O desenvolvimento de novos materiais com baixa densidade e propriedades mecânicas estáveis em altas temperaturas é necessário para reduzir o consumo de combustível e consequentemente a emissão de gases no setor aeroespacial. Uma nova classe de materiais chamada HEAs (Ligas de Alta Entropia), que combinam elementos refratários e alumínio podem ser candidatas para superar esse desafio. Ligas de Alta Entropia contendo Al-Zr-Nb-Ti-V estão sendo estudadas em nosso grupo de pesquisa. Os diagramas de fases são uma ferramenta necessária para o desenvolvimento e otimização dessas ligas. O objetivo do presente trabalho é a investigação experimental do sistema ternário Al-V-Zr a 1200°C. Ligas do sistema foram fundidas em um forno a arco com cadinho de cobre refrigerado a água e eletrodo não consumível de tungstênio sob atmosfera de argônio. Pedaços das amostras foram embrulhados em folhas de Zr e tratadas a 1200°C por 10 dias usando tubos de sílica em vácuo primário para alcançar o equilíbrio termodinâmico. Para a observação das microestruturas, as amostras foram preparadas pelo método metalográfico padrão. A composição e microestrutura das amostras foram analisadas por microscopia eletrônica de varredura (MEV) e espectroscopia por energia dispersiva (EDS). A caracterização microestrutural das amostras foi complementada por difratometria de raios X (DRX) utilizando pó e radiação de Cu-K?. No trabalho publicado por Guzei (1993) foi proposto a existência de duas fases ternárias com estequiometria Zr0,9V0,4Al2,7 e Zr13V2Al5. Entretanto neste trabalho, apenas a fase ternária Zr0,9V0,4Al2,7 foi observada. Em contrapartida, observou-se a estabilidade uma outra fase ternária com estequiometria aproximada (Zr,Al)2V e protótipo Ti2Ni. Uma nova seção isotérmica a 1200°C foi proposta baseada no equilíbrio termodinâmico determinado pelas medições das composições das fases. / The development of new materials with low density and stable mechanical properties at high temperature is necessary to reduce fuel consumption and consequently the emission of gases. A new class of material called HEA combining refractory elements and aluminum can be good candidate to overcome this challenge. High entropy alloys in the Al-Zr-Nb-Ti-V system are being investigated in our research group. The phase diagram data are a necessary tool for the design and optimization of the alloys. The objective of these study is an experimental research of the Al-V-Zr ternary system at 1200°C. Several alloys were melted in an arc furnace using non-consumable tungsten electrode in a water cooled copper crucible, under an inert atmosphere of argonium. Parts of the samples were treated at 1200 °C for 10 days using silica tubes sealed under primary vacuum in order to achieve the thermodynamic equilibrium. For the observation of microstructures, the specimens were prepared following conventional metallographic methods. The compositions and microstructures of the alloys were investigated by scanning electron microscopy (SEM) and electronic microanalysis (EDS). The microstructural characterization was complemented by X-ray diffractrometry (XRD) on powder using Cu-k? radiation. In the work published by Guzei (1993) the existence of two ternary phases with the stoichiometry Zr0,9V0,4Al2,7 and Zr13V2Al5 is indicated. However, in this work only the ternary phase Zr0,9V0,4Al2,7 was observed. In addition, another ternary phase with approximate stoichiometry (Zr,Al)2V and prototype Ti2Ni was observed. A new isothermal section at 1200°C is proposed based on the thermodynamic equilibria determined to measured compositions of the phases.

O desenvolvimento de novos materiais com baixa densidade e propriedades mecânicas estáveis em altas temperaturas é necessário para reduzir o consumo de combustível e consequentemente a emissão de gases no setor aeroespacial. Uma nova classe de materiais chamada HEAs (Ligas de Alta Entropia), que combinam elementos refratários e alumínio podem ser candidatas para superar esse desafio. Ligas de Alta Entropia contendo Al-Zr-Nb-Ti-V estão sendo estudadas em nosso grupo de pesquisa. Os diagramas de fases são uma ferramenta necessária para o desenvolvimento e otimização dessas ligas. O objetivo do presente trabalho é a investigação experimental do sistema ternário Al-V-Zr a 1200°C. Ligas do sistema foram fundidas em um forno a arco com cadinho de cobre refrigerado a água e eletrodo não consumível de tungstênio sob atmosfera de argônio. Pedaços das amostras foram embrulhados em folhas de Zr e tratadas a 1200°C por 10 dias usando tubos de sílica em vácuo primário para alcançar o equilíbrio termodinâmico. Para a observação das microestruturas, as amostras foram preparadas pelo método metalográfico padrão. A composição e microestrutura das amostras foram analisadas por microscopia eletrônica de varredura (MEV) e espectroscopia por energia dispersiva (EDS). A caracterização microestrutural das amostras foi complementada por difratometria de raios X (DRX) utilizando pó e radiação de Cu-K?. No trabalho publicado por Guzei (1993) foi proposto a existência de duas fases ternárias com estequiometria Zr0,9V0,4Al2,7 e Zr13V2Al5. Entretanto neste trabalho, apenas a fase ternária Zr0,9V0,4Al2,7 foi observada. Em contrapartida, observou-se a estabilidade uma outra fase ternária com estequiometria aproximada (Zr,Al)2V e protótipo Ti2Ni. Uma nova seção isotérmica a 1200°C foi proposta baseada no equilíbrio termodinâmico determinado pelas medições das composições das fases. / The development of new materials with low density and stable mechanical properties at high temperature is necessary to reduce fuel consumption and consequently the emission of gases. A new class of material called HEA combining refractory elements and aluminum can be good candidate to overcome this challenge. High entropy alloys in the Al-Zr-Nb-Ti-V system are being investigated in our research group. The phase diagram data are a necessary tool for the design and optimization of the alloys. The objective of these study is an experimental research of the Al-V-Zr ternary system at 1200°C. Several alloys were melted in an arc furnace using non-consumable tungsten electrode in a water cooled copper crucible, under an inert atmosphere of argonium. Parts of the samples were treated at 1200 °C for 10 days using silica tubes sealed under primary vacuum in order to achieve the thermodynamic equilibrium. For the observation of microstructures, the specimens were prepared following conventional metallographic methods. The compositions and microstructures of the alloys were investigated by scanning electron microscopy (SEM) and electronic microanalysis (EDS). The microstructural characterization was complemented by X-ray diffractrometry (XRD) on powder using Cu-k? radiation. In the work published by Guzei (1993) the existence of two ternary phases with the stoichiometry Zr0,9V0,4Al2,7 and Zr13V2Al5 is indicated. However, in this work only the ternary phase Zr0,9V0,4Al2,7 was observed. In addition, another ternary phase with approximate stoichiometry (Zr,Al)2V and prototype Ti2Ni was observed. A new isothermal section at 1200°C is proposed based on the thermodynamic equilibria determined to measured compositions of the phases.

Les “alliages à haute entropie (de mélange)” (AHE) sont une nouvelle famille de matériaux prometteurs. Ils sont caractérisés par la formation d'une solution solide à 5 éléments (en proportions équiatomiques) de structure cristalline simple. Dans cette thèse, la composition cubique centrée TiZrNbHfTa est étudiée, proposant une caractérisation en profondeur d’un alliage considéré « de référence » dans la famille des AHE réfractaires.Tout d'abord, la microstructure et la structure de l’alliage (dans son état brut de coulée ou recristallisé) sont étudiées. L’environnement local de sous-systèmes de TiZrNbHfTa est analysé par EXAFS. Le traitement des données est effectué par une double approche d’affinement EXAFS et de simulation Reverse Monte-Carlo couplée à une approche d’algorithme génétique. Un mélange quasi-parfait des différents éléments est obtenu à l’échelle locale et la distribution de distance des premiers voisins devient moins bien définie sous l’effet de l’augmentation des différences entre rayons atomiques.Ensuite, l’impact de la solution solide concentrée sur les propriétés mécaniques et les mécanismes de déformation de l’alliage est étudié. Des essais mécaniques spécifiques sont effectués, conduisant à l’obtention des volumes d’activation et à la partition de la contrainte d’écoulement. Une étude MET complémentaire permet d'analyser les microstructures de déformation. Une très haute limite d’élasticité est obtenue, mais la force de friction de Peierls contrôle de manière classique la déformation de cet alliage à la température ambiante, ce qui conduit à un taux d’écrouissage limité. Une nouvelle approche visant à augmenter cette propriété est finalement proposée / High entropy alloys (HEA) are a new promising type of materials. Breaking with the traditional alloying concepts, solid solution(s) based on 5 elements in equiatomic concentration with simple crystal structures are obtained. In this study, the equiatomic composition TiZrNbHfTa is investigated, in order to provide an in-depth characterization of a “reference” body centered cubic refractory HEA.First, the microstructure and structure of the alloy are studied. Thermomechanical treatments procedures are established to access recrystallized microstructures. The local environment is studied by EXAFS in sub-components of the TiZrNbHfTa system. The double approach used, based on EXAFS fit and reverse Monte-Carlo coupled with evolutionary algorithm allowed to quantify both the mixing of the elements at the atomic scale and the lattice distortion. For all the investigated compositions, good mixing is achieved, and the distance distribution of first nearest neighbors becomes less precise with increasing atomic size mismatch.Then, the impact of such concentrated multi-element solid solution on the mechanical properties and the deformation mechanism of the material is investigated by specific tests. The activation volumes and the flow stress partition are extracted. The mechanical results are coupled with a TEM study. This part evidences that the alloy displays an impressive yield strength. However, the high lattice friction controlling the dislocation glide does not differ from classical bcc structures, leading to a rather low work hardening. A new design approach aiming at increasing the work-hardening in such materials is finally proposed, and a proof of concept is given

High entropy alloys (HEAs) is a concept wherein alloys are constructed with five or more elements mixed in equal proportions; these are also known as multi-principle elements (MPEs) or complex concentrated alloys (CCAs). This PhD thesis dissertation presents research conducted to develop precipitation-hardenable high entropy alloys using a much-studied fcc-based equi-atomic quaternary alloy (CoCrFeNi). Minor additions of aluminium make the alloy amenable for precipitating ordered intermetallic phases in an fcc matrix. Aluminum also affects grain growth kinetics and Hall-Petch hardenability. The use of a combinatorial approach for assessing composition-microstructure-property relationships in high entropy alloys, or more broadly in complex concentrated alloys; using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys as a candidate system. The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ~25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. With increasing Al content, there was a gradual change from an fcc-based microstructure (including the ordered L12 phase) to a bcc-based microstructure (including the ordered B2 phase), accompanied with a progressive increase in microhardness. Based on this combinatorial assessment, two promising fcc-based precipitation strengthened systems have been identified; Al0.3CuCrFeNi2 and Al0.3CoCrFeNi, and both compositions were subsequently thermo-mechanically processed via conventional techniques. The phase stability and mechanical properties of these alloys have been investigated and will be presented. Additionally, the activation energy for grain growth as a function of Al content in these complex alloys has also been investigated. Change in fcc grain growth kinetic was studied as a function of aluminum; the apparent activation energy for grain growth increases by about three times going from Al0.1CoCrFeNi (3% Al (at%)) to Al0.3CoCrFeNi. (7% Al (at%)). Furthermore, Al addition leads to the precipitation of highly refined ordered L12 (γ′) and B2 precipitates in Al0.3CoCrFeNi. A detailed investigation of precipitation of the ordered phases in Al0.3CoCrFeNi and their thermal stability is done using atom probe tomography (APT), transmission electron microscopy (TEM) and Synchrotron X-ray in situ and ex situ analyses. The alloy strengthened via grain boundary strengthening following the Hall-Petch relationship offers a large increment of strength with small variation in grain size. Tensile strength of the Al0.3CoFeNi is increased by 50% on precipitation fine-scale γ′ precipitates. Furthermore, precipitation of bcc based ordered phase B2 in Al0.3CoCrFeNi can further strengthen the alloy. Fine-tuning the microstructure by thermo-mechanical treatments achieved a wide range of mechanical properties in the same alloy. The Al0.3CoCrFeNi HEA exhibited ultimate tensile strength (UTS) of ~250 MPa and ductility of ~65%; a UTS of ~1100 MPa and ductility of ~30%; and a UTS of 1850 MPa and a ductility of 5% after various thermo-mechanical treatments. Grain sizes, precipitates type and size scales manipulated in the alloy result in different strength ductility combinations. Henceforth, the alloy presents a fertile ground for development by grain boundary strengthening and precipitation strengthening, and offers very high activation energy of grain growth aptly suitable for high-temperature applications.

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Master‘s thesis deals with the preparation and evaluation single-phase high-entropy alloy based on cobalt, chromium, iron, nickel and manganese and its variants strengthened by dispersion of oxidic particles. High-entropy alloy was prepared in powder form by mechanical alloying from the equiatomic proportions of atomic powders. Obtained powder was subsequently compacted by spark plasma sintering. By the method of mechanical alloying were successfully prepared single-phase high-entropy alloy and its variant strengthened by dispersion of nanometric yttria oxides. It has been found that the oxide particles present in the microstructure of high-entropy alloy significantly block mobility of grain boundary and dislocation at elevated temperatures. As a result of this behavior were observed doubling of alloy strength and decreasing of creep rate at 800 °C.

Multi-component coatings and high entropy alloys have in recent years attracted great interest for research, since they have shown to exhibit properties greater than the com- ponents of their parts. Today’s climate challenges requires transitioning from fossil fuels to renewable energy sources which demands use of new technology and new innovations. The hydrogen fuel cell is a technology which produces no carbon emissions, and the drive for innovation has led researchers to apply multi-component (high entropy alloys) coatings to invent the next generation hydrogen fuel cells and help the transition to renewable energy sources. This thesis has investigated the process-structure-property relationships of four deposi- tion growth parameters: target current (Itarget), argon pressure (PAr). substrate bias (Vsubstrate) and deposition time (tdeposition) on TiNbZrTa-coatings, grown by magnetron sputtering using an industrial deposition system. The range of the parameters have been: Itarget from 2.5 to 6 A, PAr from 1 to 17 mTorr, Vsubstrate from 30 to 200 V and tdeposition from 3.6 to 12 minutes (depending on Itarget). Coatings have been grown on Si (001) and stainless steel 304 and 316L substrates. The coating microstructure was analyzed by X-ray diffraction and electron microscopy. The results have yielded that all coatings are equimolar and that the coatings exhibit three different morphologies, two different topologies and two different corresponding structures. The different morphologies are wave, coarse columnar and fine columnar morphology. The two topologies are nodular and dune surface topology. The two different structures are a solid solution BCC (110) phase and an amorphous or nanocrystalline phase. The results indicate that parameters affecting the temperature of the substrate (Tsubstrate) is the prime decider for the final morphology of the coatings. High Itarget and Vsubstrate, low PAr and long tdeposition all increases Tsubstrate and results in a coating which exhibits a fine columnar morphology, dune topology and a solid solution BCC phase. These types of coatings have also proven to have improved corrosion resistance compared to the other type of coatings seen in this thesis. The other kind of coating is grown with low Itarget and Vsubstrate, high PAr and short tdeposition, which causes minimal increase of Tsubstrate. These growth parameters result in a coating with coarse columnar morphology, nodular topology and amorphous or nanocrystalline phase, with less corrosion resistance. / FunMat II

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21767号 / 工博第4584号 / 新制||工||1714(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 乾 晴行, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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