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Powder Metallurgy of Hybrid Materials for Advanced Applications
Kozlík, Jiří ; Stráský, Josef (advisor) ; Kim, Hyoung Seop (referee) ; Novák, Pavel (referee)
Powder Metallurgy of Hybrid Materials for Advanced Applications The field-assisted sintering technology (FAST) is a versatile powder consolidation method, which uses electrical current for rapid heating of the sintered material. It is routinely used for sintering of various metallic materials, but mostly with preal- loyed powders as a feedstock. The aim of the thesis is to explore the capabilities of FAST for sintering of composite materials and for using blended elemental powders as a raw material in following areas: 1. Manufacturing of architectured composites 2. Consolidation and ageing in a single processing run (in-situ ageing) 3. Rapid alloy prototyping using blended elemental powders 4. High-throughput experimental investigations of multicomponent phase dia- grams Al6061 + Ti-6Al-4V architectured composite was produced, but issues with the pow- der penetration into the lattice have arisen due to the small lattice dimensions. Me- chanical properties could have been assessed by a three-point bending only. The composite has shown improved strength over the plain, unannealed Al6061 matrix, but a premature fracture when the Al matrix was aged properly. Using FAST was found to be inconvenient for architectured composite manufacturing. A long, fully dense rod was manufactured from the Ti-5Al-5V-5Mo-3Cr...
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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 (advisor) ; Chráska, Tomáš (referee)
Bulk commercially pure titanium was prepared by powder metallurgy, namely by cryogenic milling and spark plasma sintering, with aim to produce ultra-fine grained material with enhanced strength. The microstructure of milled powders was investigated in detail by a novel method called transmission EBSD, which allowed the first direct observation of texture within the powder particles. This texture is similar to rolling texture, because of the similar nature of the defor- mation during milling. Microstructure observations revealed grains with the size under 100 nm. The influence of sintering parameters on material properties were studied by scan- ning electron microscopy including EBSD, X-ray diffraction and by microhardness measurements. The trade-off relationship between porosity and grain size was identified, fully dense material with ultra-fine grained microstructure could not be produced. Increased oxygen content was identified as a main strengthening factor, while porosity has significant deteriorating effect on mechanical properties. The texture of powder was retained in the bulk material. The possibility of stabilizing the microstructure by mechanical alloying of Ti with yttrium oxide nanoparticles was investigated with mixed results. The stabiliza- tion was successful, but several issues...
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Titanium and titanium alloys prepared by cryogenic milling
Kozlík, Jiří ; Stráský, Josef (advisor) ; Janeček, Miloš (referee)
Ultra-fine grained materials are presently thoroughly investigated due to their enhanced mechanical properties. Cryogenic milling is one of the severe plastic deformation methods, which allow production of these materials. Titanium powder was processed by cryogenic milling in liquid nitrogen and argon and consequently consolidated via spark plasma sintering method. In this work, the influence of milling conditions (liquid nitrogen vs. liquid argon, material of balls, duration and speed of milling, usage of stearic acid) on size and shape of powder particles, contamination and mechanical properties was investigated. Particle size reduction was generally not observed, while their morphology changed significantly. Using liquid nitrogen as a cooling medium leads to strong contamination of prepared material and consequently to its hardening and embrittlement. Stearic acid supresses cold welding of particles during milling and enhances its efficiency. It is possible to eliminate stearic acid from powder by cleaning in acetone before sintering, to prevent contamination of processed material. Microhardness increased, depending on milling efficiency (in liquid argon), from original 178 HV to 200-300 HV range. Increase of yield and ultimate stength was observed in compression tests while maintaining ductility....
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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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