Neste trabalho foram avaliados os efeitos da adição de polímeros precursores na sinterização, microestrutura, dureza e na tenacidade à natura de materiais cerâmicos à base de carbeto de silício e de AI2O3. As matérias-primas cerâmicas foram caracterizadas por análise semi-quantitativa por espectrometria de fluorescência de raios X, difração por laser, adsorção gasosa e microscopia eletrônica de varredura. Para os polímeros utilizou-se análise termogravimétrica. A cinética de sinterização das amostras compactadas foi estudada por dilatometria. A caracterização dos materiais sinterizados incluiu medidas de densidade aparente pelo princípio de Arquimedes e/ou por picnometria de He, porosimetria por intrusão de Hg, análises de difração de raios X, de carbono total, avaliação da microestrutura e por microscopia eletrônica de varredura e de transmissão, análise por espectrometria de raios X por dispersão de energia, ensaios de impressão Vickers para determinação de dureza e tenacidade à fiatura. No estudo de cerâmica à base de carbeto de silício foram utilizados os aditivos óxidos AI2O3 (4% em peso) e Y2O3 (4% em peso) e os aditivos poliméricos PMHS (polimetilhidrogenossiloxano) e polimetilhidrogenossiloxano com D4Vi. O processamento envolveu a cura do material, pirólise e sinterização (1850 °C e 1950 °C/l h. Ar ou N2). Nas amostras à base de carbeto de silício houve elevada perda de massa, principalmente quando a atmosfera de sinterização foi argônio. As amostras à base de carbeto de silício, com adição de polímeros atingiram densidade de até 3,15 g/cm3 quando pirolisadas a 900 °C em N2 e sinterizadas a 1950 °C, em atmosfera de nitrogênio. Para as amostras à base de alumina foram utilizados os aditivos poliméricos PMHS, PMS (polimetilsilsesquioxano) e PPS (polifenihnetilvinilhidrogenosilsesquioxano) e as sinterizações foram realizadas a 1650 °C e 1700 °C, não havendo variações significativas nas densidades obtidas nas duas temperaturas. Nos materiais com adição de PMHS foram obtidos compósitos de alumina e mulita, sendo que os grãos de mulita foram formados intergranularmente. Nas amostras contendo PMS ou PPS a distribuição das feses formadas, Si2Al4O4N4 e Si2ON2, foi bastante heterogênea, A obtenção de compósitos cerâmicos utilizando pequenas adições de polímeros precursores cerâmicos mostrou-se viável para materiais à base de alumina, sendo uma rota simples de conformação, com grande potencial para a obtenção de peças com geometria complexa. / The effects of the addition of precursor polymers on sintering, microstructure, hardness and fiacture toughness of silicon carbide and alimiina ceramics were studied. The ceramic raw materials were characterized by semi-quantitative analysis by X-ray fluorescence, particle size by laser diffraction, specific surface area by gas adsorption and microstructural analysis by scanning elecfron microscopy. The polymers were analyzed by thermogravimetry. The sintering kinetics of cold-pressed specimens was studied by dilatometry. The sintered materials were characterized by evaluation of apparent density by the Archimedes technique and/or helium picnometry, by mercury porosimetry, by X-ray diflftaction, by evaluation of total carbon content, by scanning and transmission electron microscopy, by enetgy dispersion X-ray spectrometry, and by Vickers indentation analysis for determining hardness and fiacture toughness. AI2O3 (4wt.%) and Y2O3 (4wt.%) and polymetylhydrogenosiloxane and polymetylhydrogenossiloxane with D4Vi were the sintering aids for SiC. The processing procedures were material cure, pyrolysis and sintering (1850 X and 1950 °C/1 h, Ar or N2). High mass loss was measured in silicon carbide based ceramics, mainly under argon. Silicon carbide based ceramics with polymer sintering aids achieved 3.15 g/cm3 density after pyrolysis at 900 °C under N2 and sintering at 1950 °C under nitrogen. PMHS, PMS and PPS polymer sintering aids were used for almnina based ceramics sintering carried out at 1650 °C and 1700 °C, without significant difference in the final density. Addition of PMHS yielded alumina and mullite composites, with intergranular mullite grains. Heterogeneous Si2AI4O4N4 and Si2ON2 phases were obtained in specimens with PMS or PPS, The preparation of ceramic composites using small amounts of precursor polymers showed a suitable process for alumina-based ceramics, a simple forming route, with high potential for the fabrication of complex shape pieces.

As células a combustível são dispositivos eletroquímicos que produzem energia elétrica limpa. Entretanto, o seu uso em larga escala ainda é limitado devido às enormes perdas de potencial que ocorrem no cátodo em função da lenta cinética da reação de redução de oxigênio (RRO). Portanto, é necessário o desenvolvimento de novos catalisadores que sejam viáveis comercialmente, apresentem alta atividade eletrocatalítica e grande estabilidade para esta reação. Neste trabalho, a RRO foi estuda em meio ácido em eletrocatalisadores à base de platina suportadas em carbetos de tungstênio (WC) e molibdênio (MoC) com diferentes estruturas cristalinas. Os carbetos foram preparados pelos métodos de decomposição térmica (MDT) e carburação (MC) e, subsequentemente, utilizados para a preparação dos catalisadores do tipo Pt-carbeto pelo método do ácido fórmico ou borohidreto de sódio. Estes materiais foram caracterizados por difração de raios-X (XRD), espectroscopia de energia dispersiva de raios-X (EDX), microscopia eletrônica de transmissão (TEM), espectroscopia de absorção de raios-X (XAS) in situ e espectroscopia de fotoelétrons excitados por raios-X (XPS). As medidas eletroquímicas foram realizadas em meio ácido usando a técnica de eletrodo disco-anel rotativo de camada fina. Os resultados de XRD, em conjunto com as medidas de TEM, indicaram que o MDT e MC fornecem carbetos com estruturas cúbicas (&beta;-WC1-x e &delta;-MoC) e hexagonais (&alpha;-WC e &alpha;-Mo2C) com tamanho de partícula < 2 nm e entre 10 e 40 nm, respectivamente. Os estudos eletroquímicos evidenciaram que os catalisadores Pt-carbeto/C apresentam diferenças de atividade específica (SA) e mássica (MA) para a RRO em função da estrutura e composição e que todos os catalisadores do tipo Pt-&beta;-WC1-x/C, Pt- &alpha;-Mo2C/C e Pt-&delta;-MoC/C e Pt2Ni- &alpha;-WC/C apresentam maior SA em comparação à Pt/C. A concentração de carbeto na matriz de carbono também influenciou significativamente os valores de SA, porém não há uma tendência clara que permita unificar um efeito comum para todos os catalisadores. Dentre todos estes catalisadores, o Pt-&beta;-WC1-x40/C e o Pt-&delta;-MoC40/C são 3,6 e 2,5 vezes mais ativos (SA) em comparação ao catalisador Pt/C. Também foi observado que a via predominante da RRO envolve cerca de 4 elétrons por molécula de oxigênio, com baixa produção de H2O2 em todos os casos. No caso dos materiais baseados em Pt-&beta;-WC1-x/C, análises de XANES in situ mostraram um pequeno aumento na ocupação da banda 5d da Pt, o que deve levar a uma interação Pt-OHx mais fraca, aumentando a cinética da RRO como observado. Além disso, os resultados de XPS evidenciam que a melhora na atividade específica está relacionada a um efeito sinérgico entre Pt ou Pt2Ni com as espécies do tipo WOx ou MoOx; exceto para os catalisadores de ligas Pt2Ni, para o qual os resultados de XPS indicaram a presença de WC na superfície do catalisador. Os testes de estabilidade mostraram que apesar do Pt/C ser mais ativo após 12.000 ciclos, o catalisador Pt-&alpha;-WC20/C é o mais estável em relação a sua AS, indicando que as interações benéficas das fases remanescente de Pt e WC/WOx são mantidas após o teste de durabilidade. / Fuel cells are electrochemical devices that produce clean electrical energy. However, their use on large scale is still limited due to the enormous potential losses that occur at the cathode due the slow kinetics of the oxygen reduction reaction (ORR). Therefore, the development of new catalysts that are commercially viable, present high electrocatalytic activity and great stability for this reaction is still necessary. In this work, the ORR was studied in acid medium on platinum-based electrocatalysts supported on tungsten (WC) and molybdenum (MoC) carbides with different crystalline structures. Carbides were prepared by the thermal decomposition (TDM) and a carburizing (CM) methodes and, subsequently used for the preparation of the Pt-carbide type catalysts by the formic acid or sodium borohydride method. These materials were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The electrochemical measurements were performed in acid medium using the thin-layer rotating disk-ring electrode technique. The results of XRD, together with those of TEM, indicate that the MDT and MC provide carbides with cubic (&beta;-WC1-x and &delta;-MoC) and hexagonal ( &alpha;-WC and &alpha;-Mo2C) structures with particle size < 2 nm and between 10 and 40 nm, respectively. The Pt-carbide/C catalysts exhibit differences in the specific activity (SA) and mass (MA) for the ORR as a function of structure and composition. All catalysts of the types Pt-&beta;-WC1-x/C, Pt- &alpha;-Mo2C/C, Pt-&delta;-MoC/C and Pt2Ni- &alpha;-WC/C presented higher SA as compared to Pt/C. The concentration of carbide in the carbon matrix significantly influence the SA, but no tendency was clearly found so to identify a common effect catalysts overall. Among all catalysts, Pt-&beta;-WC1-x40/C and Pt-&delta;-MoC40/C are 3.6 and 2.5 times more active (SA) compared to the Pt/C catalyst. It was also observed that the predominant ORR mechanism involve &cong; 4 electrons per oxygen molecule, and so with negligible production of H2O2. In the case of the materials based on Pt-&beta;-WC1-x/C, in situ XANES analyzes evidenced a small increase in the Pt 5d-band, occupancy leading to a weaker Pt-OHx interaction, increasing ORR kinetics as observed. In addition, XPS results show that the improvement in SA is related to a synergistic effect between Pt or Pt2Ni with WOx or MoOx species, except for the Pt2Ni alloy catalysts, for which the XPS results indicated the presence of some WC on the catalyst surface. Stability tests show that although Pt/C is more active after 12,000 cycles, the Pt-&alpha;-WC20/C catalyst is the most stable relative to its SA, indicating that the beneficial interactions of the remaining phases of Pt and WC/WOx are maintained after the durability test.

Made available in DSpace on 2017-07-21T20:42:38Z (GMT). No. of bitstreams: 1 JOEVERTON IURP PEREIRA.pdf: 10346836 bytes, checksum: 7a9cb51468fd6451b35517266553a7b7 (MD5) Previous issue date: 2012-09-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The alloys of Ni-Cr-Al-C system were synthesized by mechanical alloying, as a proposal of an alternative processing route, in order to improve the microstructure control regarding to a more homogeneous distribution of both Ni3Al precipitates and dispersed chromium carbides, characteristics phases of NiCrAlC alloys. It were chosen 15 compositions, comprised within a range present in the literature, with carbon content variations between 0,5 to 1,5%wt, chromium between 7,5 to 11%wt, and aluminum in the same way as chromium. The milling products were compressed and sintered at 1200°C for a period of 2 hours, and the results of the analysis by Xray diffraction, optical microscopy, scaning electron microscopy (SEM), energy dispersion analysis (EDS) and hardness were compared with some foundry products presents in the literature. The NiCrAlC alloys studied at this work presented nanometric Ni3Al ( phase) precipitates, which provided a hardness that is comparable to that of alloys made by casting followed by heat treating, in spite of exhibit a significant porosity. At the same way, the chromium carbides showed finely dispersed in the matrix, and homogeneously distributed. / Ligas do sistema Ni-Cr-Al-C foram sintetizadas por moagem de alta energia, como proposta de uma rota alternativa de processamento, visando um melhor controle da microestrutura no que concerne a uma distribuição mais homogênea tanto dos precipitados de Ni3Al quanto dos carbetos de cromo dispersos, fases características das ligas NiCrAlC. Foram escolhidas 15 composições compreendidas dentro de uma faixa presente na literatura, com variações de carbono entre 0,5 e,1,5%, de cromo entre 7,5 e 11%, e alumínio da mesma forma que o cromo. Os produtos de moagem foram compactados e sinterizados à 1200°C por um período de 2h, e os resultados das análises de difração de raios x, microscopia ótica e eletrônica, análise por dispersão de raios x e dureza realizadas foram comparados com algumas ligas fundidas presentes na literatura. As ligas NiCrAlC estudadas apresentaram precipitados nanométricos de Ni3Al (fase ), que proporcionaram uma dureza comparável à das ligas produzidas por fusão seguidas de tratamento térmico, apesar de exibirem uma porosidade significativa. Do mesmo modo os carbetos de cromo mostraram-se finamente dispersos na matriz, e homogeneamente distribuídos.

Primary Water Stress Corrosion Cracking (PWSCC) of Alloy 600 and similar Ni-Cr-Fe alloys is regarded as one of the most important challenges to nuclear power plant operation. During the past decades the majority of research has focused on PWSCC crack growth rate measurements in order to assess the lifetime of real components and to develop empirical models for crack propagation. However, the incubation and initiation stages of PWSCC have the same or even greater importance than the propagation stage, particularly because SCC can be undetected for more than 20 years before the occurrence of a rapid and catastrophic failure. There is, therefore, the scientific need to understand the mechanisms playing a fundamental role in the formation and development of intergranular cracks embryo, the so-called SCC initiation "precursor events", in order to be able to predict and mitigate the occurrence of PWSCC. Amongst all the models proposed for SCC initiation, the internal oxidation mechanism proposed by Scott and Le Calvar in 1992 appears to be the most comprehensive. Although the internal oxidation mechanism is widely accepted, it still requires further elucidation, especially in terms of enhanced grain boundary diffusivity and the role of intergranular carbides on the oxidation mechanism. The present work has focused on the initial stages of intergranular oxidation of solution-annealed (SA) and thermally-treated (TT) Alloy 600 with the aim of understanding the active mechanism responsible for the enhanced intergranular oxide penetration kinetics. The material was tested in simulated PWR primary water at 320°C, high-pressure hydrogenated-steam at 400°C and low-pressure H2-steam environment at 480°C at potential more reducing than the Ni/NiO equilibrium. The detailed microstructural characterization was conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and analytical transmission electron microscopy (ATEM) and demonstrated that Alloy 600SA is susceptible to diffusion-induced grain boundary migration (DIGM), preferential intergranular oxidation (PIO) and localised Cr and Fe depletions at the grain boundaries. The similar analyses performed on Alloy 600TT demonstrated reduced susceptibility to PIO and grain boundary migration. Further, detailed analyses confirmed that intergranular carbides were readily oxidized/consumed in all 3 environments and acted as Cr reservoir/O trap. These results shed additional light on the "precursor events" for PWSCC of Alloy 600, especially on the mechanism responsible for the enhanced Cr and O diffusivity and on the mechanism responsible for the enhanced Alloy 600TT SCC initiation resistance. Moreover, the strong similarities in the Alloy 600 oxidation behaviour observed for the 3 different environments and at the 3 different temperatures suggested that the same PIO mechanism is active in both steam and water and at temperatures between 320°C and 480°C. These results strongly support the possibility of using the low-pressure H2-steam environment as a substitute environment to accelerate PWSCC initiation without changing the mechanism.

<p>The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials.</p><p>Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics.</p><p>The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface.</p><p>Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO₂, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.</p>

In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF2single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr3C2 were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach. In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)7C3 carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software. In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M23C6, M7C3 and M5C2 (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M7C3/graphite was partly constructed at 1073 K. In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)4(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased. Keywords: Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;

In this thesis 9-12 % Cr steels, used for high-temperaturecomponents in fossil-fired power plants are considered. Thecreep strength of thees steels depend on their microstructurethat consists of a matrix of tempered lath martensite withdensely distributed precipitates. The mechanical properties arestrongly influenced by precipitates present in the matrix andthe more densely distributed they are the higher is thehardening effect. These particles nucleate, grow and coarsenduring use in power plants, leading to a degradation ofmechanical properties. In this thesis the nucleation andcoarsening behaviour of the precipitates in the Cr steels aresimulated by new models and the results are compared withtransmission electron microscopy (TEM) observations of testedmaterials. A model of the nucleation of MX, which mainly isvanadium-nitrides, is presented. MX precipitates are assumed tonucleate on dislocations during annealing. The model takes intoaccount the full multicomponent thermodynamical behaviour ofthe system as well as the strain energy caused by a puredilatoric strain and the shape of the nucleus. The calculationsyield a critical size and shape represented by an oblatespheroid with a radius of a few nanometers and a thickness ofan atomic layer. This is in agreement with TEM observations ofsupercritical particles. A new coarsening model, which takes into account themulticomponent effects, is presented and it has been tested ondifferent systems to validate the model.     Four different carbides, Cr7C3, Mo6C, VC and NbC, in austenitic matrix as well asγ ' in ternary Ni-base super-alloy systems (Ni-Al-Mo)were compared with measurements from literature.     TiC-particles in austenitic stainless steel, ASTM 316Ti,were considered. The simulations were performed taking intoaccount iron and 7 alloying elements. The measurements wereperformed with TEM on samples that had been heat-treated at900° C.     MX and M23C6in a 9 % Cr steel were investigated with energyfiltering transmission electron microscopy (EFTEM) andcompared with simulations. The used samples had been heattreated for various periods of time at 600 and 650° Cfor up to 26 000 h. The agreement of the simulations with the experiments wasgood in all cases when reasonable values of the interfacialenergy, the only adjustable parameter, were chosen. Coarsening simulations were also performed to investigatethe influence of changes in composition on the coarsening rate.For MX, in a 9 % Cr steel, the coarsening rate is almostindependent of the V/Nb ratio but highly dependent on theN-content. Also the effect on the coarsening rate for M23C6by adding Co to a Cr steel was investigated bysimulation. Co is known to increase the resistance totempering. The results show that a final average radius of thecarbides after 30 000 h at 600° C decreases with 30 % witha Co addition of 10 mass %. <b>Keywords:</b>Cr steels, nucleation, coarsening, model,DICTRA, precipitates, carbides, carbo-nitrides, MX, VN, M23C6, TiC, TEM, EFTEM, Curie-temperature

The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials. Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics. The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface. Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO2, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.

The present thesis deals with a theoretical description of issues regarding chemical bonding, structure and stability of transition metal carbides and multilayered structures. First principles density functional theory has been used extensively to investigate the properties of alloyed solutions of transition metal carbides. Joint theoretical and experimental investigations have shown that there is a driving force for carbon to be released from these ternary carbide systems as a response to the alloying. This release of carbon was shown to yield favorable lubricating properties in the case of alloyed solutions of Ti-Al-C, that were not present in the case of pure TiC, a property that can be used to design new materials that combine high hardness with favorable tribological properties. From calculations of the activation energy of C diffusion in the vicinity of substitutional transition metal impurities (M) in TiC, it is found that the mobility of C atoms is increased due to the presence of the impurities. The lowering of the activation energy barriers suggests that the mobility of C in alloyed solutions of Ti-M-C is increased and will be more pronounced at lower temperature than for C diffusion in TiC. The magnetic properties of alloyed solutions of Ti-Fe-C has been investigated using both theory and experiment. Theoretical calculations reveal that the magnetic moment and the critical temperature increase when increasing the Fe content as well as when lowering the C content in the system. Furthermore, the magnetic exchange parameters between Fe atoms were found to clearly reflect changes in the chemical bonding when varying the C content. Experimentally the magnetic properties were found to be rather substantial. Furthermore, the magnetic properties changes upon annealing due to the formation of Fe-rich and Fe-poor regions in the system. After long enough annealing times precipitates of α-Fe are formed which is consistent with theoretical predictions. The interaction between TiC(111) surfaces and C in the form of graphite has also been investigated. For these systems it was found that graphite was rather strongly bonded to the carbide surface and that the atomic as well as electronic structure at the interface depend on the termination of the carbide surface. This research was motivated by the recent interest in graphene, but also to investigate how carbide grains interacts with C when dispersed in a carbon matrix. A model for the calculation of structural parameters in multilayer structures has been presented and evaluated. The model is based on classical elasticity theory and uses the elastic constants of the materials constituting the multilayer as the only input.

Advanced alloys often present precipitated carbides and nitrides in their microstructure following exposure to elevated temperatures. These secondary phases are usually undesirable, because potentially deleterious for the corrosion and mechanical performances of the material. Carbides and nitrides are enriched in key alloying elements that are subtracted from their surrounding matrix areas, creating alloying element depleted zones, which might become initial sites for corrosion initiation. In this study, the influence of micro- and nano-sized precipitated carbides and nitrides on the corrosion initiation of biomedical CoCrMo alloys and duplex stainless steels has been investigated at microscopic scale, by using a combination of local probing techniques. The microstructures of the alloys were first characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and magnetic force microscopy (MFM). The Volta potential mapping of carbides and nitrides revealed their higher nobility compared to the matrix, and particularly compared to their surrounding areas, suggesting the occurrence of some alloying element depletion in the latter locations, which may lead to a higher susceptibility for corrosion initiation. In-situ electrochemical AFM studies performed at room temperature showed passive behavior for large potential ranges for both alloy families, despite the presence of the precipitated carbides or nitrides. At high anodic applied potential, at which transpassive dissolution occurs, preferential dissolution started from the areas adjacent to the precipitated carbides and nitrides, in accordance with the Volta potential results. Thus, the presence of carbides and nitrides doesn’t largely affect the corrosion resistance of the tested advanced alloys, which maintain passive behavior when exposed to highly concentrated chloride solutions at room temperature with no applied potential. The effect of nitrides on the corrosion initiation of duplex stainless steels was investigated also at temperatures above the critical pitting temperature (CPT). Depending on the type, distribution and size range of the precipitated nitrides different corrosion behaviors were observed. Intragranular (quenched-in) nano-sized nitrides (ca. 50-100 nm) finely dispersed in the ferrite grains have a minor influence on the corrosion resistance of the material at temperatures above the CPT, while larger intergranular (isothermal) nitrides (ca. 80-250 nm) precipitated along the phase boundaries cause a detrimental reduction of the corrosion resistance of the material, in particular of the austenite phase / <p>QC 20130927</p>