No description available.

No description available.

The friction stir processing is applied in mixing elemental thin sheets of Mg, Al, and Zn in various portions to result in hard intermetallic alloys with Vicker¡¦s hardness in excess of 350. The Mg3Al2Zn3

Cette thèse est consacrée à l'étude de composés riches en magnésium innovants destinés au stockage solide de l'hydrogène. Le but est de déstabiliser l'hydrure de Mg et d'accélérer sa cinétique de sorption par des effets d'alliage et de nano-structuration. La première famille de composés concerne les phases pseudo-binaires Mg6Pd1-xMTx (MT = Ni, Ag, Cu). Leurs propriétés structurales et les effets de substitution du Pd ont été étudiés par diffraction des rayons X, microscopie électronique à balayage et microsonde de Castaing. Les propriétés thermodynamiques et cinétiques d'hydrogénation de ces matériaux ont ensuite été déterminées par réaction solide-gaz. Différents mécanismes d'hydrogénation sont mis en jeu en fonction de l'élément de substitution. La nature des phases formées lors de la réaction d'hydrogénation modifie la stabilité des systèmes métal-hydrogène. Ainsi, la transformation de métal à hydrure est caractérisée par au moins deux plateaux de pression. Le premier plateau a lieu à une pression proche de celle de Mg/MgH2, alors que le second se produit à pression plus élevée. La détermination des valeurs d'enthalpie et d'entropie de réaction ont permis de quantifier la déstabilisation atteinte. Les meilleures cinétiques de désorption sont obtenues pour l'alliage au Ni, grâce à l'effet catalytique de la phase Mg2NiH4 formée lors de l'hydrogénation. La seconde approche vise à combiner les effets d'alliage et de nano-structuration. Des nanoparticules de Mg6Pd atteignant des tailles aussi petites que 3 nm sont confinées dans des matrices carbonées nano-poreuses. En comparant leurs propriétés d'hydrogénation à celles de l'alliage massif équivalent, on démontre non seulement que la cinétique de (dés)hydrogénation des nanoparticules est bien plus rapide, mais aussi que leur état hydrogéné est déstabilisé. Enfin, des nano-composites MgH2-TiH2 ont été synthétisés par broyage mécanique sous atmosphère réactive. L'ajout d'un catalyseur (TiH2) et la nano-structuration du Mg permettent de considérablement accélérer les cinétiques d'absorption et désorption d'hydrogène dans le Mg. Afin de comprendre le rôle de la phase TiH2 sur les propriétés cinétiques remarquables de ces nano-composites, leurs propriétés structurales ont été déterminées par diffraction des rayons X et des neutrons. L'existence d'une interface cohérente entre les phases Mg et TiH2 est d'importance majeure pour faciliter la mobilité de H au sein du nano-composite. De plus, il est démontré que les inclusions de TiH2 freinent la croissance de grain de Mg/MgH2, permettant ainsi de maintenir la nano-structuration des composés lors de leur cyclage / This thesis is dedicated to the study of novel magnesium-rich compounds for solid state hydrogen storage. The aim is to destabilize Mg hydride and accelerate its sorption kinetics by alloying and nanostructuration. The first family of compounds concerns the Mg6Pd1-xTMx (TM = Ni, Ag, Cu) pseudo-binary phases. Their structural properties and the effects of Pd substitution have been studied by X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Their thermodynamics and kinetics of hydrogenation have been determined by solid-gas reaction. Different hydrogenation mechanisms take place depending on the substituting element. The stability of the metal-hydrogen system is altered by the nature of the phases formed during hydrogenation reaction. Thus, metal to hydride transformation is characterized by at least two absorption plateau pressures. The pressure of the first plateau is similar to that of Mg/MgH2 while the second one occurs at higher pressure. The enthalpy and entropy of reaction are determined to quantify the destabilizing effect of Pd by TM substitution. Best desorption kinetics are found for the Ni containing alloy thanks to the catalytic effect of the Mg2NiH4 phase formed on hydrogenation. The second approach aims to combine alloying with nanostructuration effects. Nanoparticles of Mg6Pd as small as 3 nm are confined into nanoporous carbon matrix. By comparing their hydrogenation properties with those of the bulk alloy, we demonstrate that not only the (de)hydrogenation kinetics are much faster for the nanoparticles, but also that their hydrided state is destabilized. Finally, MgH2-TiH2 nanocomposites were synthesized by mechanical milling under reactive atmosphere. The addition of a catalyst (TiH2) and Mg nanostructuration allow strongly accelerating the sorption kinetics of hydrogen in Mg. To understand the role of the TiH2 phase on the outstanding kinetics of these nanocomposites, their structural properties have been determined by X-ray and neutron diffraction. The existence of a coherent interface between Mg and TiH2 phases is of major importance to facilitate H-mobility within the nanocomposite. Furthermore, it is shown that the TiH2 inclusions inhibit the Mg/MgH2 grain growth, thus maintaining the composites nanostructure during their cycling

In this study, firstly, in order to achieve fine grains in solid solution strengthened AZ31 magnesium alloy by friction stir processing (FSP), various efforts have been made. It has found that with a newly designed cooling system, the microstructure of commercial AZ31 alloy can be refined dramatically by carefully controlling the FSP parameters. It is of scientific interest that nanometer grains have been observed in the resultant microstructure for the AZ alloy experienced by two-pass FSP. Besides, in order to modify the microstructure and mechanical properties, FSP is also applied to incorporate AZ31 Mg alloy with nano-ZrO2 particles, nano-SiO2 particles and different fractions of Al and Zn elements. The microstructure and mechanical properties of the modified alloy and composite samples are investigated and compared. By one-pass FSP coupled with rapid heat sink from liquid nitrogen cooling approach, the ultrafine grain size in AZ31 Mg alloy is successfully achieved. The grain boundaries are well defined and the mean grain size can be refined to 100~300 nm from the initial 75 £gm of commercial AZ31 Mg alloys sheets. The ultrafine grained structure can drastically increases the microhardness from the initial 50 up to 120 Hv, or an increment factor of 2.4 times. Furthermore, the nanometer grains can be even achieved by two passes FSP coupled with rapid heat sink. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm. The nanocrystalline grains can be characterized by the TEM observations and the diffraction rings in SAD patterns. The highest hardness point can reach ~150 Hv which is equal to triple of the AZ31 matrix, and the mean hardness also increases up to around 134 Hv. Bulk Mg-AZ31 based composites with 10~20 vol% of nano-ZrO2 particles and 5~10 vol% of nano-SiO2 particles are also successfully fabricated by FSP. The average grain size of the resultant composites could be effectively refined to 2~4 £gm, and it demonstrates much higher hardness values compared to commercial AZ31 billet. Moreover, for the Mg/ZrO2 composite fabricated by one pass and subsequent cooling pass FSP, the recrystallized grain size could be further refined to 0.4 £gm with the hardness value of 135 Hv. As for multi-element Mg base alloys fabricated by FSP, high fractions of Al and Zn elements can result in apparent grain refinement, this can be proved by the broadening of diffraction peaks. Multi-passes FSP can induce the appearance of intermetallic compounds, however, some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the resultant alloys reachs nearly 350 in Hv scale due to the generation of intermetallic compounds and grain refinement.

No description available.

Le présent travail de thèse se propose de profiter de la description cristallographique à N dimensions pour rediscuter les structures des alliages métalliques périodiques dont les atomes sont situés sur un sous-ensemble ordonné d’un Z-module — c’est-à-dire sur des positions qui sont des combinaisons linéaires entières de N>3 vecteurs arithmétiquement indépendants— exemplifiés par les quasicristaux et les phases approximantes. La description cristallographique à N dimensions permet ainsi de révéler des symétries supplémentaires, cachées dans l’espace physique, susceptibles d’engendrer de nouveaux types de défauts, dits défauts de module, dans les alliages métalliques périodiques dont les atomes se localisent sur un Z-module non trivial dans l’espace direct. Ces défauts correspondent aux opérations de symétrie interne du Z-module qui sont perdues lors de la projection rationnelle dans l’espace physique en raison de la périodicité de l’alliage. Ce sont des macles, des défauts de translations et les dislocations qui les bordent et que nous désignons par dislocations de modules pour les différencier des usuelles dislocations de réseau. Elles apparaissent comme des dislocations partielles bordant une ou plusieurs fautes d’empilement. Des dislocations particulières peuvent exister avec des vecteurs de Burgers ayant une composante nulle dans l’espace physique. Ces dislocations, que nous appelons dislocations scalaires, n’engendrent aucun champ de contrainte et ne sont sensibles à aucun champ de déformation. La phase orthorhombique NiZr de symétrie Cmcm a une maille cristalline dont tous les atomes sont situés sur une fraction d’un même Z-module pentagonal ; elle, est à ce titre, parfaitement descriptible à 5 dimensions. A parti de cette analyse, nous avons prédit l’existence de nouveaux défauts tels que les macles quinaires et défauts de translations, tous observés et confirmés par nos observations en microscopie électronique haute résolution HREM et STEM-HAADF. / The present work is an attempt to take advantage of the N-dimensional crystallographic description to rediscuss the structures of periodic metal alloys whose atoms are located on an ordered subset of a Z-module — positions that are integer linear combinations of N> 3 arithmetically independent vectors — like those encountered in quasicrystals and approximants. This N-dimensional crystallographic description makes it possible to reveal hidden symmetries in periodic metallic alloys, that can generate new types of possible defects by symmetry breaking during the projection in the physical space. Thus, in addition to the usual defects due to a local breaking of periodicity, these alloys may contain new defects corresponding to internal symmetry operations of the Z-module which are lost due to the periodicity of the alloy. These defects are twins, translation defects and dislocations that border them and that we refer to as dislocations of modules to differentiate them from the usual network dislocations and which appear as partial dislocations bordering one or more stacking faults. Particular dislocations may exist with Burgers vectors having a null component in the physical space. These dislocations, which we call scalar dislocations, generate no stress field and are not sensitive to any deformation field. The orthorhombic phase of NiZr has a crystal structure the atoms of which are located on a fraction of the pentagonal Z-module and is therefore fully describable at 5 dimensions. We can thus describe new defects such as quinary twins, but also defects of translations that all keep the Z-module invariant. All these defects have indeed been observed and recognized by high resolution electron microscopy (HREM and STEM-HAADF).

The thesis introduces a novel alloy system based on submicron distributions of intermetallic phases realised through eutectic solidification in the ternary system Ni-Al-Zr. Various compositions in this system comprising of intermetallic phases distributed in different eutectic structures show ultra-high strength at temperatures upto 700°C combined with reasonable tensile plasticity, exceptional oxidation resistance and high temperature structural stability. Intermetallics have long been used in high temperature alloys systems such as in the classical Ni-base superalloys that derive their strength from nanoscale dispersions of the aluminide, Ni3Al(γ’) in a matrix of disordered fcc Ni (γ), alloyed with expensive, high density refractory elements such as Re and Ru. The high temperature applications of intermetallics derive from their strength retention to high temperatures, creep resistance enabled by low diffusion rates, and attractive oxidation resistance based on high concentration of elements such as Al that forms stable oxides. Several decades of effort on the development of new generation of intermetallic alloys through the 80’s and 90’s have gone unrewarded, with the exception of TiAl based alloys that are now used in recent generation aircraft engines. The promise of intermetallics as high temperature candidate materials is limited by their poor ductility or toughness arising from several intrinsic properties such as low grain boundary cohesive strength (in the case of Ni3Al) or an insufficient number of slip systems (as in NiAl) or extrinsic effects such as embrittlement by hydrogen (Fe3Al) that derive fundamentally from the existence of directionality in bonding. However, low ductility or toughness can often be alleviated by limiting the length scale for slip. We have therefore examined the possibility of combining intermetallics in the form of eutectic structures, potentially limiting slip lengths within each intermetallic constituent. Eutectic structures in binary systems limit the choice of intermetallic combinations so that finding such combinations with engineering potential is difficult. On the other hand combinations of three elements or more would enable a significantly larger set of permutations of eutectic intermetallics, provided the constituent binary phase diagrams contain either eutectic or peritectic reactions involving intermetallic phases, as well as intermediate intermetallic phases. The ternary Ni-Al-Zr system met our criterion in several ways. The Ni-Al binary phase diagram shows a peritectic reaction from liquid and NiAl (Pm 3m, B2 with a lattice parameter of 0.288nm) to form Ni3Al (Pm 3m, L12 with a lattice parameter of 0.356 nm), intermetallics that have been extensively investigated in earlier literature. The Ni-Zr system shows a peritectic reaction between liquid and the Ni7Zr2 (C12/m1 with a lattice parameters a=0.469nm, b=0.823nm, c=1.219nm) phase to form the intermetallic Ni5Zr (F 43m with a lattice parameter of 0.670nm). Further the NiAl and Ni7Zr2 are both intermediate phases and should therefore form a mono-variant eutectic on the composition line joining these two phases in the ternary system. We note that Zr participates in many glass forming systems. In the Ni-Zr system, for example, glass forming ability has been associated with the structure of the liquid phase and associated low diffusivity. As a consequence, a fine scale eutectic structure may be expected. Zr has also been reported to strengthen and ductilise Ni3Al and NiAl. Finally, both Al and Zr form stable oxides and might promote oxidation resistance. After introducing the thesis in Chapter 1, the experimental details are outlined in the Chapter 2. The experimental results and subsequent discussions are presented in three subsequent chapters. Chapter 3 reports the microstructural aspects of as cast alloys in this ternary system Previous literature and our analysis of phase equilibria in the Ni-Al-Zr system based on Thermo-Calc, suggested that solidification from the liquid to form the Ni3Al + Ni5Zr, Ni3Al + Ni7Zr2 and NiAl+ Ni7Zr2 eutectics is possible. We obtained eutectic structures involving combinations of these intermetallic phases along a constant zirconium section at 11 at. %. The alloy A (Ni-77 at.%, Zr-11at.% and rest Al) contains eutectic structures containing the Ni3Al and Ni5Zr phases in two morphologies, a planar, lamellar structure and a more irregular form. The alloys B (Ni-74 at.%, Zr-11at.% and rest Al) and C (Ni-71 at.%, Zr-11at.% and rest Al) contain two different eutectic structures that combine the Ni3Al and Ni7Zr2 phases, and the NiAl and Ni7Zr2 phases. These phases were identified by a combination of X-ray diffraction, transmission electron microscopy coupled with energy dispersive spectroscopy and electron probe microanalysis. The volume fraction of each eutectic constituent is different in the two compositions in that alloy B(Ni-74 at.%, Zr-11at.% and rest Al) contains significantly higher volume fractions of the eutectic containing the Ni3Al and Ni7Zr2 phases than the alloy C (Ni-71 at.%, Zr¬11at.% and rest Al). In order to understand effect of individual phases we have melted several other alloys (alloy D to I) bounding these eutectic alloys (7-25 at.% Al, 5-15 at.% Zr and rest Ni) that form primary solidification phases of the intermetallic structures that constitute the eutectics. Chapter 4 discusses the mechanical behaviour of the fully eutectic alloys alloys as well as alloys with a combination of primary phases along with a eutectic. Mechanical behaviour was assessed in vacuum arc melted and suction cast material. The compressive strength of eutectic and off-eutectic compositions has been evaluated as a function of temperature. Very high strength levels of around 2 GPa could be achieved accompanied by reasonable room temperature tensile plasticity in the range 3-4%. The introduction of the respective primary phases of NiAl, Ni3Al, Ni5Zr and Ni7Zr2 results in decrease of strength. We have explored the origins of strength and tensile plasticity in alloys through micro and pico indentation (hardness) measurements and an examination of slip lines and crack initiation on pre-polished surface of the tensile tested samples as well as by transmission electron microscopy. Chapter 5 explores the oxidation resistance of these alloys in isothermal tests. The oxidation resistance of alloys compares well with recently developed cast single crystal alloys. Clearly, the oxide scale is extremely adherent and no spalling occurs. Electron microprobe analysis shows the presence of a fine scale, layered oxide structures and reaction zones within the substrate. The oxidation behaviour has been characterized using TGA, XRD and EPMA. We have attempted to understand the mechanism of oxidation through analysis of rate constants and activation energy coupled with microstructural observations. Chapter 6 presents a summary of the current work and present the scope for further work.

Diversos pesquisadores vêm sugerindo o uso de glicerol e outros alcoóis como matéria-prima para produção de bens de maior valor agregado e para geração de energia elétrica, através de células a combustível. Contudo, o sucesso dessas tecnologias de conversão depende do desenvolvimento de catalisadores mais eficientes. Nesse aspecto, abordagens teóricas se apresentam como ferramentas auxiliares, capazes de fornecer informações difíceis de serem acessadas experimentalmente e que são fundamentais para o projeto de materiais mais eficientes. Nesta tese, foram investigados, via teoria do funcional da densidade (DFT), o papel de defeitos de superfície e efeitos de tensão na adsorção de alcoóis como metanol, etanol e glicerol, sobre superfícies Pt(111) contendo defeitos e ligas de Pt3Ni(111). Para melhorar a descrição dos sistemas de adsorção, foi adicionada a correção de dispersão DFT-D3 à abordagem da DFT. Através da dinâmica molecular empregando o potencial ReaxFF, foram estudados os efeitos de temperatura sobre glicerol, em diversos níveis de recobrimento de superfície, interagindo com os substratos Pt3Ni(111). Os resultados mostram que o glicerol se liga através do oxigênio dos grupos hidróxi aos sítios top de metais de transição (TM), orientando sua cadeia carbônica quase que totalmente paralela à superfície. Os cálculos de energia de adsorção indicam que o glicerol interage mais fortemente com sítios de baixa coordenação, presentes em superfícies com defeitos, o que pode ser compreendido por meio do modelo da banda d. Além disso, a presença de múltiplos sítios de baixa coordenação favorece configurações onde o glicerol se liga à superfície por dois grupos hidróxi, um central e um terminal. Entretanto, existe uma clara preferência de alcoóis se ligarem a sítios de adsorção catiônicos, indicando que a influência de interações Coulombianas é um fator preponderante no processo de adsorção de alcoóis sobre TM. Análises de densidade eletrônica dos sistemas adsorvidos sugerem que a adsorção promove perturbações na densidade eletrônica dos alcoóis, como o deslocamento de densidade eletrônica das ligações C-O e O-H para a região de interação entre a molécula e o substrato, que estão associadas ao estiramento/enfraquecimento das ligações C-O e O-H observados através de análises estruturais. Os resultados DFT também demonstraram que a adição da correção de dispersão DFT-D3 melhorou a descrição das energias de adsorção e se mostrou essencial para reproduzir a tendência do crescimento da energia de adsorção com o tamanho molecular dos alcoóis, enquanto sua natureza atrativa promoveu a diminuição das distâncias atômicas entre alcoóis e substratos. O estudo de dinâmica molecular mostrou que a configuração de adsorção DFT se mantém apenas em temperaturas próximas de 0 K e que outras configurações são favorecidas a temperaturas mais altas. A presença de outras moléculas de glicerol promove, mediante o aquecimento do sistema, a formação de aglomerados de moléculas ligadas através de interações de hidrogênio, o que estabiliza as moléculas e, provavelmente, retarda seu processo de dessorção. Nos sistemas com maior densidade de moléculas, observou-se, ao final da simulação, a formação de fragmentos CH3OH-CHOH-CH2O- e átomos de H adsorvidos na superfície, indicando a quebra de ligações O-H do grupo hidróxi terminal. / Several researchers have been suggesting the use of glycerol and other alcohols as a feedstock to produce higher value-added goods and electricity through fuel cells. However, the success of these conversion technologies depends on the development of efficient catalysts. In this context, theoretical approaches are useful tools that are able to yield important insights that could not be easily obtained from experiments and are fundamental for the future design of more efficient materials. Hence, in this thesis, we investigated via density functional theory (DFT) the role of surface defects and strain effects on the adsorption of methanol, ethanol and glycerol on defected Pt(111) and Pt3Ni-based surfaces. To improve the description of the adsorbed systems, we added the van der Waals (vdW) correction DFT-D3 to the DFT approach. We also studied through molecular dynamics, employing the ReaxFF potential, the effects of temperature on the glycerol, considering different levels of surface coverage, interacting with the Pt3Ni(111) substrates. Our results show that the glycerol binds through the oxygen from a terminal hidroxi group to top sites of transition-metals (TM) with the carbon chain almost parallel to the surface. The calculations of adsorption energy indicate that glycerol interacts strongly with low-coordinated sites, such as those of surface defects, which can be rationalized through the d-band model. Furthermore, the presence of multiple low-coordinated sites was related with configurations where the glycerol binds to the substrates by two hidroxi groups, the central and a terminal one. However, there is a clear preference of the alcohols to bind on cationic adsorption sites, which indicates that the Coulomb interactions play a major role on the adsorption process of alcohols on TM. Electron density analyzes suggest that the adsorption promotes perturbations in the electronic density of the alcohols, such as a partial displacement of electron density from the C-O and O-H bonds to the region between the molecule and the substrate, which are related with the stretching/weakening of the C-O e O-H, as found in the structural analyzes. The DFT results also show that the addition of the DFT-D3 dispersion correction enhanced the adsorption energies and was essential to reproduce correctly the dependence of the binding energy with the molecule size, while its attractive nature promoted the decrease of the atomic distances between alcohols and substrates. The molecular dynamics showed that the glycerol DFT lowest energy adsorption configuration is maintained for temperatures close to 0 K whereas different configurations are favored in higher temperatures. In the presence of multiple glycerol molecules, the heating of the system promotes the formation of molecular clusters bound through hydrogen interactions, which stabilize the molecules and, probably, delay the desorption process. In the systems with higher molecular density, we found that CH3OH-CHOH-CH2O- fragments and H atoms are formed in the end of the simulation, which indicates that the breaking of O-H bonds from the terminal hidroxi groups is promoted.

In this thesis, I study the electronic and structural phase diagrams of the superconducting 122 iron pnictides systems Ba1-xKx(Fe1-yCoy)2As2 and EuFe2(As1-xPx)2 by means of the local probe techniques 57Fe Mössbauer spectroscopy (MS) and muon spin relaxation (muSR). For both isovalent substitution strategies - Co/K for Fe/Ba and P for As, respectively - the antiferromagnetic Fe ordering and orthorhombic distortion of the parent compounds BaFe2As2 and EuFe2As2 are subsequently suppressed with increasing chemical substitution and superconductivity arises, once long-range and coherent Fe magnetic order is sufficiently but not entirely suppressed. For Ba1-xKx(Fe1-yCoy)2As2 in the charge compensated state (x/2=y), a remarkably similar suppression of both, the orthorhombic distortion and Fe magnetic ordering, as a function of increasing substitution is observed and a linear relationship between the structural and the magnetic order parameter is found. Superconductivity is evidenced at intermediate substitution with a maximum Tsc of 15 K coexisting with static magnetic order on a microscopic length scale. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density. Within this model, the experimental findings are compatible with the predicted s± pairing symmetry. For EuFe2(As1-xPx)2, the results from 57Fe MS and ZF-muSR reveal an intriguing interplay of the local Eu 2+ magnetic moments and the itinerant magnetic Fe moments due to the competing structures of the iron and europium magnetic subsystems. For the investigated single crystals with x=0.19 and 0.28, 57Fe MS evidences the interplay of Fe and Eu magnetism by the observation of a transferred hyperfine field below Tafm at which the Eu subsystem orders into a canted A-type AFM magnetic structure. Furthermore, an additional temperature dependent out-of-plane tilting of the static Fe hyperfine field is observed below the onset of static Eu ordering. ZF-muSR shows a strong increase of the local field at the muon site below Tafm=20 K and a crossover from isotropic to anisotropic Eu spin-dynamics between 30 and 10 K. The temperature dependence of the spin dynamics, as derived from the muSR dynamic relaxation rates, are related to a critical slowing down of Eu-spin fluctuations which extends to even much higher temperatures (~100 K). They also effect the experimental linewidth observed in the 57Fe MS experiments. The strong influence of the Eu magnetic order onto the primary observables in both methods prevents conclusive interpretation of the experimental data with respect to a putative interplay of Fe magnetism and superconductivity.