Thèse (M.Eng.) -- Université du Québec à Chicoutimi, 2003. / Bibliogr.: f. [223]-230. Document électronique également accessible en format PDF. CaQCU

Thèse (D.Eng.) -- Université du Québec à Chicoutimi, 2006. / La p. de t. porte en outre: Thèse présentée à l'Université du Québec à Chicoutimi pour l'obtention du doctorat en ingénierie. CaQCU Bibliogr.: f. 130-147. Document électronique également accessible en format PDF. CaQCU

Thesis (Ph.D.)--Mississippi State University. Department of Physics and Astronomy. / Title from title screen. Includes bibliographical references.

Thesis (Ph.D.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 220-225).

Magnesium and magnesium alloys are used in a wide variety of structural applications including automotive, aerospace, hand tools and electronic industries thanks to their light weight, high specific strength, adequate corrosion resistance and good castability. Al and Zn are the primary alloying elements in commercial Mg alloys and commonly used in automotive industries. AZ91 is one of the most popular Mg alloys containing 9% Al and 1% Zn. Hence, lots of research have been done during last decades on AZ91D. However, the existing data concerning mechanical properties and microstructural features showed large scatter and is even contradictory. This work focused on the correlation between the microstructure and the mechanical properties of as-cast AZ91 alloy. An exhaustive characterization of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 using optical and electron backscattered diffraction (EBSD) was performed. These microstructural parameters were correlated to offset yield point (Rp0.2), fracture strength and elongation to fracture. It was understood that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy at temperature range from room temperature up to 190oC. It was realized that by increasing the Mg17Al12 content above 11% a network of intermetallic may form. During deformation this rigid network should break before any plastic deformation happen. Hence, increase in Mg17Al12 content resulted in an increase in offset yield point. The presence of this network was supported by study of thermal expansion behaviour of the alloy containing different amount of Mg17Al12. A physically-based model was adapted and validated in order to predict the flow stress behaviour of as-cast AZ91D at room temperature up to 190ºC for various microstructures. The model was based on dislocation glide and climb in a single-phase (matrix) material containing reinforcing particles. The temperature dependant variables of the model were quite well correlated to the underlying physics of the material.

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No. of bitstreams: 1 DissRBS.pdf: 2973698 bytes, checksum: 37eb711fcbda392563dc3155916b5d95 (MD5) Previous issue date: 2017-03-31 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Magnesium is considered a promising candidate in applications of storage hydrogen in the solid state, due, among other factors, to its low relative cost and high gravimetric capacity (7,6 wt%.). For magnesium, the use of transition metals as additives may show great improvements in the kinetics of hydrogen absorption/desorption. Different additives can have different functionalities, for example, reducing the energy barrier for H2 adsorption and acting as a heterogeneous nucleating agent in the kinetics of phase transformation. In this research, the main effects of the use of zirconium (Zr) as an additive of the magnesium for the purposes of hydrogen storage in bulks produced by rapid solidification in melt-spinning furnace and cold rolling were studied in detail. Basically, structural characteristics were related to the behavior of absorption / desorption. The alloy produced by cold rolling showed behavior of activation and absorption similar to the pure Mg which was produced by the same route. For the route processed by melt spinning, the use of zirconium as additive has presented a deleterious effect on the activation. However, after activation, the kinetics of absorption exhibit behavior similar to pure Mg. The activation had significantly improved after mechanical or thermal processing, however, the structural characteristics that permeate this effect still have a lack of studies. For both process route, the desorption kinetics presented excellent results, reducing the complete reaction in a few minutes. With the results obtained it is possible to conclude that the use of zirconium as an additive in magnesium alloys has no beneficial effect on absorption. In the other hand, the desorption is extremely attractive. After exposure to air, it was verified that both compositions presented formation of contaminants on the surface and this made the activation a difficult way. However, this effect was more latent in pure magnesium. / O magnésio é considerado um candidato promissor em aplicações de armazenagem de hidrogênio no estado sólido, devido, entre outros fatores, ao seu baixo custo relativo e elevada capacidade gravimétrica, 7,6% em peso. Para o magnésio, o uso de metais de transição como aditivos pode apresentar melhoras significativas na cinética de absorção/dessorção de hidrogênio. Diferentes aditivos podem ter diferentes funcionalidades, por exemplo, reduzir a barreira energética para a adsorção de H2 e atuar como agente inoculante de nucleação heterogênea na cinética de transformação de fase. Neste trabalho, foram estudados em detalhes os principais efeitos provenientes da utilização de zircônio como aditivo ao magnésio para fins de armazenagem de hidrogênio em massas metálicas volumosas (do inglês, bulks) produzidas por solidificação rápida em forno melt-spinning (da sigla em inglês, MS) e laminação a frio (do inglês, CR, cold rolling). Basicamente, relacionaram-se características estruturais ao comportamento de absorção/dessorção. A liga produzida por laminação a frio apresentou comportamento de ativação e absorção semelhante ao Mg puro produzido pela mesma rota. Para o material processado por melt-spinning, o uso do zircônio como aditivo apresentou efeito deletério na ativação. Contudo, após a ativação, a cinética de absorção exibe comportamento semelhante ao Mg puro. Posteriores processamentos mecânico ou térmico melhoraram significativamente a ativação, contudo, as características estruturais que permeiam tal efeito ainda carecem de estudos. Para ambas as rotas de processamento, a cinética de dessorção apresentou ótimos resultados, reduzindo em poucos minutos a reação completa. Com os resultados obtidos se conclui que o uso do zircônio como aditivo em ligas de magnésio não apresenta efeitos benéficos na absorção. Todavia, a dessorção da liga com Zr é extremamente atrativa. Após exposição ao ar, verificou-se que ambas as composições apresentaram formação de contaminantes sobre a superfície e, isso dificultou a ativação, porém, este efeito foi mais evidente no magnésio puro.

Les alliages de magnésium représentent une alternative à l’utilisation d’alliages d’aluminium ou de matériaux composites, en particulier dans le secteur aéronautique dans l’objectif de réduire la masse des structures. Ces travaux de thèse ont pour but de participer au développement de nouvelles méthodes de protection des alliages de magnésium, plus efficaces et respectueuses de l’environnement. Pour mener à bien ces travaux, des techniques électrochimiques, en particulier la spectroscopie d’impédance électrochimique, ont été couplées à des mesures par microscopie à force atomique (AFM), à des analyses par spectroscopie d’émission atomique à plasma induit (ICP-AES) et par spectroscopie de masse d’ions secondaires à temps de vol (Tof-SIMS) ainsi que des essais normalisés industriels. Tout d’abord, la résistance à la corrosion en milieu Na2SO4 de trois alliages de magnésium contenant des terres rares (WE43, EV31 et ZE41) a été étudiée et comparée à celle de deux alliages riches en aluminium (AZ31 et AZ91) et à celle du magnésium pur. Pour tous les alliages, il a été montré que les particules intermétalliques agissent comme des cathodes locales. Cet effet de couplage galvanique est plus marqué pour les particules riches en terres rares, en particulier dans le cas de l’alliage EV31. Conjointement, la corrosion est contrôlée par la dissolution de la matrice riche en magnésium et par le recouvrement progressif de la surface métallique par un film d’oxydes/hydroxydes. Ce film est plus protecteur pour les alliages que pour le magnésium pur mais cet effet bénéfique n’est toutefois pas suffisant pour compenser le rôle néfaste joué par les particules intermétalliques. Au final, l’ajout de terres rares augmente la vitesse de corrosion des alliages de magnésium en milieu Na2SO4 par rapport à celle des alliages AZ ou celle du magnésium pur. Dans le cas de l’alliage WE43, qui a été retenu pour la suite de l’étude, il a été montré que le film protecteur d’oxydes est plus mince et plus stable que celui formé sur le Mg pur, en particulier en présence d’ions chlorure. Ces résultats ont été expliqués par l’incorporation des éléments d’alliages, comme l’yttrium, qui serait responsable de la formation d’un film d’oxydes plus compact. Puis, plusieurs méthodes de protection ont été envisagées dans le but d’obtenir une résistance à la corrosion compatible avec les exigences de l’industrie aéronautique. Un traitement d’anodisation, développé par la société Prodem et appelé CEP, en combinaison avec plusieurs primaires de peinture sans chromate, proposés par la société Mapaéro (hydrodiluable ou haut-extrait sec) ont été évalués et comparés aux solutions de référence chromatées. Il a été montré que les traitements de conversion actuels, même en présence de primaire chromaté, ne permettent pas une protection efficace des alliages de magnésium. Le traitement CEP, de par sa structure poreuse, permet une bonne adhésion avec les primaires. Les meilleures performances ont été obtenues pour le traitement CEP revêtu par le primaire haut-extrait sec. Des analyses supplémentaires ont montré que l’ajout d’un vernis permettait d’obtenir un système de protection prometteur pour le remplacement des systèmes de référence sur la base des exigences clés aéronautiques. / Magnesium alloys represent an alternative to aluminum alloys and composite materials – especially in the aeronautic sector with the aim of reducing the structural mass. The main goal of this thesis is to participate in the development of new protection methods that would be more efficient and environmentally-friendly. To this end, electrochemical techniques – such as electrochemical impedance spectroscopy (EIS) – were used together with atomic force microscopy, inductively coupled plasma atomic emission spectrometry, time-of-flight secondary ion mass spectrometry and standardized industrial tests. First, the corrosion resistance in Na2SO4 solution of three magnesium alloys containing rare earths (WE43, EV31 and ZE41) was studied and compared to those of two aluminum-rich magnesium alloys (AZ31 and AZ91) and pure magnesium. For all the alloys, it was shown that intermetallic particles act as local cathodes. This galvanic coupling was stronger in the case of rare earths magnesium alloys, especially for the EV31 alloy. Their corrosion was controlled at the same time by the dissolution of magnesium-rich matrix and by the progressive covering of the alloy surface by an oxides/hydroxides layer. This film was more protective in the case of the alloys; but it is not sufficient to counterbalance the detrimental effect of intermetallic particles. Ultimately, the addition of rare earths increases the corrosion rate of magnesium alloys in Na2SO4 solution compared to aluminum magnesium alloys or pure magnesium. Concerning the WE43 alloy, it was shown that the protective oxide film was thinner and more stable than the one formed on pure magnesium, especially in the presence of chloride ions. These results were explained by the incorporation of alloying elements, like yttrium, that should be responsible of the formation of a more compact film. Then, several protection methods were considered to obtain a corrosion-resistant system that would be compatible with key requirements of the aeronautic industry. The combination of the anodization treatment developed by Prodem Company, called CEP, and the water-based or high-solid coatings offered by Mapaero Company was assessed and compared to classical chromated coatings. It was concluded that current chemical conversion treatments did not provide enough protection for magnesium alloys. The CEP treatment porous morphology acts as an efficient primer base. The best performances were obtained for the CEP treatment in combination with high solid chromate-free coatings. Complementary analysis showed that, with an additional varnish layer, the developed protection system is compatible with aeronautic key requirements.

Les alliages de Magnésium (Mg) sont une nouvelle génération de matériaux biodégradables ayant une bonne ostéointégration et un module d'élasticité similaire à celle de l'os humain. Ces propriétés rendent ces matériaux attrayant pour produire des implants temporaires pour la réparation osseuse. Toutefois, les alliages Mg se dégradent rapidement in vivo, rendant nécessaire de contrôler leur vitesse de corrosion pour accompagner la régénération tissulaire. Parmi les approches proposées pour réduire la corrosion et la biocompatibilité des alliages Mg, les plus utilisées sont les couches de conversion et les revêtements de surface. Dans ce travail une approche synergique qui combine une réduction du taux de corrosion avec l'amélioration de la biocompatibilité des alliages Mg est proposée. De nouveaux revêtements bicouches ont été déposés sur la surface d'alliages AZ31 et ZE41 : (i) un revêtement de silane-TiO2 déposé par dip-coating et (ii) des couches supérieures de collagène de type I et/ou de chitosane. Le revêtement inférieur a été efficace pour réduire la corrosion des alliages dans un fluide corporel simulé et en milieu de culture. La culture cellulaire in vitro de fibroblastes et ostéoblastes, a révélé que le dépôt additionnel de biopolymères a amélioré la réponse biologique du revêtement de silane-TiO2. Ces résultats montrent qu'il existe un effet combiné des revêtements bicouches et de la composition des alliages sur la réponse à la corrosion et sur le comportement cellulaire. Ce travail apporte donc une nouvelle contribution à la compréhension de l'évolution de la corrosion des alliages Mg dans des environnements biologiques. / Magnesium (Mg) alloys are a new generation of biodegradable materials with good osseointegration and elastic modulus similar to that of human bone. These properties make them attractive materials to produce biodegradable implants for bone repairing applications that require temporary support. However, Mg alloys degrade rapidly in the in vivo environment making necessary to control their corrosion rate to accompany the tissue healing processes. Several approaches have been proposed for reducing corrosion rate and improving biocompatibility of Mg alloys. The most used ones are conversion films and surface coatings. This project proposes a synergistic approach that combines both decreased corrosion rate and improved biocompatibility of Mg alloys: we developed novel bi-layered coatings to functionalize the surface of AZ31 and ZE41 Mg alloys for bone repair applications. First, a bottom silane-TiO2 coating was formulated and deposited on both alloys by the dip-coating technique. The silane-based coating was effective in slowing down the corrosion rate of the substrates in simulated body fluid (SBF) and in Dulbecco’s Modified Eagle’s Medium (DMEM). Secondly, top layers of type I collagen and/or chitosan were developed. Cell in vitro tests, with fibroblasts and osteoblasts, revealed that the biopolymers enhanced the biological response of the silane-TiO2 coating. Furthermore, the findings showed that there is a combined effect of the bi-layered coatings and the nature of the alloys on their final corrosion response and on the fate of the cells. In the same way, this work contributes to elucidating corrosion processes of Mg alloys in organic solutions in the long-term.

Title: Influence of Solid Solution Elements and Precipitate Formation on the Mechanical Behaviour of Magnesium Alloys Author: Mgr. Kseniya ILLKOVÁ Department / Institute: Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague Supervisor of the doctoral thesis: Doc.RNDr.František Chmelík, CSc. Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague Abstract: Mg-1 wt. % Mn alloys containing about 1 wt.% of calcium (MX10), aluminium (AM11), cerium(ME11), yttrium (MW11), neodymium (MN11) and 8 wt.% of aluminium (AM81) were indirectly extruded with two different speeds. The partially recrystallised microstructures obtained in MX10, AM11, ME11 and MW11 after slow extrusion exhibited bimodal grain size distributions. Only in the case of AM81 and MN11 a fully recrystallised microstructure was found, indicating differences in the grain nucleation rate during recrystallisation. In these alloys, the weakest texture and, correspondingly, the lowest asymmetry in mechanical properties were also observed. The acoustic emission technique was used to reveal dislocation or twin controlled deformation in differently textured fractions of the microstructure during tensile and compression testing. A direct correlation between the acoustic...