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Es werden ab-initio Rechnungen des Supermagnetwiderstands-Effektes von Fe/Cr-Multilagen vorgestellt. Die Elektronenstruktur wurde im Rahmen einer LCAO-Superzellen-Rechnung bestimmt. Als Störung der idealen Schichtstruktur wurden Cr-Defekte in Fe angenommen, die durch spinabhängige Relaxationszeiten beschrieben werden. Die elektrischen Transportkoeffizienten wurden durch Lösung der linearisierten Boltzmann-Gleichung in Relaxationszeitnäherung unter Verwendung des Mott-schen Zweistrommodells berechnet. Bei den betrachteten Systemen variierte die Dicke der Fe-Schicht zwischen 3 und 9 Monolagen, die der Cr-Schicht zwischen 1 und 13 Monolagen. In Abhängigkeit von der Fe- bzw. Cr-Schichtdicke ergeben sich in Übereinstimmung mit den Experimenten charakteristische Oszillationen des Supermagnetwiderstandes. Es wird der Einfluß der Spinanisotropie der Streuung auf den Effekt untersucht. Insbesondere kann gezeigt werden, daß der Effekt auch für spinunabhängige Streuung existiert. / Ab-initio calculations of the Giant Magnetoresistance (GMR) for Fe/Cr multilayers are presented. The electronic structure of the Fe/Cr superlattice is calculated within an optimized LCAO scheme using the local spin density approximation. The scattering of the electrons by Cr impurities in an Fe environment is taken into account by spin dependent relaxation times. The transport is described quasiclassically by solving the linearized Boltzmann equation in relaxation time approximation. In agreement with experiments characteristic oscillations of the GMR are obtained in dependence on the Cr and Fe layer thickness. It can be shown, that the GMR can be reduced or increased by the spin anisotropy of the scattering, but the phenomenon still exists for spin-independent scattering.

Les aciers appelés ODS (pour Oxide Dispersion Strengthened), renforcés par une dispersion homogène de nano-oxydes, sont des matériaux de structure avancés pour les futurs réacteurs nucléaires de fusion et de fission. En effet ces nano-oxydes, à base d’Y et Ti, servent comme centres de recombinaison de défauts ponctuels et d'obstacles aux mouvements des dislocations, améliorant de ce fait leur résistance aux radiations et aux températures élevées. La fabrication conventionnelle des aciers ODS est réalisée par broyage mécanique suivi de traitements thermo-mécaniques, et ne permet pas facilement de comprendre les mécanismes physiques conduisant à la précipitation des nano-oxydes, ce qui serait potentiellement utile pour optimiser leur production. La cinétique de formation de ces nano-oxydes peut être étudiée en utilisant une technique alternative, à savoir la synthèse par faisceaux d’ions, qui présente de nombreux avantages, notamment le contrôle précis des paramètres expérimentaux et la possibilité de décorréler divers facteurs contribuant à la cinétique de précipitation. Au cours de cette thèse, cette technique a été utilisée pour étudier la coprécipitation d'ions métalliques (Y et/ou Ti) et d'oxygène implantés dans un alliage modèle Fe-Cr de composition proche de celle typique des aciers ODS commerciaux. Des ions de Y, Ti et O à basse énergie ont été implantés dans des échantillons d'alliage Fe10wt%Cr de haute pureté à température ambiante. Les échantillons implantés ont ensuite été recuits à diverses températures entre 600 à 1100°C pour favoriser la précipitation de nano-oxydes, conformément au principe de cette technique. La microscopie électronique à transmission a été utilisée pour caractériser la structure cristallographique et la composition chimique des nano-oxydes formés lors de trois séries d'expériences. Tout d'abord, l'implantation séquentielle d'ions Ti et O a été mise en œuvre. Un recuit ultérieur a révélé qu’il n’y avait pas de précipitation d'oxyde de titane jusqu’à des températures inférieures à 1000°C, mais la présence de nano-oxydes riches en chrome avec une structure hexagonale de type corundum, qui contiennent une certaine quantité de Ti à des températures suffisamment élevées. Ce n’est qu’après le recuit à 1100°C que des nano-oxydes d’un autre type à cœur enrichi en Ti et coquille enrichie en Cr ont également été observés. Deuxièmement, l'implantation séquentielle d’ions Y et O a entraîné la formation à 800°C de nano-oxydes probablement riches en yttrium. Le recuit à 1100°C a favorisé la croissance des particules identifiées comme étant des nano-oxydes d’yttrium avec une coquille enrichie en Cr. Enfin, une implantation ionique séquentielle de deux ions métalliques (Y et Ti) a été réalisée, suivie d'une implantation d’O. L'ordre d'implantation des ions métalliques s'est révélé crucial pour la précipitation de nano-oxydes lors du recuit ultérieur. Lors de la séquence avec une implantation de Ti en premier, une précipitation d'oxyde riche en chrome de structure corundum hexagonale a été observée, très similaire au cas de l'implantation d’ions Ti et O. En revanche, la séquence avec une implantation d’ions Y en premier a produit des nano-oxydes d'yttrium-titane qui possèdent une structure non identifiable. En résumé, l’étude a démontré la faisabilité de la formation de nano-oxydes de Y, Ti et (Y, Ti) par implantation ionique. La thèse présente la caractérisation détaillée de ces nano-oxydes, ainsi que certaines de leurs caractéristiques spécifiques, telles que la présence de relations d'orientation entre les nano-oxydes et la matrice FeCr, qui ont été observées même dans le cas de nano-oxydes de type corundum riches en Cr. Enfin, les résultats obtenus, combinées avec les données de la littérature, sont discutées pour une meilleure compréhension des mécanismes impliqués dans la formation des nano-oxydes dans les aciers ODS. / Oxide Dispersion Strengthened (ODS) steels, that is steels reinforced with a homogeneous distribution of (Y,Ti) oxide nano-particles, are advanced structural materials for nuclear applications. The oxide particles serve as point defect recombination centres and obstacles to dislocation motion thereby improving radiation resistance and high-temperature strength of these steels making them perfect candidate materials for future fusion and fission nuclear reactors. The conventional fabrication of ODS steels is achieved by mechanical alloying followed by thermomechanical heat treatments. This way of ODS steel production seems complicated to understand the physical mechanisms leading to the precipitation of nano-oxide particles. The kinetics of nanoparticle formation can be much better studied using an alternative technique of nanoparticle growth, namely Ion Beam Synthesis (IBS). This approach has many advantages including the precise control of experimental parameters and the ability to de-correlate various factors contributing to precipitation kinetics. A better knowledge gained in this way would be potentially helpful for optimization of ODS steel production routines. In the course of this PhD study, the IBS approach was applied to investigate the co-precipitation of metal (Y and/or Ti) and oxygen ions implanted into a model Fe-Cr alloy with the composition close to those typical for commercial ODS steels. Following the standard IBS schedule, consisting of ion implantation followed by high-temperature heat treatment, ions of Y, Ti and O at low energies were implanted into high-purity Fe10wt%Cr alloy samples at room temperature. The implanted samples were then annealed at various temperatures ranging from 600 to 1100°C to promote the precipitation of nano-oxide particles. A range of Transmission Electron Microscopy techniques were used to characterize the crystallographic structure and chemical composition of the nanoparticles. The study has been performed following three sets of experiments. First of all, the sequential implantation of Ti and O ions was implemented. Subsequent annealing at temperatures below 1000°C revealed that precipitation of titanium oxide was suppressed. Instead, chromium-rich nano-oxide particles with corundum hexagonal structure were found to precipitate. At sufficiently high temperatures these corundum particles were found to contain certain amount of Ti. Only after annealing at the highest temperature of 1100°C, particles of another type with Ti enriched core and Cr enriched shell were additionally fixed. Secondly, sequential Y and O ion implantation resulted in the formation of probable yttrium-rich oxides at 800°C. Annealing at 1100°C promoted their growth to larger sized yttria (Y₂O₃) particles with a Cr enriched shell. Finally, sequential ion implantation of both metal ions (Y and Ti) was performed, followed by O implantation. The order of metal ion implantation has been found to be crucial for subsequent oxide precipitation at the annealing stage. With the Ti implantation first in the sequence, the precipitation of corundum hexagonal chromium-rich oxide was observed, very similar to the case of Ti and O implantation. In contrast, implantation starting with Y produced yttrium-titanium oxide particles with unidentifiable structure. Summing up, the study has demonstrated the feasibility of the formation of Y, Ti and (Y,Ti) oxides by ion implantation. The thesis presents the detailed characterization of the nanoparticles, as well as the discovered specific features of precipitated particles, such as the presence of orientation relationships between the particles and the FeCr matrix, which was observed even for the case of Cr-rich corundum particles. Finally, the implications of the obtained results, in conjunction with the already known data from the existing literature, for the better understanding of the mechanisms involved in the formation of nano-oxide particles in ODS steels are discussed.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / Ligas Fe-Cr-Mo sÃo amplamente utilizadas na fabricaÃÃo de tubulaÃÃes para a indÃstria petroquÃmica nos processos de transporte e refino do petrÃleo devido a uma combinaÃÃo de propriedades como dureza, tenacidade e resistÃncia à fluÃncia e à corrosÃo em altas temperaturas. Atualmente, a liga Fe-9Cr-1Mo à a mais comumente utilizada. Contudo, em refinarias de petrÃleo que apresentam altos Ãndices de acidez, tem-se observado, de maneira acentuada, processos de corrosÃo nessas ligas, principalmente processos relacionados à acidez naftÃnica. Um aumento no teor de molibdÃnio da liga tem sido apontado como uma possÃvel soluÃÃo para este problema, visto que a adiÃÃo desse elemento na liga resulta em uma melhor resistÃncia à corrosÃo. Nesse contexto, foram desenvolvidas no LACAM - LaboratÃrio de CaracterizaÃÃo de Materiais da UFC, ligas Fe-9Cr-XMo com trÃs diferentes percentuais em peso de molibdÃnio (X = 5, 7, 9 wt%). A caracterizaÃÃo dessas ligas, bem como dos precipitados que nelas surgem devido aos tratamentos tÃrmicos, e o estudo da sua resistÃncia à corrosÃo serÃo feitos atravÃs de vÃrias tÃcnicas. Neste trabalho, especificamente, foi utilizada a Espectroscopia MÃssbauer e a DifraÃÃo de Raios-x para uma melhor compreensÃo das mudanÃas na estrutura das ligas quando submetidas Ãs diferentes condiÃÃes de serviÃo dentro da faixa de temperatura de 450ÂC a 650ÂC. Os resultados obtidos permitem concluir que a distribuiÃÃo dos elementos de liga na matriz à influenciada pelo teor de molibdÃnio, pela temperatura e o pelo tempo de tratamento. TambÃm foram extraÃdos e caracterizados os precipitados da liga comercial ASTM A213 da classe T91 utilizada para efeito de comparaÃÃo com as ligas E1, E2 e E3. A presenÃa destes precipitados altera a distribuiÃÃo dos elementos de liga na matriz. A anÃlise da influÃncia da concentraÃÃo dos elementos de impureza na estrutura das ligas estudadas foi investigada atravÃs de um modelo de primeiros e segundos vizinhos. O valor do campo magnÃtico hiperfino varia proporcionalmente ao nÃmero de vizinhos de impureza nos sÃtios de ferro. A distribuiÃÃo de elementos de liga na matriz està diretamente relacionada à distribuiÃÃo de campo magnÃtico hiperfino. As probabilidades associadas aos valores de campo hiperfino permitiram uma melhor compreensÃo da estrutura das ligas. / Fe-Cr-Mo alloys are widely used in the manufacture of piping for the petrochemical industry in oil transport and refining processes due to a combination of properties such as hardness, toughness, creep resistance and corrosion resistance at high temperatures. Nowadays the Fe-9Cr-1Mo alloy is the most common alloy in use. However, in some refineries with higher acidity levels, strong corrosion processes have been observed, specially in processes related to naphthenic acid corrosion. An increasing in the Molybdenum content has been pointed out as a possible solution to this problem, since the addition of this alloying element results in a better corrosion resistance. In this context, Fe-9Cr-XMo alloys with three different Molydbenum contents (X = 5, 7, 9 wt%) have been developed at LACAM - LaboratÃrio de CaracterizaÃÃo de Materiais at the Universidade Federal do CearÃ. The characterization of these alloys, as well as the precipitates that appear on them due to heat treatment and studies of their corrosion resistance were made through various techniques. Specifically in this work, Transmission MÃssbauer Spectroscopy and X-Ray Diffraction were used in order to achieve a better understanding of the structure changes of the alloys when they are subjected to different service conditions in the temperature range from 450ÂC to 650ÂC. The results led to the conclusion that the distribuition of the alloy elements in the matrix is influenced by the molibdenum content, the temperature and the treatment time. The precipitates present in the ASTM A213 grade T91, used for comparation with E1, E2 and E3 alloys, have been extracted and characterized as well. The presence of this precipitates changes the distribuition of the alloy elements in the matrix. The analysis of the influence of the impurity elements in the structure of the alloys was investigated by a model of first and second neighbors. The changes in the magnetic hyperfine field is proportional to the number of impurity atoms in the neighborhood of the iron sites. The distribuition of the alloy elements in the matrix is direct related to the distribution of the magnetic hyperfine field. The probabilities associated to the values of hiperfine field allow for a better understanding of the structure of the alloys.

In the absence of a fusion neutron source, research on the structural integrity of materials in the fusion environment relies on current fission data and simulation methods. Through investigation of the Fe-Cr system, this detailed study explores the challenges and limitations in the use of currently available radiation sources for fusion materials research. An investigation of ion-irradiated Fe12%Cr using nanoindentation with a cube corner, Berkovich and spherical tip, and micro-cantilever testing with two different geometries, highlighted that the measurement of irradiation hardening was largely dependent on the type of test used. Selected methods were used for the comparison of Fe6%Cr irradiated by ions and neutrons to a dose of 1.7dpa at a temperature of 288°C. Micro-cantilever tests of the Fe6%Cr alloy with beam depths of 400 to 7000nm, identified that size effects may significantly obscure irradiation hardening and that these effects are dependent on radiation conditions. Irradiation hardening in the neutron-irradiated alloy was approximately double that of the ion-irradiated alloy and exhibited increased work hardening. Similar differences in hardening were observed in an Fe5%Cr alloy after ion-irradiation to a dose of 0.6dpa at 400°C and doses rates of 6 x 10^-4dpa/s and 3 x 10^-5dpa/s. Identified by APT, it was shown that increased irradiation hardening was likely to be caused by the enhanced segregation of Cr observed in the alloy irradiated with the lower dose rate. These observations have significant implications for future fusion materials research in terms of the simulation of fusion relevant radiation conditions and micro-mechanical testing.