Additive manufacturing (AM) techniques are being studied for their application in the aerospace industry. Numerous benefits come from the already in shape final piece, which needs reduced amount of prime material for its production and can have its shape numerically optimized for weight reduction. Austenitic stainless steels (AuSS) are widely used in aerospace and their manufacturing through AM is a popular research topic in order to accelerate their effective incorporation in air-crafts and spaceships. The special microstructures of AM has been observed with characterization techniques. The present work studies the high temperature stability of three AuSS (316L, MOD-316 and 21-6-9) considering two approaches; surface corrosion and microstructure evolution. First, for high temperature corrosion, thermogravimetric analysis has been performed from 850°C to 1150°C. From the results, kinetic analysis were performed and the activation energy was extracted from Arrhenius fits. Two mechanism were found for alloy 316L (first 435.41 kJ/mol and second 593.24 kJ/mol) and MOD-316 (first 740.01 kJ/mol and second 495.58 kJ/mol). Further SEM observations on the scales have shown Ni diffusion through the chromia scale in MOD-316 alloy, which could explain the higher oxidation rates at 1150°C. Alloy 21-6-9 has the best passivation behaviour with an activation energy of 190.47 kJ/mol. Secondly, long heat treatment (HT) at 725°C in air atmosphere has been performed, for 24 and 240h. Samples were initially as-built or annealed (900°C for 1h), to compare the effect of the HT on the microstructure evolution and precipitates formation. LOM observation showed preferable nucleation in grain boundaries (GB), an increment of the number of precipitates and a growth towards elongated shapes following GB with increased time. It was also observed a reduction in precipitates number with the annealing HT for all the alloys. XRD, SEM and EDS analysis has been carried out to identify the structure and composition of the precipitates. Various chromium, tungsten, copper, molybdenum and niobium carbides and oxides have been found in MOD-316. Higher porosity is observed in 21-6-9, that presented mainly chromium oxides, carbides and nitrides in GB and surrounding the AM defects. / SeSSA

<p> This dissertation describes the recent advancements in micromechanical testing that inform how deformation mechanisms in austenitic stainless steels (SS) are affected by the presence of irradiation-induced defects. Austenitic SS is one of the most widely utilized structural alloys in nuclear energy systems, but the role of irradiation on its underlying mechanisms of mechanical deformation remains poorly understood. Now, recent advancement of microscale mechanical testing in a scanning electron microscope (SEM), coupled with site-specific transmission electron microscopy (TEM), enables us to precisely determine deformation mechanisms as a function of plastic strain and grain orientation.</p> <p> </p> <p>We focus on AISI 304L SSs irradiated in EBR-II to ~1-28 displacements per atom (dpa) at ~415 °C and contains ~0.2-8 atomic parts per million (appm) He amounting to ~0.2-2.8% swelling. A portion of the specimen is laser welded in a hot cell; the laser weld heat affected zone (HAZ) is studied and considered to have undergone post-irradiation annealing (PIA). An archival, virgin specimen is also studied as a control. We conduct nanoindentation, then prepare TEM lamellae from the indent plastic zone. In the 3 appm He condition, TEM investigation reveals nucleation of deformation-induced <i>α</i>’ martensite in the irradiated specimen, and metastable <i>ε</i> martensite in the PIA specimen. Meanwhile, the unirradiated control specimen exhibits evidence only of dislocation slip and twinning; this is unsurprising given that alternative deformation mechanisms such as twinning and martensitic transformation are typically observed only near cryogenic temperatures in austenitic SS. Surface area of irradiation-produced cavities contribute enough free energy to accommodate the martensitic transformation. The lower population of cavities in the PIA material enables metastable <i>ε</i> martensite formation, while the higher cavity number density in the irradiated material causes direct <i>α</i>’ martensite formation. In the 0.2 appm He condition, SEM-based micropillar compression tests confirm nanoindentation results. A deformation transition map with corresponding criteria has been proposed for tailoring the plasticity of irradiated steels. Irradiation damage could enable fundamental, mechanistic studies of deformation mechanisms that are typically only accessible at extremely low temperatures. </p>

La thèse est inscrite au sein du projet ASTRID, qui est un démonstrateur technologique pour les réacteurs de quatrième génération (Gen-IV). Le premier matériau choisi pour constituer les gaines de cœur est un acier inoxydable austénitique stabilisé au titane (type 15Cr-15Ni Ti). L’écrouissage à froid des gaines permet la précipitation de nano-carbures de titane en service sur les dislocations, retardant ainsi les phénomènes de restauration par effet d’épinglage. En conditions accidentelles (T > 650°C), et plus particulièrement dans le cas d’une perte de réfrigérant primaire, le comportement en fluage de ces gaines est très mal connu. L’objectif des travaux de thèse est donc de déterminer les mécanismes de déformation et de rupture en fluage, entre 650°C et 950°C, de cet acier à l’état non irradié.Dans un premier temps, les microstructures d’échantillons après différents recuits ont été comparées afin d’étudier l’influence de la température sur les évolutions métallurgiques. L’étude de la précipitation et des cinétiques de restauration et de recristallisation, ont permis de dresser les évolutions microstructurales sans charge appliquée.En plus d’étudier le comportement en fluage uniaxial de l’acier à haute température, les caractérisations des éprouvettes après essais ont permis de déterminer les évolutions microstructurales au cours et après essais de fluage (contributions simultanées de la température et de la contrainte). La comparaison avec les microstructures obtenues après recuits a mis en évidence une accélération de la cinétique de recristallisation sous charge, rendant l’effet de la contrainte sur ces évolutions non négligeable.Après fluage sous air aux plus basses températures (650°C et 750°C), les fractographies présentent une rupture globalement transgranulaire avec certaines zones intergranulaires. Après fluage sous vide secondaire à plus hautes températures (850°C et 950°C), un fort amincissement des éprouvettes et une striction quasiment complète dans l’épaisseur ont été observés. Ce fort amincissement se traduit par un alignement de cupules, caractéristique de ruptures 100% ductiles à très haute température. / The ASTRID project aims at designing a fast-reactor prototype for the 4th generation of nuclear power plants. The material to be used for fuel cladding is a cold-worked austenitic stainless steel stabilized with titanium (15Cr-15Ni Ti type) and optimized in minor elements. This material was developed to limit recovery and irradiation-induced swelling and to improve microstructural stability and mechanical properties in normal operating conditions. In case of incidental situations (irradiation temperature > 650°C), the cladding might rapidly reach higher temperatures up to 950°C where its stability could be affected. The present work aims at improving knowledge and understanding of the microstructural evolution and creep behaviour of this steel at these temperatures (650°C-950°C).Microstructural characterizations of thermally-aged samples have been performed in order to study the effect of temperature on metallurgical evolutions (precipitation, recovery and recrystallization). A phenomenological model including recovery and recrystallization processes was set up to reproduce hardness measurements versus ageing time and temperatures.Isothermal creep tests up to 950°C under a wide range of stress levels allowed investigation of viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of high-temperature loading, microstructural characteristics of stress-free thermally-aged samples were compared with post-mortem examinations of creep specimens.At 650°C and 750°C the value of stress exponent is higher than 7. The main deformation mechanism during creep test is power-low creep, which is consistent with the results found in the literature.Beyond 850°C, the increase in dislocation mobility promotes recovery and recrystallization processes. As a consequence, a competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950°C, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. The comparison between microstructures of crept specimens and stress-free, thermally-aged samples leads to the conclusion that the recrystallization kinetics is accelerated by application of a mechanical loading.As for the fracture behaviour, creep tests under air environment at lower temperatures (650°C-750°C), led to predominating ductile fracture but some intergranular zones were observed on fracture surfaces. Creep tests under high vacuum at higher temperatures (850°C-950°C) lead to a high fracture elongation with a reduction of area up to 100%.

A crescente demanda de aplicações de aços inoxidáveis austeníticos e a constante pressão para redução de custo nas empresas siderúrgicas, devido à alta volatilidade no custo do níquel, resultaram em novos desenvolvimentos de aços da série 200. Esta nova classe de aços inoxidáveis austeníticos contém elevados teores de manganês e nitrogênio em substituição ao elemento níquel. A justificativa para a realização deste trabalho é a escassez de estudos comparativos entre aços inoxidáveis austeníticos da série 200 e série 300 disponíveis na literatura em relação ao comportamento da transformação de fase induzida pela deformação e da resistência à corrosão. Os principais fatores que afetam a microestrutura no endurecimento por deformação são: a energia de defeito de empilhamento, composição química, temperatura, grau, taxa e modo de deformação. Realizou-se uma análise crítica e adequação dos conceitos de níquel e cromo equivalente para os aços AISI 201 e AISI 304. Amostras desses aços foram solubilizadas, laminadas e racionadas em diferentes condições para caracterização microestrutural com o auxílio de técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, ferritoscópio e microdureza. Curvas de endurecimento em função do grau de deformação, fração volumétrica de martensita em função do grau de deformação, assim como a evolução microestrutural e sua respectiva identificação de fase com o grau de deformação foram resultados obtidos deste trabalho. Em geral, aumentando a deformação plástica a frio, maior é a dureza para ambos os aços e maior é a fração volumétrica de martensita induzida por deformação. O aço AISI 201 é mais susceptível a transformação de fase do que o aço AISI 304 devido a sua menor EDE. Ensaios eletroquímicos de espectroscopia de impedância eletroquímica e polarização potenciodinâmica anódica foram realizados para avaliação da resistência a corrosão e para avaliar o comportamento da repassivação. Ambos os aços apresentaram comportamento similares quanto à resistência à corrosão, além de apresentarem potenciais de corrosão da ordem de 10-8 A/cm², típico de materiais passivos. / The continuous increase in the application demand of austenitic stainless steels and the constant pressure for cost reduction in the steelmaking industry, due to the high instability of nickel price, has conduced to new developments of the AISI 200 series steels. This new austenitic stainless steel series employes high manganese and nitrogen contents in substitution to nickel. The reason of this work is the lack of comparative studies in the literature between austenitic stainless steels of 200 and 300 series relative to the martensite strain induced phase transformation and its corrosion resistance. The main factors that affect microstructure on strain-hardening are: stacking fault energy, chemical composition, temperature, strain and strain rate. A critical analysis of the concept related to the nickel and chrome equivalents for the AISI 201 and AISI 304 steels has been carried out. Samples of these steels were heat treated and cold rolled to different strains for subsequent microstructural evaluation using equipments such as optical microscope, scanning electron microscope, X-ray diffraction, microhardness and ferritoscope. Strain hardening versus strain, martensite volume fraction versus strain, as well as microstructure evolution and its respective phase identification with strain are some of the main results obtained in this study. In general, increasing the strain hardening, the higher will be the hardness of both stainless steels and higher is the induced martensite volume fraction. The AISI 201 steel presented higher susceptibility to induced phase transformation in comparison to the AISI 304 steel due to its lower stacking fault energy. Electrochemical impedance spectroscopy and anodic potenciodynamic polarization were the techniques used in this work to evaluate the corrosion resistance and passivation behavior respectively. Both steels presented similar corrosion resistance, apart from presenting a corrosion potential of about 10-8 A/cm² , which is typical for passivated materials.

Nos últimos anos, devido ao elevado preço do níquel, uma nova série de aços inoxidáveis austeníticos com um menor teor de níquel foi criada. A essa nova série foi dado o nome de série 200. Dentre os aços dessa classe, o AISI 201 tem sido utilizado em aplicações onde a elevada resistência à corrosão não é tão necessária. Neste trabalho de Mestrado investigou-se a formação e a reversão da martensita induzida por deformação em um aço inoxidável austenítico AISI 201 laminado a frio em 20, 40 e 60% de redução em espessura. Das chapas laminadas foram retiradas amostras que foram recozidas em várias temperaturas (200-800oC) por 1 hora. Amostras do material laminado em 60% de redução em espessura também foram recozidas por várias temperaturas (200-800oC) e por vários tempos (5-180min). Com isso, avaliou-se a evolução microestrutural do material durante a laminação frio e durante o recozimento por meio de medidas de microdureza Vickers, microscopias óptica, eletrônica de varredura e eletrônica de transmissão, difração de elétrons retroespalhados, difração de raios X e medidas de magnetização. Além disso, foram realizados cálculos termodinâmicos para a previsão da formação de fases nesse material. Constatou-se que o material de partida não era completamente austenítico, possuindo uma pequena fração de ferrita ? residual em sua microestrutura. Com relação às medidas de magnetização, observou-se que a fração de fase ferromagnética (martensita) aumenta com o aumento da deformação, aumentando a magnetização de saturação (Ms) do material. Para pequenas deformações (20% de redução em espessura) houve a ocorrência de um pico no valor de campo coercivo do material (Hc). Com o aumento da deformação (40 e 60%) os valores de Hc diminuíram. Com relação à reversão da martensita induzida por deformação durante os recozimentos, observou-se que ela ocorre na faixa de temperatura de 500-700oC para o material laminado em 60% de redução em espessura. O comportamento do material nesse estudo corrobora o que tem sido reportado na literatura para os aços da série 300. Entretanto, pouco tem sido publicado com relação às propriedades magnéticas do aço inoxidável austenítico AISI 201, principalmente com relação ao campo coercivo. Neste trabalho também foram realizadas medidas de magnetização durante o recozimento das amostras (condição in situ). Os parâmetros obtidos desses experimentos in situ foram comparados com aqueles obtidos para as amostras recozidas isotermicamente. / In the last years, since nickel price increased, another series of austenitic stainless steel with less amount of nickel has emerged: the series 200. The AISI 201 stainless steel has been used where intermediated corrosion resistance is needed. In this work, the formation of strain-induced martensite and its reversion in an AISI 201 austenitic stainless steel were studied. The material was characterized in terms of microstructure and then cold rolled up to 20, 40 and 60% of thickness reduction. For all degree of reduction, samples were annealed at several temperatures (200-800oC) for 1 hour. Additional samples taken from the 60% cold-rolled material were also annealed at several temperatures (200-800oC) for several times (5-180minutes). The microstructural evolution during cold rolling and annealing was evaluated using microhardness Vikers testing, light optical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, X-Ray diffraction and magnetization measurements. Phase predictions were also performed using software Thermo-calc©. It was observed that the as-received material was not fully austenitic. It has a small fraction of ?-ferrite within its matrix. The amount of ferromagnetic phase (martensite) increases with increasing deformation. For small deformation (20%), there is a peak in the coercive field of the material (Hc). As deformation increases, Hc values decrease. It was also observed that the martensite reversion takes place at 500-700oC. The behavior of the material is in accordance with what has been reported in the literature for the 300 series. However, only few works have been reported concerning AISI 201 stainless steel and its magnetic properties. In this work, magnetic measurements were also carried out during annealing (in situ condition). The obtained parameters from the in situ magnetic measurements were compared to those ones obtained from the isothermally annealed samples.

A crescente demanda de aplicações de aços inoxidáveis austeníticos e a constante pressão para redução de custo nas empresas siderúrgicas, devido à alta volatilidade no custo do níquel, resultaram em novos desenvolvimentos de aços da série 200. Esta nova classe de aços inoxidáveis austeníticos contém elevados teores de manganês e nitrogênio em substituição ao elemento níquel. A justificativa para a realização deste trabalho é a escassez de estudos comparativos entre aços inoxidáveis austeníticos da série 200 e série 300 disponíveis na literatura em relação ao comportamento da transformação de fase induzida pela deformação e da resistência à corrosão. Os principais fatores que afetam a microestrutura no endurecimento por deformação são: a energia de defeito de empilhamento, composição química, temperatura, grau, taxa e modo de deformação. Realizou-se uma análise crítica e adequação dos conceitos de níquel e cromo equivalente para os aços AISI 201 e AISI 304. Amostras desses aços foram solubilizadas, laminadas e racionadas em diferentes condições para caracterização microestrutural com o auxílio de técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, ferritoscópio e microdureza. Curvas de endurecimento em função do grau de deformação, fração volumétrica de martensita em função do grau de deformação, assim como a evolução microestrutural e sua respectiva identificação de fase com o grau de deformação foram resultados obtidos deste trabalho. Em geral, aumentando a deformação plástica a frio, maior é a dureza para ambos os aços e maior é a fração volumétrica de martensita induzida por deformação. O aço AISI 201 é mais susceptível a transformação de fase do que o aço AISI 304 devido a sua menor EDE. Ensaios eletroquímicos de espectroscopia de impedância eletroquímica e polarização potenciodinâmica anódica foram realizados para avaliação da resistência a corrosão e para avaliar o comportamento da repassivação. Ambos os aços apresentaram comportamento similares quanto à resistência à corrosão, além de apresentarem potenciais de corrosão da ordem de 10-8 A/cm², típico de materiais passivos. / The continuous increase in the application demand of austenitic stainless steels and the constant pressure for cost reduction in the steelmaking industry, due to the high instability of nickel price, has conduced to new developments of the AISI 200 series steels. This new austenitic stainless steel series employes high manganese and nitrogen contents in substitution to nickel. The reason of this work is the lack of comparative studies in the literature between austenitic stainless steels of 200 and 300 series relative to the martensite strain induced phase transformation and its corrosion resistance. The main factors that affect microstructure on strain-hardening are: stacking fault energy, chemical composition, temperature, strain and strain rate. A critical analysis of the concept related to the nickel and chrome equivalents for the AISI 201 and AISI 304 steels has been carried out. Samples of these steels were heat treated and cold rolled to different strains for subsequent microstructural evaluation using equipments such as optical microscope, scanning electron microscope, X-ray diffraction, microhardness and ferritoscope. Strain hardening versus strain, martensite volume fraction versus strain, as well as microstructure evolution and its respective phase identification with strain are some of the main results obtained in this study. In general, increasing the strain hardening, the higher will be the hardness of both stainless steels and higher is the induced martensite volume fraction. The AISI 201 steel presented higher susceptibility to induced phase transformation in comparison to the AISI 304 steel due to its lower stacking fault energy. Electrochemical impedance spectroscopy and anodic potenciodynamic polarization were the techniques used in this work to evaluate the corrosion resistance and passivation behavior respectively. Both steels presented similar corrosion resistance, apart from presenting a corrosion potential of about 10-8 A/cm² , which is typical for passivated materials.

Environmentally-assisted cracking (EAC) phenomena affect the in-service behaviour of austenitic stainless steels in nuclear power plants. EAC includes such degradation phenomena as Stress Corrosion Cracking (SCC) and Corrosion Fatigue (CF). Factors affecting EAC include the material type, microstructure, environment, and stress. This is an important degradation issue for both current and Gen III+ light water reactors, particularly as nuclear power plant lifetimes are extended ( > 60 years). Thus, it is important to understand the behaviour of the alloys used in light water reactors, and phenomena such as SCC to avoid failures. Although there is no agreement on the mechanism(s) of SCC, the importance of localized electrochemical reactions at the material surface is widely recognised. Considerable research has been performed on SCC and CF crack growth, but the initiation phenomena are not fully understood. In this project, novel in situ analytical TEM techniques have been developed and applied to explore localised reactions in Type 304 austenitic stainless steel. In situ transmission electron microscopy has become an increasingly important and dynamic research area in materials science with the advent of unique microscope platforms and a range of specialized in situ specimen holders. In metals research, the ability to image and perform X-ray energy dispersive spectroscopy (XED) analyses of metals in liquids are particularly important for detailed study of the metal-environment interactions with specific microstructural features. To further facilitate such studies a special hybrid specimen preparation technique involving electropolishing and FIB extraction has been developed in this thesis to enable metal specimens to be examined in the liquid cell TEM specimen holder using both distilled H2O and H2SO4 solutions. Furthermore, a novel electrode configuration has been designed to permit the localized electrochemical measurement of electron-transparent specimens in the TEM. These novel approaches have been benchmarked by extensive ex situ experiments, including both conventional electrochemical measurements and microcell measurements. The results are discussed in terms of validation of in situ test data as well as the role of the electron beam in the experiments. In situ liquid cell TEM experiments have also explored the localized dissolution of MnS inclusions in H2O, and correlated the behaviour with ex situ experiments. Based on the research performed in this thesis, in situ liquid cell and in situ electrochemical cell experiments can be used to study nanoscale reactions pertaining to corrosion and localized dissolution leading to "precursor" events for subsequent EAC phenomena.

Objetivo principal desta dissertação foi verificar os efeitos da temperatura de laminação na formação de martensita induzida por deformação e na sua posterior reversão da martensita para austenita no aço inoxidável austenítico AISI 304L. O estudo foi predominantemente microestrutural e para análise e caracterização foram utilizadas as técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, medidas de dureza Vickers e medidas de fases ferromagnéticas por ferritoscopia. As amostras foram inicialmente solubilizadas a 1100 ºC por uma hora, visando a dissolução de uma pequena quantidade residual de ferrita encontrada nas amostras na condição como recebida, depois laminadas em diferentes temperaturas, determinando-se curvas de endurecimento por deformação e de formação de martensita induzida por deformação em função do grau de deformação. Em seguida, foram realizados pré-recozimentos a 600 ºC, favorecendo apenas a reversão da martensita para austenita, de maneira que não ocorresse a recristalização. A quantidade e a temperatura de deformação apresentaram forte influência na quantidade de martensita formada, no endurecimento por deformação e na cinética de amolecimento durante o recozimento. Os pré-tratamentos realizados a 600 ºC causaram acentuada reversão da martensita, algum amolecimento e pequeno efeito no tamanho de grão recristalizado durante o posterior recozimento a 600 ºC. / The main objective of the present dissertation was to verify the effects of the rolling temperature on the formation of strain induced martensite and in its subsequent martensite reversion to austenite in a AISI 304L stainless steel. The study was predominantly microstructural and, for the analysis and characterization, several techniques have been used, namely optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness measurements and magnetic phase measurements, using the ferritoscope. The samples were initially solution annealed at 1100 ºC for one hour, aiming at the dissolution of a small quantity of the residual -ferrite found in the samples in the as-received condition; then rolling was performed at different temperatures, evaluating strain hardening and the strain induced martensite as a function of strain. Following, pre-annealing treatments at 600 ºC have been performed, favoring only the martensite to austenite reversion, in a way that no recrystallization would occur. Strain and temperature had a strong influence on the amount of formed martensite, on the strain hardening and on the softening kinetics during annealing. The pre-annealing treatments at 600 ºC caused an accentuated effect on the martensite reversion, some softening and a small effect on the recrystallized grain size during the subsequent annealing at 600 ºC.

Bien que l’amorçage de fissure joue un rôle important en fatigue, ses mécanismes ne sont pas encore pleinement compris. Des critères d'amorçage basés sur des mécanismes physiques de déformation plastique ont été proposés mais ne sont pas faciles à utiliser et à valider, car ils nécessitent des variables locales à l'échelle du grain. L'étude présente vise à établir un critère d'amorçage en fatigue oligocyclique, utilisable sous chargement d'amplitude variable.Le comportement mécanique de l'acier inoxydable AISI 316L étudié a été caractérisé en fatigue oligocyclique. Il a été modélisé par un schéma autocohérent utilisant une loi de plasticité cristalline basée sur l'évolution des densités de dislocations. L'endommagement de surface a été suivi pendant un essai de fatigue à l'aide d'un dispositif de microscopie optique in situ. Les fissures présentes après 2000 cycles ont été analysées et leurs caractéristiques cristallographiques calculées.Comme les grains de surface montrent une déformation plus importante à cause d'un moindre confinement par les grains voisins, il est nécessaire de définir une loi de localisation spécifique aux grains de surface. La forme proposée fait intervenir une variable d'accommodation intergranulaire, sur le modèle de la loi de localisation de Cailletaud-Pilvin. Elle a été identifiée à partir de simulations par éléments finis. L'état des contraintes et des déformations dans les grains de surface a alors été simulé. Des indicateurs d'amorçage potentiels ont ensuite été comparés sur une même base expérimentale. Deux indicateurs pertinents de l'endommagement en fatigue ont pu être obtenus. / Although crack initiation is proved to play an important role in fatigue, its mechanisms have not been fully understood. Some crack initiation criteria based on physical mechanisms of plastic deformation have been defined. However, these criteria are not easy to use and valid, as they need local variables at the grain scale. The present study aims at establishing a crack initiation criterion in low-cycle fatigue, which should be usable under variable amplitude loading conditions.Tension-compression fatigue tests were first carried out to characterize the mechanical behavior of the stainless steel AISI 316L studied. The mechanical behavior was simulated using a self-consistent model using a crystalline plastic law based on dislocation densities. The evolution of surface damage was observed during a fatigue test using an in situ optical microscopic device. Cracks were analyzed after 2000 cycles and their crystallographic characteristics calculated. As surface grains exhibit larger strain because they are less constraint by neighbor grains, a specific numerical frame is necessary to determine stress state in surface grains. A localization law specific to surface grains under cyclic loading was identified from finite element simulations. The proposed form needs an intergranular accommodation variable, on the pattern of the localization law of Cailletaud-Pilvin. Stress-strain state in surface grains was simulated. Potential indicators for crack initiation were then compared on a same experimental data base. Indicators based on the equivalent plastic strain were found to be suitable indicators of fatigue damage.

Objetivo principal desta dissertação foi verificar os efeitos da temperatura de laminação na formação de martensita induzida por deformação e na sua posterior reversão da martensita para austenita no aço inoxidável austenítico AISI 304L. O estudo foi predominantemente microestrutural e para análise e caracterização foram utilizadas as técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, medidas de dureza Vickers e medidas de fases ferromagnéticas por ferritoscopia. As amostras foram inicialmente solubilizadas a 1100 ºC por uma hora, visando a dissolução de uma pequena quantidade residual de ferrita encontrada nas amostras na condição como recebida, depois laminadas em diferentes temperaturas, determinando-se curvas de endurecimento por deformação e de formação de martensita induzida por deformação em função do grau de deformação. Em seguida, foram realizados pré-recozimentos a 600 ºC, favorecendo apenas a reversão da martensita para austenita, de maneira que não ocorresse a recristalização. A quantidade e a temperatura de deformação apresentaram forte influência na quantidade de martensita formada, no endurecimento por deformação e na cinética de amolecimento durante o recozimento. Os pré-tratamentos realizados a 600 ºC causaram acentuada reversão da martensita, algum amolecimento e pequeno efeito no tamanho de grão recristalizado durante o posterior recozimento a 600 ºC. / The main objective of the present dissertation was to verify the effects of the rolling temperature on the formation of strain induced martensite and in its subsequent martensite reversion to austenite in a AISI 304L stainless steel. The study was predominantly microstructural and, for the analysis and characterization, several techniques have been used, namely optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness measurements and magnetic phase measurements, using the ferritoscope. The samples were initially solution annealed at 1100 ºC for one hour, aiming at the dissolution of a small quantity of the residual -ferrite found in the samples in the as-received condition; then rolling was performed at different temperatures, evaluating strain hardening and the strain induced martensite as a function of strain. Following, pre-annealing treatments at 600 ºC have been performed, favoring only the martensite to austenite reversion, in a way that no recrystallization would occur. Strain and temperature had a strong influence on the amount of formed martensite, on the strain hardening and on the softening kinetics during annealing. The pre-annealing treatments at 600 ºC caused an accentuated effect on the martensite reversion, some softening and a small effect on the recrystallized grain size during the subsequent annealing at 600 ºC.