The evolution of materials at an atomistic level may have vital consequences for the properties of materials. Therefore, modelling long time scale behaviour of defects in a material is very important, particularly for those used in nuclear power plants. The materials used in nuclear power plants should have good mechanical properties to overcome the corrosive environment and high temperature. Examples of these materials are the austenitic stainless steel and the Ni-based alloys due to their high temperature properties. Molecular Dynamics (MD) and on the fly Kinetic Monte Carlo (otf-KMC) techniques have been used to model the radiation damage in austenitic stainless steel and the Ni-based alloys. This thesis represents the main findings obtained. Three potentials were implemented and used to study radiation damage in austenitic stainless steel. Structural properties such as the elastic constants for the point defects in the pure metals were first calculated. This was followed by calculating the formation energies and migration energies of vacancy and self interstitial defects in the pure metals. Different calculations were performed using each potential on the ternary alloy (Fe with 10 at.% Ni and 20 at.% Cr) and the binary alloy (Ni with 20 at.% Cr) . For example, the segregation in these alloys was investigated using Monte Carlo simulations and results obtained for both alloys at high temperature MD. Furthermore, the vacancy formation energies were calculated for both alloys using all the potentials. Radiation damage at Grain Boundaries (GBs) in fcc Ni and a Ni-Cr binary alloy has been studied using MD and otf-KMC techniques. From the results obtained, the mobility of interstitials were found to be higher than that of vacancies and tend to move quickly to the GB. Vacancies are found to migrate to the GB if they are near otherwise they tend to form clusters in the bulk. During the simulations, interesting mechanisms were observed for the point defects migration and recombinations. Large roughening at the GB was observed, especially in the alloy system and overall the total number of defects accumulated on the GB after multiple collision cascades were relatively small. The radiation in fcc Ni resulting from low energy collision cascades was also modelled using MD and otf-KMC techniques. This part of work aimed replicating the observations seen in experiment and trying to understand them. Recombinations between vacancies and interstitials were found to happen from large distances with low barriers. Most defects produced from low energy collision cascades were found to recombine or interstitials were found to form clusters. Modelling the evolution of the vacancies shows the possibility of producing Stacking Fault Tetrahedra (SFT) which were found to dissociate at 200°C.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Este trabalho teve como objetivo avaliar as caracterÃsticas operacionais (geometria da solda e diluiÃÃo) e metalÃrgicas (microestrutura, composiÃÃo quÃmica, microdureza e resistÃncia à corrosÃo) de revestimentos compostos pela mistura das ligas de nÃquel Inconel 625 e Hastelloy C-276 depositadas simultaneamente pelo processo MIG/MAG duplo arame sobre chapas de aÃo ASTM 516 Gr 60. Inicialmente foi realizado um estudo preliminar para conhecer os parÃmetros iniciais de soldagem e os limites operacionais do processo. Em seguida, foram realizados ensaios em simples deposiÃÃo atravÃs da aplicaÃÃo do MÃtodo Taguchi com intuito de selecionar os melhores parÃmetros de cada fator de controle avaliado em um nÃmero reduzido de experimentos. Posteriormente, foram realizados revestimentos iniciais a partir dos parÃmetros selecionados. Em seguida foram realizados revestimentos definitivos nos quais foram feitas anÃlises das caracterÃsticas geomÃtricas e diluiÃÃo, alÃm de uma caracterizaÃÃo metalÃrgica com uso de microscopia Ãtica, microscopia eletrÃnica de varredura e anÃlise de composiÃÃo quÃmica. Ainda foram realizadas anÃlises do perfil de microdureza dos revestimentos, bem como anÃlise de resistÃncia à corrosÃo. Os resultados mostraram que os nÃveis de diluiÃÃo global mÃnimos alcanÃados nos ensaios definitivos foram de aproximadamente 22%. A microestrutura dos revestimentos soldados com a mistura das ligas Inconel 625 e Hastelloy C-276 foi constituÃda de uma maneira geral por uma matriz gama com precipitaÃÃo de fases secundÃrias ricas em niÃbio, molibdÃnio e titÃnio. A anÃlise de composiÃÃo quÃmica revelou que o percentual de Fe na solda à crescente com o aumento da diluiÃÃo. O aumento de Mo, W e reduÃÃo do Fe no metal de solda contribuiu para a formaÃÃo de fases secundÃrias. Em relaÃÃo à microdureza, nÃo houve diferenÃas significativas entre o primeiro e o Ãltimo cordÃo de solda e os valores obtidos foram adequados para revestimento. Jà para a resistÃncia à corrosÃo, algumas amostras atingiram temperatura crÃtica de pite (TCP) de 80ÂC e houve uma relaÃÃo entre os fatores de controle os principais elementos de liga presentes nos revestimentos. / The aim of this work was to evaluate the operational (weld geometry and dilution) and metallurgical characteristcs (microstructure, chemical composition, microhardness and corrosion resistance) of Inconel 625 and Hastelloy C-276 nickel-based alloys weld overlay deposited simultaneously by GMAW Double Wire process on steel plate ASTM 516 Gr 60. Initially a previous study was carried out in order to know the early welding parameters and the operational limits of process. Next, single passes welding trials was conducted by applying Taguchi method aiming to select the best parameters of each control factor studied in a reduced numbers of experiments. After, weld overlays were made based on selected parameters. Next, the definitive weld overlays were deposited on steel plates and thus were analysed the geometric features, dilution and metallurgical characterization using optical microscopy and scanning electron microscopy, also a chemical composition analysis was carried out. The profile of microhardness were also performed, as well as analysis of corrosion resistance. The results showed that the minimal global dilution levels measured on definitive weld overlays were 22% approximately. The microstructure of the weld overlays deposited by mixing Inconel 625 and Hastelloy C-276 was constituted generally by a gamma matrix and secondaries phases enriched in niobium, molybdenum and titanium. The chemical composition analysis revealed that the the percentage of Fe on weld increases when dilution levels rises. The increase in Mo, W and reduction in Fe content on the weld metal contributed to the formation of secondaries phases. Regarding the microhardness, no significative differences between first and last weld bead and the microhardness values obtained were appropriate for weld overlays. For corrosion resistance, some of the samples supported Critical Pitting Temperature (CPT) of 80 ÂC and there were a relation between the controled factors and principals alloy elements presents in the weld overlays.

AgÃncia Nacional do PetrÃleo / CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / A soldagem de revestimento com ligas à base de nÃquel constitui uma interessante opÃÃo para a construÃÃo e o reparo de equipamentos para aplicaÃÃes em ambiente de corrosÃo severa, permitindo alcanÃar as caracterÃsticas desejadas (resistÃncia à corrosÃo) com uma significativa reduÃÃo de custos. Neste contexto, alguns estudos indicam que o processo de soldagem TIG com alimentaÃÃo de arame pode ser uma boa opÃÃo, desde que os parÃmetros de soldagem sejam corretamente ajustados. O objetivo do presente trabalho foi avaliar e otimizar os principais parÃmetros de soldagem, bem como avaliar o comportamento metalÃrgico de revestimentos de ligas de nÃquel depositados pelo processo TIG com alimentaÃÃo de arame frio sobre chapas de aÃo C-Mn. TrÃs ligas à base de nÃquel foram avaliadas no presente estudo: AWS ER Ni-Cr-Mo 3; AWS ER Ni-Cr-Mo 4 e AWS ER Ni-Cr-Mo 14. As soldagens foram realizadas numa primeira anÃlise em simples deposiÃÃo e, posteriormente, na formaÃÃo de camadas de revestimento, variando diversos parÃmetros operacionais. A caracterizaÃÃo microestrutural consistiu de anÃlises de microscopia Ãtica, microscopia eletrÃnica de varredura (MEV) e de transmissÃo (MET), espectroscopia de raio-X dispersivo em energia (EDS), difraÃÃo de elÃtrons retroespalhados (EBSD) e difraÃÃo de raios-X (DRX). Ensaios de polarizaÃÃo eletroquÃmica e desgaste com roda de borracha foram realizados para avaliar a resistÃncia à corrosÃo e ao desgaste, respectivamente. Os resultados mostraram que o processo TIG com alimentaÃÃo automÃtica de arame frio demonstrou ser uma opÃÃo interessante para a soldagem de revestimentos de ligas de nÃquel, podendo depositar em apenas uma camada, revestimentos que alcanÃam nÃveis de diluiÃÃo inferiores ou muito prÃximos de 5%, caracterÃstica favorÃvel para garantir boas propriedades aos revestimentos. A microestrutura dos revestimentos depositados com a liga AWS ER Ni-Cr-Mo 3 foi constituÃda por uma matriz γ com fases secundÃrias ricas em Nb (fase Laves e carboneto/nitreto de Ti e Nb). Estruturas complexas constituÃdas por um nÃcleo de nitreto de titÃnio-niÃbio (TiNbN) e uma casca de carboneto de niÃbio (NbC) ou niÃbio-titÃnio (NbTiC) foram observadas. A microestrutura dos revestimentos depositados com as ligas AWS ER Ni-Cr-Mo 4 e AWS ER Ni-Cr-Mo 14 foi constituÃda por uma matriz γ e fases secundÃrias ricas em Mo (fases σ, P e μ). Foi identificado um empobrecimento de Mo ao redor dos precipitados ricos em Mo, tornando esta regiÃo susceptÃvel à corrosÃo. O ensaio de polarizaÃÃo eletroquÃmica nÃo se mostrou capaz de avaliar o comportamento dos revestimentos quanto à corrosÃo por pites. O baixo nÃvel de dureza da matriz associada com uma pequena fraÃÃo volumÃtrica e dimensÃo dos precipitados resultou numa baixa resistÃncia ao desgaste abrasivo. / The weld overlay with nickel-based alloys is an interesting alternative for the construction and repair of equipments for applications in severe corrosive medium, allowing reach the desirable characteristics (corrosion resistance) with a significant reduction of cost. In this context, the GTAW cold wire feed process can be a good option for overlay deposition, since the welding parameters are correctly adjusted. The main objective of the present work was to evaluate and to optimize the operational and metallurgical characteristics of nickel-based alloys weld overlays deposited by the GTAW cold wire feed on C-Mn steel plates. The nickelbased alloys AWS ER Ni-Cr-Mo 3; AWS ER Ni-Cr-Mo 4 and AWS ER Ni-Cr-Mo 14 were studied in the this work. The welds were performed initially in single weld bead and later in multipass to produce the coating, changing several operational parameters. The microstructure was investigated using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive of X-ray spectroscopy (EDS), electron backscattering diffraction (EBSD) and X-ray diffraction (XRD). Potentiodynamics polarization analysis and wheel rubber wear test were used to evaluate the corrosion and abrasive wear resistance, respectively. The results showed that the GTAW cold wire feed process demonstrated to be an interesting option for the weld overlay with nickel-base alloys, could deposit in just one layer, coatings with dilution levels inferior or very close to 5%, important characteristics to assure a good performance of the coatings. The microstructure of the coatings deposited with AWS ER NiCrMo-3 alloy was constituted by the γ-matrix and secondary phases Nb-rich (Laves phase and Nb-Ti carbide/nitrides). It was observed that the precipitates with cuboidal and cruciform morphologies are formed by a complex structure constituted by titanium-niobium nitride (TiNbN) as nucleus and niobium carbide (NbC) or niobium-titanium (NbTiC) carbide as shell. The microstructure of the coatings deposited with the AWS ER NiCrMo-4 and AWS ER NiCrMo-13 alloys were constituted by the γ-matrix and secondary phases Mo-rich (σ, P and μ phases). It was identified a Mo depletion around the precipitates rich in Mo, causing a drop in the corrosion resistance. The potentiodynamic polarization was not shown capable to evaluate the behavior of the coatings for the pitting corrosion. The low level of hardness of the matrix associated with a small volumetric fraction of secondary phases (hard precipitates) resulted in a low abrasive wear resistance.

Molten salt reactors (MSRs) using chloride-based salt-matrixes as coolants or fuels are a promising option for advanced nuclear reactors, but the extreme temperatures and corrosivity of molten salts pose a challenge for implementation. Molten MgCl2-NaCl-KCl is a viable candidate for MSRs that is considered in this work. Thermochemical properties are derived from electrochemical tests that aid in characterizing the properties of salts. To study these properties, some work has proposed using a three-electrode system with a reference electrode housed in a ceramic membrane. This research aims to develop a stable high-temperature reference electrode using a ceramic membrane that is then applied to develop an on-line monitoring system of Ni-Cr alloy corrosion in chloride salt. A mullite tube used as the membrane of a Ni(II)/Ni reference electrode in molten MgCl2-NaCl-KCl is studied. The performance of two different membrane thicknesses (1.325mm and 0.255mm) was studied in temperature ranges from 635oC to 835oC and data collected on the calculated formal potential of the Ni(II)/Ni system. Tests indicated that the results were stable and repeatable, and the formal potential for both systems differed from the previous experimental data by 0.12V at most, indicating that the system can be applied as an effective reference electrode. Using the reference electrode, on-line monitoring the corrosion of Ni-15wt.%Cr, Ni-20wt.%Cr, and Ni-30wt.%Cr was studied for 120 hours in MgCl2-NaCl-KCl. The on-line measurements showed the concentration changes of dissolved Cr and Ni by corrosion in the bulk molten salt. This work confirms that Ni(II)/Ni reference electrodes with a mullite tube membrane are stable and effective in molten chloride salt systems, particularly MgCl2-NaCl-KCl. The mullite membrane prepared by the manufacturer may be used directly for electrochemical applications without polishing, simplifying the reference electrode manufacturing process, and making it easier to replicate. The use of a Ni(II)/Ni reference electrode provides an avenue to study a different range of salt systems than previous reference electrodes allowed, particularly alloys in chloride salts at high temperatures. This work also confirms that the mullite tube may be used to perform on-line analysis of alloy corrosion in high temperature molten chloride salts. The study of Ni-Cr alloys in chloride salts better prepares the nuclear industry to select coolant salts and alloy containers with the best set of thermochemical and corrosion resistant characteristics for MSRs. / Master of Science / The United States receives approximately 18% of its energy from nuclear technology. Many of the reactors supplying this energy are at the end of their lifecycle and the decommissioning of some of these plants has already begun. In order to replace this older generation of nuclear reactors, a safer and cheaper option has been suggested: Molten Salt Reactors. Molten salt reactors (MSRs) using high temperature salts as a fuel or coolant are a promising option, but the extreme conditions of molten salts pose a challenge for construction and use of MSRs. Molten MgCl2-NaCl-KCl is a salt being considered for MSR application, and is considered in this work. Properties of the salts considered for MSRs are being studied diligently before implementation of these reactors. Electrochemical tests are used to study and monitor these properties. These electrochemical tests use a three-electrode system with a reference electrode housed in a membrane. In this work, a mullite tube is used as a ceramic membrane for a reference electrode in molten MgCl2-NaCl-KCl. The performance of two different membrane thicknesses (1.325mm and 0.255mm) was studied in temperature ranges from 635oC to 835oC. Results indicate that the system is an effective reference electrode. Using this innovative reference electrode, a method of monitoring on-line corrosion of Ni-15wt.%Cr, Ni-20wt.%Cr, and Ni-30wt.%Cr alloys was studied for 120-hour time periods during exposure to MgCl2-NaCl-KCl. This work confirms that reference electrodes with a mullite membrane may be used for electrochemical applications when studying molten chloride salts. The use of a Ni(II)/Ni reference electrode with a mullite membrane provides an avenue to study a different range of salt systems than previous reference electrodes and ceramics allowed, particularly chloride salts. Additionally, this mullite membrane Ni(II)/Ni reference electrode system may be used for monitoring on-line corrosion of Ni-Cr alloys in chloride salt systems.

No description available.

No description available.

En esta tesis, se analiza la relación entre el envejecimiento y la evolución microestructural en las aleaciones refractarias conocidas como HP40-Nb y ET45 micro. Se presentan resultados obtenidos por microscopía óptica y electrónica de barrido con microanálisis por rayos X, así como ensayos isotérmicos de dilatometría. Además, se aplicó el modelo cinético de Johnson-Mehl-Avrami-Kolmogorov para estudiar la nucleación y crecimiento de la precipitación secundaria de carburos del tipo Cr23C6. Estas aleaciones, de microestructura dendrítica, presentan una red de carburos primarios de tipo NbC y Cr7C3 en el caso de la ET45 micro, y de tipo NbC y Cr23C6 en el caso de la aleación HP40-Nb. En ambas aleaciones, se observa una zona de deplexión libre de precipitados secundarios adyacente a la red de carburos eutécticos primarios y que presenta menor dureza que la matriz austenítica. En la aleación HP40-Nb, se encontraron zonas ricas en silicio y níquel en los bordes de los carburos primarios de niobio, mientras que no se halló presencia de estos átomos en las zonas centrales de estas partículas. Ensayos isotérmicos de dilatometría indicaron que, a 1073 y 1098 K, la aleación sufre una contracción, la cual se estima que está asociada a la diferencia entre coeficientes de dilatación térmica entre la matriz y los carburos eutécticos primarios, mientras que a 1123 y 1148 K la aleación experimenta una expansión térmica. La transformación del carburo de niobio en el siliciuro rico en níquel y niobio, induce un cambio de volumen significativo en la celda unitaria, lo que se manifiesta en aquellos ensayos de dilatometría para temperaturas en las cuales la muestra se expande. Se encontraron indicios de la presencia de un siliciuro coincidente con la fase G para muestras envejecidas a 1123 K a tiempos equivalentes a la duración del ensayo de dilatometría a dicha temperatura. La transformación del carburo de niobio hacia el siliciuro rico en níquel y niobio, comienza en la interfaz de los carburos de niobio con la matriz, progresando hacia el interior de los mismos con el desarrollo del envejecimiento. Luego de tratamientos térmicos de envejecimiento a temperaturas de 1073, 1173 y 1273 K por cortos períodos de tiempo, se halló que la precipitación secundaria de carburos de tipo Cr23C6 en la matriz ocurre en una sola etapa. En el caso de la aleación HP40-Nb, este fenómeno presentó una energía de activación de 194 kJ/mol. Análogamente, para la aleación ET45 micro, se encontró que dicha energía es de 188 kJ/mol. / In this thesis, the relationship between exposure to high temperatures and microstructural evolution in centrifugal cast alloys HP40-Nb and ET45 micro is analysed. Results obtained by optical and scanning electron microscopy with X-ray microanalysis are presented, as well as isothermal dilatometry tests. In addition, the Johnson-Mehl-Avrami-Kolmogorov kinetic model was applied to study the nucleation and growth of secondary precipitation of Cr23C6 carbides. These alloys, of dendritic-type microstructure strengthened by a network of NbC and Cr7C3 primary carbides of type in the case of the ET45 micro, and of NbC and Cr23C6 primary carbides in the case of the HP40-Nb alloy. In both alloys, there is a depleted zone which is free of secondary precipitates adjacent to the network of primary eutectic carbides and presenting lower hardness than the austenitic matrix. In the HP40-Nb alloy, areas rich in silicon and nickel were found at the edges of primary niobium carbides, while no presence of these atoms was found in the central areas of these particles. Isothermal dilatometry tests showed that at 1073 and 1098 K, the alloy undergoes a contraction, which is estimated to be associated with the difference between thermal expansion coefficients between the matrix and primary eutectic carbides, while at 1123 and 1148 K alloy undergoes thermal expansion. The transformation of niobium carbide into nickel and niobium-rich silicide induces a significant change in volume in the unit cell. Such effect is manifested during those dilatometry tests for temperatures in which the sample expands. Indications of the presence of a silicide coinciding with the G phase were found for samples aged at 1123 K at times equivalent to the duration of the dilatometry test at said temperature. It has been found that the transformation of niobium carbide into silicon rich in nickel and niobium begins at the interface of niobium carbides with the matrix, progressing inwards with the development of aging. After thermal aging treatments at temperatures of 1073, 1173 and 1273 K for short periods of time, it was found that secondary precipitation of Cr23C6 carbides in the matrix occurs in a single stage. In the case of the HP40-Nb alloy, this phenomenon had an activation energy of 194 kJ/mol. Similarly, for the ET45 micro alloy, the activation energy was found to be 188 kJ/mol. / TEXTO PARCIAL en período de teletrabajo

Neste trabalho, visando-se melhorar o desempenho ao desgaste e/ou à corrosão de revestimentos de ligas auto-fluxantes, amostras de aço SAE 1020 foram revestidas pelo processo de aspersão térmica, sendo que uma delas foi revestida apenas com uma liga auto-fluxante a base de níquel (\'NI\"CR\"B\"SI\"C\') e outras duas, às quais aplicou-se essa liga com adições de carboneto de boro ou carboneto de silício. As camadas foram caracterizadas por meio de ensaios micrográficos ópticos e eletrônicos, microdurezas, corrosão e de microdesgaste. A camada adicionada com carboneto de boro apresentou o maior nível de dureza e melhor desempenho ao desgaste, seguida pela camada com carboneto de silício e a liga auto-fluxante pura. As amostras adicionadas de carboneto de boro e silício apresentaram comportamentos similares à corrosão, com potenciais de corrosão superiores ao da amostra sem adição. / In this work, aiming to improve the wear and/or corrosion performance of self-fluxing alloys coatings, samples SAE 1020 steel were coated by thermal spray process, one of then was coated only with a self-fluxing nickel-based (\'NI\"CR\"B\"SI\"C\') alloy and two others, in which was applied this alloy with additions of boron carbide or silicon carbide. The coatings were characterized by means of optical and electronic micrography, microhardness, corrosion and microwear tests. The coating added with boron carbide showed the highest level of hardness and the better wear performance, followed by the coating with silicon carbide and the pure self-fluxing alloy. The samples added with boron or silicon carbide showed similar corrosion performance, with higher corrosion potential in comparison to that presented by the sample without addition.

Les travaux portent sur deux nuances d’alliages Ni-Cr-W, l’une contenant des précipités de W (EM722) et l’autre non (EM721). Les matériaux forgés, fournis par Aubert & Duval, présentaient des microstructures hétérogènes avec de très gros grains. Dans un premier temps, une procédure thermomécanique a été développée afin d’obtenir un état de référence avec une microstructure homogène avec une faible taille de grains et une texture relativement peu accusée. A partir de cet état de référence, l’évolution de la texture, au cours du laminage à froid (différents taux de réduction), a été analysée par diffraction des neutrons et a montré un renforcement de la texture de type Laiton. Les mécanismes de déformation ont été étudiés en microscopie électronique en transmission et par EBSD (Electron BackScatter Diffraction) afin de comprendre la formation de ce type de texture. Les bandes de cisaillement sont supposées être un facteur important pour la formation de ce type de texture. A partir des états déformés, la cinétique de recristallisation a été déterminée pour deux températures (700°C et 900°C) à partir de mesures de microdureté et par EBSD. Les mécanismes de recristallisation ont été étudiés par MET. Le mécanisme de SIBM (Strain Induced grain Boundary Migration) est le mécanisme prépondérant pour la nuance EM721. Pour l’autre nuance, EM722, possédant des précipités de W, le mécanisme de SIBM est présent, mais en plus, un second mécanisme de PSN (Particle Stimulated Nucleation) a été mis en évidence. De plus, quel que soit le mécanisme de recristallisation, une fois que les germes sont formés, ils maclent très rapidement. Par conséquent, une étude complémentaire sur l’évolution des joints de macle pendant la recristallisation primaire a été réalisée, en employant l’EBSD et surtout le système ASTAR installé dans un MET. Il a alors été montré que cette évolution dépend à la fois de la taille des grains et de l’énergie stockée par les grains pendant la déformation. Enfin, une simulation de la recristallisation primaire de type Monte-Carlo a également été mise en œuvre. Son originalité réside notamment dans sa capacité à rendre compte de la formation des joints de macles cohérents et rectilignes. / This work focuses on two grades of Ni-Cr-W alloys, one containing W precipitates (EM722) and the other not (EM721). The forged materials, provided by Aubert & Duval, had heterogeneous microstructures with very large grains. At first, a thermo-mechanical procedure was developed to obtain a reference state with a homogeneous microstructure with a small grain size and a texture relatively weak. From this reference state, the texture evolution during cold rolling (different reduction rates), was analyzed from neutron diffraction measurements and showed the strengthening of the Brass-type texture. The deformation mechanisms were studied by TEM (transmission electron microscopy) and EBSD (Electron Backscatter Diffraction) to understand the formation of this type of texture. The formation of shear bands is assumed to be the most important factor for the formation of such a texture. From the deformed states, the recrystallization kinetics was determined for two temperatures (700°C and 900°C) from micro-hardness and EBSD measurements. The recrystallization mechanisms were investigated by TEM. The SIBM (Strain Induced Grain Boundary Migration) mechanism is the predominant mechanism for the EM721 material. For the other one (EM722), which has W precipitates, the SIBM mechanism is present, but a second one (PSN: Particle Stimulated Nucleation) has also been evidenced. Furthermore, regardless of the mechanism of recrystallization, once the nuclei are formed, they twin quickly. Therefore, a further study on the evolution of twin boundaries during primary recrystallization was performed, using EBSD and especially the ASTAR system installed in a TEM. It has been shown that the twin development depends both on the grain size and the stored energy during the deformation. Finally, a Monte Carlo simulation of the primary recrystallization was also implemented. Its originality lies especially on its ability to create coherent and straight twin boundaries.

Neste trabalho, visando-se melhorar o desempenho ao desgaste e/ou à corrosão de revestimentos de ligas auto-fluxantes, amostras de aço SAE 1020 foram revestidas pelo processo de aspersão térmica, sendo que uma delas foi revestida apenas com uma liga auto-fluxante a base de níquel (\'NI\"CR\"B\"SI\"C\') e outras duas, às quais aplicou-se essa liga com adições de carboneto de boro ou carboneto de silício. As camadas foram caracterizadas por meio de ensaios micrográficos ópticos e eletrônicos, microdurezas, corrosão e de microdesgaste. A camada adicionada com carboneto de boro apresentou o maior nível de dureza e melhor desempenho ao desgaste, seguida pela camada com carboneto de silício e a liga auto-fluxante pura. As amostras adicionadas de carboneto de boro e silício apresentaram comportamentos similares à corrosão, com potenciais de corrosão superiores ao da amostra sem adição. / In this work, aiming to improve the wear and/or corrosion performance of self-fluxing alloys coatings, samples SAE 1020 steel were coated by thermal spray process, one of then was coated only with a self-fluxing nickel-based (\'NI\"CR\"B\"SI\"C\') alloy and two others, in which was applied this alloy with additions of boron carbide or silicon carbide. The coatings were characterized by means of optical and electronic micrography, microhardness, corrosion and microwear tests. The coating added with boron carbide showed the highest level of hardness and the better wear performance, followed by the coating with silicon carbide and the pure self-fluxing alloy. The samples added with boron or silicon carbide showed similar corrosion performance, with higher corrosion potential in comparison to that presented by the sample without addition.