků byla tvořena dendrity. Žíhání pak mělo za následek zhrubnutí jednotlivých fází a došlo ke změnám v chemickém složení způsobené difúzí prvků.
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NEW APPROACH TO STRESS-STRAIN CURVE PREDICTION USING BALL INDENTATION TEST
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
This work is concerned with the method for predicting stress-strain behavior of material using instrumented indentation technique. High strength low alloy steel with different thermal treatment was taken into the analysis. Heat treatment for the steel was performed to obtain different mechanical properties. Assessment of mechanical properties was done by using inverse technique of the finite element analysis. The results were confronted with conventional test parameters and prediction procedure defined such Automated Ball Indentation Technique (ABIT). Comparison of the material curves shows good agreement with tensile test properties which makes this non-destructive method suitable for industrial application.
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Prediction of the Traction Separation Law of Ceramics Using Iterative Finite Element Modelling
Kozák, Vladislav ; Chlup, Zdeněk ; Padělek, P. ; Dlouhý, Ivo
Specific silicon nitride ceramics, the influence of the grain size and orientation on the bridging mechanisms was found. In ceramic matrix composites, crack-bridging mechanisms can provide substantial toughness enhancement coupled with the same and/or increased strength. The prediction of the crack propagation through interface elements based on the fracture mechanics approach and cohesive zone model is investigated. From a number of damage concepts the cohesive models seem to be especially attractive for the practical applications. Within the standard finite element package Abaqus a new finite element has been developed; it is written via the UEL (user’s element) procedure. Its shape can be modified according to the experimental data for the set of ceramics and composites. The element seems to be very stable from the numerical point a view. The shape of the traction separation law for four experimental materials is estimated via the iterative procedure based on the FEM modeling and experimentally determined displacement in indentation experiments, J-R curve is predicted and stability of the bridging law is tested.
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Subsized Specimens for Fracture Resistance Characterisation Including Transferability Issues
Dlouhý, Ivo ; Stratil, Luděk ; Šiška, Filip
The contribution is focused on characterization of methods enabling to apply small/subsized specimens for fracture resistance characterization. The applied methods are divided into transition region and upper shelf region. The approaches used in the upper shelf region represent at the same time methods applicable for ductile materials without transition. Relating to subsized samples two basic approaches are applicable: (i) miniaturized samples based on common standard ones and (ii) specific specimens/methods, e.g. small punch test etc. The results described in the paper belong to the first group. For interpretation of data generated under low constraint conditions toughness scaling models and master curve approached are commented. In ductile region, either the sample used generate valid toughness characteristics, or, if not, there is no way how to correct measured data except damage quantification through micromechanical models.
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Fracture Toughness of Massively Transformed and Subsequently Heat Treated TiAl Intermetallic Compound
Sakurai, K. ; Hasegawa, M. ; Dlouhý, Ivo
The effects of massive transformation and subsequent heat treatments on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studied. Massive transformation occurs at the center region of the specimen by cooling from alpha single phase state. At the surface side of the specimen, alpha phase has remained. Fine convoluted microstructure with alpha 2, gamma phases and lamellar structure has formed by heating at (alpha + gamma) two phase state after massive transformation. Colony size or grain size is about 25 micrometer. Fine fully lamellar structure is obtained after heat treatment of convoluted microstructure at alpha phase for 60 s. Fracture toughness seems to be increasing with the increase in lamellar colony size. However, some massively transformed specimens show lower toughness due to the formation of microdamage present in samples before the test.
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Crack Resistance Characterization in TiAl Intermetallics with Enhanced Toughness
Dlouhý, Ivo ; Stratil, Luděk ; Fukutomi, H. ; Hasegawa, M.
The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable alpha phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of alpha+gamma phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.
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Intermetalické sloučeniny syntetizované in-situ v práškových materiálech
Hanusová, Petra ; Novák, Pavel (oponent) ; Skotnicová, Kateřina (oponent) ; Dlouhý, Ivo (vedoucí práce)
Mechanické zpracování materiálů v tuhém stavu je jedním z nejpoužívanějších a nejrozšířenějších postupů vůbec. Stejně tak jako chemické zpracování tuhých látek je široce rozšířené. Proto spojení těchto dvou způsobů do jednoho se jeví jako logické řešení. Tato metoda se pak nazývá mechanochemické zpracování materiálů. Zpracování materiálů tímto způsobem se vyznačuje mnoha výhodami. Jednak je toto zpracování ekonomicky výhodné, stejně jako technologicky snadno proveditelné. Touto cestou lze připravit dokonce i materiály, které by za normálních podmínek spolu nereagovaly. Mechanochemie/mechanochemická syntéza využívá mechanickou energii k aktivaci chemických reakcí a strukturním změnám. Zejména aluminotermické reakce indukované ve vysoko energetickém planetovém mlýnu nabírají na významu, jako potenciální cesta k přípravě mikro a nano krystalických in – situ kompozitních materiálů s kovovou matricí. Mechanickou aktivací ve vysoko energetickém mlýnu dochází ke změnám reakčních mechanismů z vzniku metastabilních materiálů. V práci byly studovány změny mechanismů reakcí během mechanického legování na čtyřech různých systémech. Jednalo se o systémy se společným základem, kde byl měněn jeden prvek: Al – B2O3 – X (X= C, Ti, Nb, Cr). Dále bylo studováno, zda po mechanickém legování bude docházet v systému k dalším změnám vyvolaným použitím zvýšené teploty a tlaku, tedy pomocí techniky spark plasma sintering (SPS). Všechny systémy byly mechanicky legovány za analogických podmínek. Po provedení legování byla u každého systému provedena analýza pomocí skenovací elektronové mikroskopie, dále byla provedena kvalitativní a kvantitativní analýza pomocí rentgenové difrakce. Pomocí nanoindentace byla vyhodnocena indentační tvrdost a indentační modul pružnosti. Všechny analýzy byly provedeny po mechanickém legování, stejně jako po provedení SPS a výsledky byly mezi sebou vzájemně porovnány. Na základě výsledků byla u sledovaných systémů navržena změna reakčních mechanismů. Bylo zjištěno, že dochází k tvorbě kompozitních materiálů s kovovou matricí a v případě použití chromu došlo k vytvoření hybridního kompozitního materiálu vyztuženého intermetalickou fází a boritanem hlinitým.
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COATING OF CR-V LEDEBURITIC TOOL STEEL WITH CrAgN
Jurči, P. ; Krum, S. ; Dlouhý, Ivo
The Vanadis 6 tool steel has been coated with CrN, doped with small silver addition, by reactive magnetron sputtering. Some of coated specimens were subsequently vacuum annealed at 500 degrees C. The microstructure of film has been investigated on fracture surfaces of samples. Adhesion was evaluated by scratch-test. Wear resistance was measured using a pin-on-disc method, against alumina and 100Cr6 balls, at ambient and elevated temperatures. Experimental results have shown that the CrN-layers with an addition of 3%Ag have grown in a columnar manner. The addition of small amount of silver makes the adhesion on the substrate better. At an ambient temperature, no positive effect of the Ag-addition to the wear characteristics was found. But, in the testing temperature range 400 - 500 degrees C, significant lowering of friction coefficient and improvement of wear resistance were recorded. This can be attributed to the transport of Ag towards the surface, and thereby reduced friction force.
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Metal Matrix Composites Prepared by Powder Metallurgy Route
Moravčík, Igor ; Lapin, Juraj (oponent) ; Skotnicová, Kateřina (oponent) ; Dlouhý, Ivo (vedoucí práce)
Conventionally, the alloy design, alloy production, and alloy selection are almost strictly confined to single element or one compound concept. Consequently, this alloy concept imposes a significant limit to the degrees of freedom in alloy’s composition and thus limits the development of special microstructure and properties. In the last decade, it has become particularly obvious that materials science and alloy engineering are still not fully explored due to an appearance of new class of alloys – usually called high entropy alloys (HEA). This exclusively new class of alloys caught significant scientific attention for the novelty of its approach to alloy design, as they do not contain a single base element, but rather at least 5 elements in very close atomic portions. In the recent years medium entropy alloys (MEA) appeared as a variant of HEAs with only three or four elements. The work is contributed to the research of feasibility of production of HEA and MEA alloys and composites by utilization of powder metallurgy (PM) manufacturing route, the combination of mechanical milling (MA) of elementary powders, followed by pressure or field assisted densification. Altogether three compositions have been studied: AlCoCrFeNiTi0.5, Co1.5Ni1.5CrFeTi0.5 and CoCrNi, as well as B4C metal matrix composite (MMC) with CoCrNi as matrix phase. Deep microstructural and mechanical analyses including transmission electron microscopy and tensile testing have been performed. During the whole study, the problems with the contamination of powders with oxygen have been observed, however the oxides formed relatively homogenous dispersion in all manufactured materials and they did not impair significant mechanical property reduction. AlCoCrFeNiTi0.5 exhibited relatively high hardness over 800 HV, but rather low ductility. The attempt has been made to improve the ductility with heat treatment procedure, but to no avail. The formation of in-situ TiC dispersion has been recorded, due to the utilization of carbon containing methanol as a process control agent during milling, that reacted with the present elemental Ti. In this manner metal matrix composite has been effectively produced. Additionally, the same procedure, the milling in the controlled amount of carbon containing medium, may be used also to produce other advanced composites with dispersion of in-situ formed TiC. On the other hand, CoCrNi alloy possessed very high tensile ductility (26%) and ultimate strength over 1000 MPa. Microstructure was composed of major FCC phase and BCC precipitates. The CoCrNi alloy has been due to the high ductility chosen as the best candidate for the subsequent production of metal matrix composites. The introduction of B4C resulted in the displacement reaction of Cr element with B4C, resulting in the formation of Cr5B3 boride phase. The composite possessed nano-grained microstructure and high tensile strength over 1400 MPa. However, the tensile ductility decreased to 1.9%. The AlCoCrFeNiTi0.5 alloy achieved the best combination of tensile ductility (4%) and remarkable strength over 1300 MPa, bearing pure FCC microstructure with extremely fine grain size. Therefore, the PM production route has proven to be a feasible way for the production of HEAs and MEAs, as well as HEA and MEA based metal matrix composites with remarkable combination of mechanical properties.
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