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Made available in DSpace on 2014-12-17T14:06:58Z (GMT). No. of bitstreams: 1 GilbertoM_DISSERT_PARCIAL.pdf: 494543 bytes, checksum: da2d1ceb188b373977eacdb9f150846c (MD5) Previous issue date: 2011-01-31 / In this work, was studied the formation of a composite of the refractory metal niobium with copper, through the process of high-energy milling and liquid phase sintering. The HEM can be used to synthesize composite powders with high homogeneity and fine size particle distribution. It may also produce the solid solubility in immiscible systems such as Nb-Cu, or extend the solubility of systems with limited solubility. Therefore, in the immiscible system Cu-Nb, the high-energy milling was successfully used to obtain the composite powder particles. Initially, the formation of composite particles during the HEM and the effect of preparation technique on the microstructure of the material was evaluated. Four loads of Nb and Cu powders containing 20%wt Cu were synthesized by MAE in a planetary type ball mill under different periods of grinding. The influence of grinding time on the metal particles is evaluated during the process by the withdrawal of samples at intermediate times of milling. After compaction under different forces, the samples were sintered in a vacuum furnace. The liquid phase sintering of these samples prepared by HEM produced a homogeneous and fine grained. The composite particles forming the sintered samples are the addition of a hard phase (Nb) with a high melting point, and a ductile phase (Cu) with low melting point and high thermal and electrical conductivities. Based on these properties, the Nb-Cu system is a potential material for many applications, such as electrical contacts, welding electrodes, coils for generating high magnetic fields, heat sinks and microwave absorbers, which are coupled to electronic devices. The characterization techniques used in this study, were laser granulometry, used to evaluate the homogeneity and particle size, and the X-ray diffraction, in the phase identification and to analyze the crystalline structure of the powders during milling. The morphology and dispersion of the phases in the composite powder particles, as well the microstructures of the sintered samples, were observed by scanning electron microscopy (SEM). Subsequently, the sintered samples are evaluated for density and densification. And finally, they were characterized by techniques of measuring the electrical conductivity and microhardness, whose properties are analyzed as a function of the parameters for obtaining the composite / Neste trabalho, foi estudada a forma??o de um comp?sito do metal refrat?rio ni?bio com o cobre, atrav?s do processo de moagem de alta energia e sinteriza??o por fase l?quida. A MAE pode ser usada para sintetizar p?s comp?sitos com alta homogeneidade e fina distribui??o de tamanho de part?culas. Ela tamb?m pode produzir a solubilidade s?lida em sistemas imisc?veis como o Nb-Cu, ou, estender a solubilidade de sistemas com limitada solubilidade. Portanto, no sistema imisc?vel Nb-Cu, a moagem de alta energia foi utilizada com sucesso para a obten??o das part?culas do p? comp?sito. Inicialmente a forma??o das part?culas comp?sitas durante a MAE e o efeito dessa t?cnica de prepara??o na microestrutura do material foi avaliada. Quatro cargas de p?s de Nb e Cu contendo 20% em massa de Cu foram sintetizados por MAE em um moinho de bolas tipo planet?rio, sob diferentes per?odos de moagem. A influ?ncia do tempo de moagem nas part?culas met?licas ? avaliada no decorrer do processo, atrav?s da retirada de amostras em tempos parciais da moagem. Ap?s a compacta??o sob diferentes for?as, as amostras foram sinterizadas em um forno ? v?cuo. A sinteriza??o por fase l?quida destas amostras preparadas por MAE produziu uma estrutura homog?nea e com granula??o refinada. As part?culas comp?sitas que formam as amostras sinterizadas, s?o a jun??o de uma fase dura (Nb) com alto ponto de fus?o, e uma fase d?ctil (Cu) de baixo ponto de fus?o e de elevadas condutividades t?rmica e el?trica. Com base nestas propriedades, o sistema Nb-Cu ? um material em potencial para in?meras aplica??es, como contatos el?tricos, eletrodos de solda, bobinas para gera??o de altos campos magn?ticos, dissipadores de calor e absorvedores de microondas, que s?o acoplados a dispositivos eletr?nicos. As t?cnicas de caracteriza??o utilizadas neste estudo, foram a granulometria ? laser para avaliar a homogeneidade e o tamanho das part?culas, e a difra??o de raios-X, na identifica??o das fases e an?lise da estrutura cristalina dos p?s durante a moagem. J? a morfologia e a dispers?o das fases nas part?culas do p? comp?sito, assim como as microestruturas das amostras sinterizadas, foram observadas atrav?s de microscopia eletr?nica de varredura (MEV). Posteriormente, as amostras sinterizadas foram avaliadas quanto ? densidade e densifica??o. E, finalmente, foram caracterizadas atrav?s de t?cnicas de medi??o da condutividade el?trica e microdureza Vickers, cujas propriedades s?o analisadas em fun??o dos par?metros de obten??o do comp?sito

Les alliages intermétalliques réfractaires à base de niobium sont considérés comme ayant un bon potentiel pour les applications à haute température grâce à un bon compromis entre une bonne résistance à haute température et une bonne ductilité à température ambiante. En outre, cette famille d'alliages présente également un point de fusion élevé et une faible densité. De ce fait, ils sont de bons candidats pour les applications dans les aubes de turbine à basse pression pour une plage de température comprise entre 800 et 1000 ° C. Le but de cette étude est d'étudier les changements de composition chimique, en particulier la précipitation de phase O-Ti2AlNb et les effets d'addition de silicium sur les microstructures et les propriétés mécaniques à haute température. L’étude commence par une première prospective sur les alliages montrant le plus de potentiel pour les applications visées. Les alliages étudiés doivent présenter un bon équilibre entre ductilité induite par le titane qui empêche la précipitation de la phase d-Nb3Al fragile, et une quantité élevée de niobium pour maintenir de bonnes propriétés mécaniques à haute température. Les alliages étudiés sont dopés avec 1 %at Si pour améliorer la résistance aux températures élevées et maintenir une ductilité acceptable à température ambiante. Nous nous intéressons également à l’influence des teneurs en aluminium et silicium sur les transformations de phase. Le système est renforcé par mise en ordre A2 ? B2, par effet de solution solide ou par durcissement structural de la phase O-Ti2AlNb. La cinétique de précipitation de cette phase O-Ti2AlNb est dépendante de la composition chimique. En particulier l’ajout de silicium permet un élargissement du domaine de précipitation et accélère sa cinétique de précipitation. / Niobium based refractory intermetallic alloys are considered as having great potentials for high temperature applications based on a good balance of high temperature strength and low-temperature damage tolerance. Moreover this family of alloys also exhibits a high melting point and a low density; hence they are good candidates for low pressure turbine blade applications over the temperature range of 800–1000 °C. The aim of this study is to investigate the chemical composition changes, especially the O-Ti2AlNb phase precipitation and the silicon addition effects on the microstructures and high temperature mechanical properties. This study starts with a first prospective on the chemical compositions of alloys showing the highest potential for applications wanted. The studied alloys show a good balance between the ductility at room temperature induced by titanium which stopped the fragile intermetallic d-Nb3Al precipitation; and a high niobium content to maintain good high temperature mechanical properties. Two alloys are doped with 1 %at Si to improve high temperature strength and keep an acceptable ductility at room temperature. The result indicates on the more promising alloy chemical composition. We also investigate the influence of aluminum and silicon content on phase transformation. The system is reinforced by chemical ordering A2 ? B2, by solid solution or by precipitation hardening of O-Ti2AlNb. The kinetic of precipitation of this O-Ti2AlNb relied on the chemical composition. In particular, the addition of silicon enlarges the O-Ti2AlNb precipitation domain and accelerates the kinetic of precipitation.

Durante as últimas décadas, um dos desafios da indústria aeroespacial é em relação ao aumento da eficiência dos motores de turbinas a gás. A eficiência dos motores é limitada principalmente pela temperatura dos gases de combustão, que não pode ser aumentada devido às limitações intrínsecas relacionadas ao uso das superligas à base de Ni nas partes quentes da turbina, onde as temperaturas podem atingir valores acima de 1000 °C. Este trabalho visa caracterizar novos materiais do sistema Nb-Ti para aplicações aeronáuticas, materiais de baixa massa específica que podem substituir as superligas de Ni. As ligas foram produzidas através de fusão a arco, tratadas termicamente a 1200 °C durante 48 h e expostas a temperaturas semelhantes às encontradas na seção de baixa pressão de um turborreator. Os materiais foram caracterizados em termos de composição química, propriedades mecânicas e microestrutura. Foram utilizadas as seguintes técnicas: difração de raios; microscopia eletrônica de varredura, espectrometria de raios X por energia dispersiva e Microscopia Eletrônica de Transmissão. A caracterização microestrutural revelou que as ligas expostas a 1000 °C durante 168 h apresentam uma microestrutura de duas fases composta principalmente de uma matriz ?0-BCC (Nb/Ti) com precipitados de uma segunda fase rica em titânio. Microestruturas de duas fases também foram observadas para as ligas expostas a 800 °C durante 168 h, na qual uma matriz ?0-BCC (Nb/Ti) com precipitados de uma segunda fase identificada como O2-Ti2NbAl foi observada. As ligas estudadas apresentaram massa específica inferior às superligas à base de Ni normalmente utilizadas na indústria aeronáutica. Em termos de propriedades mecânicas, as amostras expostas e testadas a 1000°C apresentaram valores baixos de resistência à compressão (100 MPa) quando comparado as amostras expostas e testadas a 800 °C (565 MPa). Pelos resultados de oxidação observou-se uma maior resistência a oxidação das ligas testadas a 800 °C, porém tanto a 1000 °C como a 800 °C não houve a formação de um filme protetor. / During the last decades, one of the challenges in the aerospace industry is with respect to increase the efficiency of gas turbine engines. The efficiency of the engines is a function of temperatures of the fluel gas, which cannot be increased because of intrinsic limitations related the use of Ni-based superalloys in the hot parts, where temperatures can reach values above 1000 °C. This work aims to investigate new materials in the Nb-Ti system, with low-density materials that may substitute Ni superalloys. The alloys were processed via arc melting, heat treated at 1200°C for 48h and exposed at temperatures similar to those encountered at the low-pressure section in a turbojet engine. The materials were characterized in terms of chemical composition, mechanical properties and microstructure. The following techniques have been used: X-ray diffraction; Scanning Electron Microscopy; Energy Dispersive X-ray Spectrometry and Transmission Electron Microscopy. The microstructural characterization have revealed that the alloys exposed at 1000 °C for 168 hours present a two-phase microstructure composed mainly of a ?0-BCC (Nb/Ti) matrix with precipitations of a second phase rich in titanium. Two-phase microstructures were also observed for the alloys exposed at 800 °C for 168 hours, where a ?0-BCC (Nb/Ti) matrix is observed with precipitates of a second phase identified as O2-Ti2NbAl. The studies alloys reported a lower density when comparing with the Ni based superalloys normally used in the aeronautical industry. In terms of mechanical properties, specimens exposed and tested at 1000 °C showed lower values of compressive strength (100 MPa) than those exposed and tested at 800 °C (565 MPa). The oxidation results allowed to observe a higher oxidation resistance of the alloys tested at 800 °C, however there was no protective film formation at 1000 °C as at 800 °C.

The Krasno deposit is only one active mine at Slavkovsky les. Mined are feldspars raw materials with high quality, commonly used in ceramic and glass industry. Mined raw material has a lot of additional components, which has influence on quality of raw material. Main additional components are unwanted coloring Fe oxides. Another additional components are apatite and topaz, which dont have influence on quality of raw material. The most interesting additional components of raw material are Ta - Nb, Li - Rb minerals. Currently the processing of raw material is under modernization for maximal separation high quality of feldspar material. The result of semi-processing operation shows the concentrate can be potential source of strategic raw materials as Nb - Ta or Li - Rb mineralization. For separation of these minerals and elements must be realized detailed research of selected processing and properties of these minerals. Powered by TCPDF (www.tcpdf.org)

La supraconductivité est intensément étudiée en physique de la matière condensée pour ses éventuelles applications. En effet, ce phénomène est caractérisé macroscopiquement par des propriétés remarquables, mais pour le moment, son exploitation est limitée par la nécessité de refroidir ces matériaux à des températures cryogéniques. Après la découverte de H₃S, dont la température de transition est de 200 K, un renouveau d’intérêt est apparu pour les matériaux supraconducteurs conventionnels. Pour ce composé, le couplage entre électrons et phonons est à l’origine de l’appariement électronique, une condition nécessaire à la supraconductivité. L’énergie associée à ce couplage se trouve généralement dans la gamme des infrarouges lointains, voire des THz, faisant de la spectroscopie infrarouge un outil idéal pour étudier ce mécanisme. Cette thèse présente les études de deux matériaux supraconducteurs dans des conditions expérimentales extrêmes de pression ou de température, permise grâce à la forte brillance du rayonnement synchrotron. Pour la phase supraconductrice H₃S à des pressions supérieures à 150 GPa, l’environnement en cellule à enclumes de diamant exclut la plupart des techniques pour déterminer la nature du mécanisme mais les études optiques restent adaptées. Les résultats spectroscopiques présentés ici démontrent un fort couplage entre électrons et phonons, qui explique l’origine d’une si haute température de transition. Des mesures complémentaires visant à caractériser NaCl sous pression sont également décrites car ce matériau est couramment utilisé comme transmetteur de pression dans les cellules `a haute pression, notamment pour le supraconducteur H₃S. La deuxième étude rapporte des résultats spectroscopiques dans le THz sur des couches nanométriques de Nb, dont les températures de transition supraconductrice sont de 4,5 K et 6,8 K. Ces mesures confirment que la nature conventionnelle du mécanisme subsiste au sein de ces films minces quasi-bidimensionnels. Pour ce travail, un ensemble instrumental permettant la mesure spectroscopique de matériaux jusqu’à des températures de 200 mK a été entièrement développé autour d’un cryostat à démagnétisation adiabatique. / Superconductivity is highly studied in condensed matter physics for its potential applications. Indeed, this phenomenon is macroscopically characterized by remarkable properties, but generally occurs in materials at cryogenic temperature thus limitating their exploitation. Recently, renewed interest has appeared for conventional superconducting materials with the discovery of H₃S, whose transition temperature is at 200 K. For this compound, the coupling between electrons and phonons is at the origin of the electronic pairing, a necessary condition for superconductivity. The associated energy for this coupling belongs to the far infrared range, even THz, making infrared spectroscopy an ideal tool to study the mechanism. This thesis presents the studies of two superconducting materials in extreme experimental conditions of pressure and temperature, allowed by the high brilliance of synchrotron radiation. For the superconducting phase H₃S under pressures superior to 150 GPa, the environment of diamond anvil cell excludes most of the techniques to determine the nature of the mechanism but the optical studies remain adapted. The spectroscopic results presented here demonstrate a strong coupling between electrons and phonons, which explains the origin of such a high transition temperature. Complementary measurements aiming at characterizing NaCl under pressure is also described because this material is often used as a pressure transmitting medium in high pressure cells, in particular for the superconducting H₃S. The second study reports spectroscopic results in the THz on nanometric layers of Nb, whose superconducting transition temperatures are 4,5 K and 6,8 K. These measurements confirm that the conventional nature of the mechanism subsists within these quasi-bidimensional thin films. For this work, an instrumental ensemble allowing the spectroscopic measurements of materials down to 200 mK has been entirely developed based on an adiabatic demagnetization cryostat.

La réussite de la mise en place d’une résistance efficace chez la plante hôte requiert la présence d’un dialogue entre les différentes voies signalétiques menant à l’induction de la défense. L’implication de l’extinction génique post-transcriptionnelle dans la résistance contre des agents pathogènes, autres que les virus, est une nouvelle avenue qui va permettre de disséquer le lien qui peut exister entre les deux voies immunitaires de la plante : celle qui est déjà importante pour lutter contre les bactéries et les champignons (voie induite à la suite de la reconnaissance des éliciteurs) et celle de l’extinction génique post-transcriptionnelle. Les résultats que nous avons obtenus ont une signification très importante concernant l’implication de l’extinction génique post-transcriptionnelle dans la résistance des plantes contre les agents pathogènes d’une manière générale et Botrytis cinerea, Alternaria brassicicola et Pseudomonas syringae en particulier. Mon travail de recherche a permis d’identifier le miR472 comme étant un acteur qui favorise la mise en place de la maladie causée par des agents pathogènes du type nécrotrophe et hémibiotrophe. De plus j’ai pu, en utilisant le modèle P. syringae pv tomato DC3000-Arabidopsis, disséquer dans le détail, comment la bactérie manipule la machinerie de la plante pour activer la voie des microARNs menant à l’accumulation du miR472, responsable de la suppression de la résistance basale et du PTI. Cette suppression passe par l’inhibition de sa cible, la protéine NB-LRR; RDS1 un nouveau régulateur impliqué dans le PTI induit par la Flg22. Cette étude permettra une avancée remarquable dans les connaissances reliées à l’immunité des plantes, ce qui constituerait une étape clé dans la découverte de nouvelles stratégies de lutte contre les agents phytopathogènes. Sur le plan fondamental, cette étude a permis de consolider le dialogue qui existe entre les voies immunes de la plante et de mettre en évidence un nouvel acteur impliqué dans la résistance basale. En matière d’application, comprendre les mécanismes de défense et de résistance des plantes aux agents pathogènes est primordial pour développer des plantes génétiquement résistantes et des stimulateurs de défense naturelle (Vaccin de plantes). Ceci constitue un enjeu majeur dans l'objectif de concevoir de nouveaux moyens de lutte phytosanitaire, à la fois plus respectueux de l’environnement et ayant moins d’impact sur la santé humaine.

<p>In this work we have investigated the intercalation of electron-donors between NbS₂ slabs in Nb-based layer sulfides.</p><p>Two series of Sr substituted Nb-based misfit sulfides belonging to the 1.5Q/1H and 1Q/1H series of misfit layer compounds have been synthesised. For large lanthanides (Ln=La, Ce), only the 1Q/1H compounds formed whereas for smaller lanthanides and yttrium, both types of phases can be obtained. The crystal structure of misfit sulfide (Pr_0.55Sr_0.45S)_1.15NbS₂ has been refined using the composite approach. In the Q-slab, Pr-atoms are partly replaced by Sr with a random distribution over one cation position. The crystal structure of misfit sulfide [(Sm_1/3Sr_2/3S)_1.5]_1.15NbS₂ belonging to the 1.5Q/1H series have also been determined. The obtained results suggest a preferred occupancy of the cation positions in the slab where Sr atoms mainly occupy positions on the exterior of the slab while Sm atoms are in the center of the slab. The (La_1-xSr_xS)_1.15NbS₂ solid solution (0.1<x<0.9) has also been studied. It was found that the maximum value of Sr substitution is 40-50% and therefore, the minimal value of charge transfer to stabilize this structure type is about 0.6ē per Nb atom. </p><p>An attempt to synthesize Sr_xNbS₂ (0.1≤x≤0.5) intercalates was made but single phases were not obtained and increasing the temperature from 1000^оС to 1100^оС leads to the decomposition of these intercalates. Single crystals of Sr_0.22Nb_1.05S₂ and Sr_0.23NbS₂ were found and their structures were determined. The structures belong to two different types of packings with statistical distribution of Sr between layers.</p><p>A new superconducting sulfide, "EuNb₂S₅", was investigated by ED and HREM and its structure model consisting of Nb₇S₁₄ and (Eu₃S₄)₂ slabs alternating along the c-axis is suggested. An attempt to suggest a model for the structure of "SrNb₂S₅" by means of X-ray single crystal diffraction was made. The proposed structure consists of two types of slabs: a Nb₇S₁₄ and a [Sr₆(NbS₄)₂S] slab with niobium in tetrahedral coordination. It is shown that "SrNb₂S₅" and "EuNb₂S₅" are have similar structures.</p><p>For the first time, single crystals of the complex sulfide BaNb_0.9S₃ have also been studied by means of X-ray single crystal diffraction. The single crystal refinement and EDX analysis showed the existence of cation vacancies at the niobium position. BaNb_0.9S₃ has also been studied by ED and no superstructure was found which implies that and the vacancies are statistically distributed.</p><p>No improvement of the magnetic properties of the studied compounds was observed in comparison to NbS₂.</p>

Superalloys based on platinum-group metals (PGMs) are being developed for high temperature applications. Currently, the optimum alloy is Pt84:Al11:Ru2:Cr3, and work is ongoing. Niobium is a possible addition to increase the melting point, but limited phase diagram data are available. Although work has been done on the Pt-Al-Nb system, there are no reported data for Pt-Cr-Nb. This is a study of the Pt-Cr-Nb system. As-cast samples of the Pt-Cr-Nb ternary system were investigated using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results were used to plot a solidification projection and all binary phases extended into the ternary, except for α´Pt, which was not identified in this investigation. The extensions of the binary phases were: (Cr) ~2 at.% Nb; (Pt) ~24 at.% Nb and 65 at.% Cr; (Nb): ~12 at.% Pt and ~17 at.% Cr; ~NbPt2: ~20 at.% Cr; ~NbCr2: ~18 at.% Pt; ~Cr3Pt: ~10 at.% Nb; ~βNbPt3: ~4 at.% Cr and ~Nb1-XPt1+X: ~13 at.% Cr. Five ternary phases were identified: τ1:~Nb17:Cr64:Pt19; τ2:~Nb28:Cr55:Pt17; τ3:~Nb30:Cr30:Pt40; τ4:~Nb45:Cr27:Pt28 and τ5:~Nb40:Cr18:Pt42. The liquidus surface was derived using the binary phase diagrams, identified primary phases, solidification sequences and the solidification projection, together with the eutectic compositions. Nineteen ternary invariant reactions were identified. Hardness measurements were made on all samples, and also toughness and fracture toughness were deduced. Alloys with (Pt) and ~NbPt2 were ductile with reasonable hardness. Alloys containing (Cr), (Nb), ~Cr3Pt, ~NbCr2, ~Nb3Pt and ~Nb2Pt were extremely brittle with cracks, whereas alloys containing ~βNbPt3 and ~Nb1-XPt1+X phases were slightly brittle.

Os aços de alta resistência e baixa liga são amplamente utilizados em dutos transportadores de óleo e gás e, atualmente, o desenvolvimento de novos projetos de liga e o uso de técnicas altamente avançadas de fabricação e processamento dos aços se tornaram essenciais para obtenção de estruturas que resistam aos danos provocados por H, principal motivo de falha de oleodutos e gasodutos em meios ricos em H2S. No presente trabalho, avaliou-se o efeito de características microestruturais na difusividade do H em dois aços grau API X65, com diferentes teores de Mn. Uma das chapas ainda está em fase experimental de desenvolvimento, tem baixo teor de Mn e foi produzida para aplicação em ambientes sour. A outra chapa tem alto teor de Mn, já é usada comercialmente há alguns anos e foi desenvolvida para trabalho em ambientes doces. Os dois materiais passaram por caracterização microestrutural nas três seções da chapa: longitudinal e transversal à direção de laminação e do topo da chapa (paralela à direção de laminação). Após a caracterização, amostras de cada seção dos aços foram submetidas a ensaios de permeabilidade ao H; o aço baixo Mn passou por análises de EBSD (Difração de Elétrons Retroespalhados), para determinação de textura. O aço baixo Mn tem microestrutura homogênea ao longo da espessura da chapa, composta por ferrita refinada e pequenas ilhas de perlita. O aço alto Mn, por sua vez, apresenta microestrutura heterogênea ao longo da espessura, formada por bandas de ferrita e perlita, com marcada presença de segregação central de elementos de liga. Os ensaios de permeabilidade ao H mostraram que os coeficientes de difusão efetiva do H, Deff, do aço baixo Mn são ligeiramente superiores aos do aço alto Mn. Outros dois importantes parâmetros que foram calculados para os dois aços são a concentração de H na sub-superfície do material, C0, e o número de traps por unidade de volume, Nt. Contrariando expectativas, o aço baixo Mn apresentou maiores valores de C0 e Nt do que o aço alto Mn. Ensaios preliminares de dessorção térmica realizados nos dois aços mostraram os mesmos resultados: o aço baixo Mn aprisiona mais H do que o aço alto Mn. Estes resultados contraditórios de C0 e Nt foram atribuídos à presença de nanoprecipitados de microadições de liga no aço baixo Mn, não detectáveis por microscopia óptica e eletrônica de varredura. Ainda, para os dois aços, os valores de Deff variaram em função da seção analisada da seguinte maneira: Deff longitudinal ? Deff transversal > Deff topo. Para entender melhor o comportamento anisotrópico da difusão do H nos dois aços calculou-se um novo coeficiente de difusão, que foi chamado de coeficiente de difusão no estado estacionário, Dss. O Dss considera que todos os traps do aço estão saturados, permitindo, assim que se avalie somente o efeito de obstáculos físicos à difusão do H. No aço alto Mn, o Dss variou da mesma maneira que o Deff: Dss longitudinal ? Dss transversal > Dss topo; este comportamento foi atribuído ao bandeamento presente no material. No aço baixo Mn, o Dss variou de forma diferente do Deff: Dss transversal > Dss longitudinal >= Dss topo, indicando que a difusão do H pode ser auxiliada por contornos de grão enquanto os traps estão sendo saturados, e que a textura cristalográfica pode influenciar a difusão após o estado estacionário ser atingido. / High strength low alloy steels are widely applied as pipelines for crude oil and natural gas transportation and, currently, new approaches to alloy design, in addition to the use of advanced steelmaking and processing techniques, have become essential for obtaining structures that resist to hydrogen damage, which is the main cause of pipelines failure in H2S-rich environments. The main objective of the present work is to evaluate the influence of microstructural features on hydrogen diffusivity in two API X65 steels, with different Mn contents. One of the steel plates has been recently developed for usage in sour environments, is on its experimental stage and has a low Mn content. The other one is a commercial plate steel, with high Mn content, developed for sweet applications. Both steel plates were characterized in its three sections, in relation to the rolling direction: longitudinal, transverse and top surface of the plate (parallell to the rolling direction). After that, samples obtained from each section of the plates were submitted to hydrogen permeation tests; the low Mn steel was also analysed with EBSD, for texture determination. The low Mn steel presents a homogeneous microstructure through plate thickness, composed of refined ferrite and small pearlite islands. The high Mn steel has a heterogeneous microstructure through the plate thickness, composed of ferrite and pearlite bands, and presents centerline segregation. Hydrogen permeation tests showed that the Deff obtained for the low Mn steel sections are slightly higher than for the high Mn steel. Another two important parameters that were calculated for both steels are the subsurface hydrogen concentration, C0, and the number of traps per unit volume, Nt. Contrary to what was expected, the low Mn steel presented the higher C0 and Nt values. Thermal dessorption spectroscopy analysis confirmed that the low Mn steel traps more H atoms than the high Mn one. These results, along with the similar Deff values, were related to the presence of nanoprecipitates of microalloying elements, that cannot be detected via optical and scanning electron microscopy. Additionally, also for both steels, the Deff values varied in function of the analyzed section as it follows: Deff longitudinal ? Deff transverse > Deff top. In order to better understand this anisotropic behavior, a new diffusion coefficient, which was called diffusion coefficient at the steady state, Dss, was determined. Dss considers that all the trapping sites are saturated, enabling, thus, the evaluation of physical obstacles to H diffusion. For the high Mn steel, the Dss varied in the same matter as the Deff: Dss longitudinal ? Dss transverse > Dss top; this behavior was associated with the microstructural banding present in the material. For the low Mn steel, the Dss exhibited a different behavior: Dss transverse > Dss longitudinal >= Dss top, suggesting that H diffusion can be aided by grain boundaries while the trapping sites are being filled and that crystallographic texture may play its role after the steady state is reached.