The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield stress, localization of inelastic deformation along narrow dislocation channels, and considerably reduced strains to failure. Further, creep rates are accelerated under irradiation. These changes in mechanical properties can be traced back to the irradiated microstructure which shows the formation of a large number of material defects, e.g., point defect clusters, dislocation loops, and complex dislocation networks. Interaction of dislocations with the irradiation-induced defects governs the mechanical behavior of irradiated metals. However, the mechanical properties are seldom systematically correlated to the underlying irradiated microstructure. Further, the current state of modeling of deformation behavior is mostly phenomenological and typically does not incorporate the effects of microstructure or defect densities. The present research develops a continuum constitutive crystal plasticity framework to model the mechanical behavior and deformed microstructure of bcc ferritic/martensitic steels exposed to irradiation. Physically-based constitutive models for various plasticity-induced dislocation migration processes such as climb and cross-slip are developed. We have also developed models for the interaction of dislocations with the irradiation-induced defects. A rate theory based approach is used to model the evolution of point defects generated due to irradiation, and coupled to the mechanical behavior. A void nucleation and growth based damage framework is also developed to model failure initiation in these irradiated materials. The framework is used to simulate the following major features of inelastic deformation in bcc ferritic/martensitic steels: irradiation hardening, flow localization due to dislocation channel formation, failure initiation at the interfaces of these dislocation channels and grain boundaries, irradiation creep deformation, and temperature-dependent non-Schmid yield behavior. Model results are compared to available experimental data. This framework represents the state-of-the-art in constitutive modeling of the deformation behavior of irradiated materials.

Transformation Induced Plasticity (TRIP) steels undergo a phase transformation when subjected to high levels of mechanical strain. This transformation from a paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible and the resultant magnetic properties may therefore be used as a measure of strain history. The transformation behaviour of TRIP steels has been recognised as a potential smart characteristic and various proposals have appeared aimed at producing a structure that performs its primary structural function as well a strain sensing function simultaneously. However the strain induced nature of the transformation implies that transformation will occur in areas of high stress concentration and therefore engineered stress concentration features will be required to provide a consistent measure of the changes in the magnetic properties of the material as a function of applied load. In order to predict the performance of smart TRIP steel sensors, an analysis method capable of quantifying the effectiveness of a component in its dual role as structure and sensor is needed. The thesis addresses the development of a methodology for correlating the changing magnetic permeability of TRIP steel sensors and structures with martensitic transformation behaviour. The prediction of the deformation behaviour including transformation is implemented by considering a mechanical analysis based on the finite element method and a constitutive model incorporating strain-induced martensitic transformation kinetics. .Extensions to the model which allow for a wide range of deformation rates and temperatures are also discussed. In order to demonstrate the application of the methodology, an analysis of a simple tensile element used in strain measurement applications is presented. The analysis also includes the effect of temperature on the performance of the sensor. An analysis of a design proposal for a smart aircraft bolt is also included to investigate the effects of geometry, particularly engineered stress concentrations, and sensor placement. / Thesis (Ph.D.)-University of Natal, Durban, 2003.

Theoretical part of this thesis includes overview of laser welding technology and possible combinations with arc welding methods for meeting desired properties of the weld. It also explains mechanical testing and metallographic examination on welds. The experimental part deals with optimisation of process parameters during welding of martensitic stainless steel, used in energetic industry using hybrid laser - TIG technology in regard of mechanical properties of the weld, its structure and proposal of technological parameters.

Au cours des procédés de mise en forme du verre creux, les outillages de verrerie sont soumis à des conditions extrêmes avec des températures pouvant dépasser largement 650 °C. De plus, ces derniers sont exposés à des phénomènes d'abrasion sévères et à des réactions physico-chimiques complexes avec le verre fondu. Ceci est particulièrement accentué par les cycles thermiques dus aux contacts répétés avec le verre. L'objectif de ce travail de thèse est de proposer des solutions innovantes de dépôts de superalliages et/ou traitements de surface destinés à augmenter la durée de vie des outillages. Trois axes d'innovation sont présentés dans ce mémoire. Le premier concerne l'étude du dépôt de poudre composite Co/NiB par soudure PTA (Plasma Transfered Arc) sur des moules en alliage cuivreux. Le second est dédié à l'étude de la faisabilité technique d'un dépôt PVD (Physical Vapor Deposition) multicouche à gradient fonctionnel, présentant des caractéristiques intéressantes pour des applications verrières en termes de dureté et de stabilité thermique, sur des poinçons en acier AISI 431. Enfin, le troisième axe de recherche fait l'objet d'une analyse plus approfondie et concerne l'étude de la nitruration par implantation d'ions azotes multichargés, également sur un acier AISI 431. On observe après implantation une augmentation significative de la dureté (+ 240 %). Une approche multi-expérimentale et multi-échelle est alors proposée afin d'étudier la microstructure du matériau implanté ainsi que son évolution après un recuit de 1h à 650 °C de manière à anticiper le comportement du matériau en production, et de corréler ceci aux propriétés de dureté / Glass moulds have to withstand very high temperatures which can far exceed 650 °C during the glass forming process. They are exposed to very tough conditions such as abrasive wear and physico-chemical reactions, accentuated by thermal shocks due to high speed contact with hot glass melt. The aim of this work is to find innovative solutions for superalloy coating or surface treatments in order to increase the service life of glass tools. Three innovation projects are presented in this report. The first one is dedicated to PTA (Plasma Transferred Arc) welded coating of composite Co/NiB powder on copper alloy moulds. The second one is about technical feasibility of a multilayer PVD (Physical Vapor Deposition) coating with properties congruent to glass production in term of hardness and thermal stability, on AISI 431 plunger stainless steel. Last but not least, the third innovative project is about nitriding by ion implantation with multicharged nitrogen ions on AISI 431 stainless steel. We notice after ion implantation a significant hardness increase (+ 240 %). A multi-experimental and multi-scale approach was carried out in order to study the implanted material microstructure and its evolution after annealing at 650 °C during 1h to predict the material behavior during production and correlate it to hardness properties

No description available.

High strength secondary hardening lath martensitic steel is a strong candidate for high performance and reliable transmission systems in aircraft and automotives. The fatigue resistance of this material depends both on intrinsic microstructure attributes, such as fine scale (M2C) precipitates, and extrinsic attributes such as nonmetallic primary inclusions. Additionally, the aforementioned attributes are affected by processing history. The objective of this research is to develop a computational framework to quantify the influence of both extrinsic (primary inclusions and residual stresses) and intrinsic (martensite laths and carbides) microstructure attributes on fatigue crack formation and the early stage of microstructurally small crack (MSC) growth that dominate high cycle fatigue (HCF) lifetime. To model the fatigue response at various microstructure scales, a hierarchical approach is adopted. A simplified scheme is developed to simulate processing effects such as shot peening that is suitable to introduce representative residual stresses prior to conducting fatigue calculations. Novel strategies are developed to couple process route (residual stresses) and microstructure scale response for comprehensive analysis of fatigue potency at critical life-limiting primary inclusions in gear steels. Relevant microstructure-scale response descriptors that permit relative assessment of fatigue resistance are identified. Fatigue crack formation and early growth is highly heterogeneous at the grain scale. Hence, a scheme for physically-based constitutive models that is suitable to investigate crack formation and early growth in martensitic steel is introduced and implemented. An extreme value statistical/probabilistic framework to assess the influence of variability of various microstructure attributes such as size and spatial distribution of primary inclusions on minimum fatigue crack formation life is devised. Understanding is sought regarding the relative role of microstructure attributes in the HCF process, thereby providing a basis to modify process route and/or composition to enhance fatigue resistance. Parametric studies are conducted to assess the effect of hot isostatic pressing and introduction of compliant coatings at debonded inclusion-matrix interface on enhancement of fatigue resistance. A comprehensive set of 3D computational tools and algorithms for hierarchical microstructure-sensitive fatigue analysis of martensitic gear steels is developed as an outcome of this research; such tools and methodologies will lend quantitative and qualitative support to designing improved, fatigue-resistant materials and accelerating insertion of new or improved materials into service.

Fretting, which can occur when two bodies in contact undergo a low amplitude relative slip, can drastically reduce the fatigue performance of a material. The extent of fretting damage is dependent on the material combination and is affected by many parameters, making it difficult to design against fretting. Some of these parameters include contact force, displacement amplitude, and contacting materials. This work develops a method for quantifying the extent of damage from fretting as a function of these parameters for a thin sheet of AISI 301 stainless steel in the full hard condition in contact with both ANSI A356 aluminum and AISI 52100 steel contacting bodies. Fretting experiments were conducted on a Phoenix Tribology DN55 Fretting Machine using a fixture which was developed for holding thin specimens. The displacement amplitude and normal force were systematically varied in order to cover a range that could typically be experienced during service. The tribological behavior was studied by analyzing friction force during cycling and inspecting the resulting surface characteristics. Fretting damaged specimens were cycled in tension in a servohydraulic test system to failure. The decrease in fatigue life caused by fretting damage was determined by comparing the stress-life (S-N) response of the fretted specimens to the S-N response of the virgin material, thus characterizing the severity of the fretting damage. The conditions that lead to the greatest reduction in life were identified in this way. Using the fracture mechanics based NASGRO model, an Equivalent Initial Flaw Size (EIFS) was used to quantify the level of fretting damage, thus separating the life of the component into crack nucleation and subsequent propagation. This method and data will allow engineers to design more robust components that resist fretting damage, thus increasing the safety and reliability of the system.

Made available in DSpace on 2014-12-17T14:07:06Z (GMT). No. of bitstreams: 1 PolianaRFS_DISSERT.pdf: 3998823 bytes, checksum: e0480a4a5d6b04450e748434898535a9 (MD5) Previous issue date: 2013-03-01 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The technique of plasma nitriding by the cathode cage mainly stands out for its ability to produce uniform layers, even on parts with complex geometries. In this study, it was investigated the efficiency of this technique for obtaining duplex surface, when used, simultaneously, to nitriding treatment and thin film deposition at temperatures below 500?C. For this, were used samples of AISI 41 0 Martensitic Stainless Steel and performed plasma treatment, combining nitriding and deposition of thin films of Ti and/or TiN in a plasma atmosphere containing N2-H2. It was used a cathodic cage of titanium pure grade II, cylindrical with 70 mm diameter and 34 mm height. Samples were treated at temperature 420?C for 2 and 12 hours in different working pressures. Optical Microscopy (OM), Scanning Electron Microscopy (SEM) with micro-analysis by Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) and analysis of Vickers Microhardness were used to investigate coating properties such as homogeneity and surface topography, chemical composition, layer thickness, crystalline phase, roughness and surface microhardness. The results showed there is a direct proportionality between the presence of H2 in plasma atmosphere and the quantity of titanium in surface chemical composition. It was also observed that the plasma treatment at lowpressure is more effective in formation of TiN thin film / A t?cnica de nitreta??o a plasma por gaiola cat?dica vem se destacando, principalmente, pela sua capacidade de produzir camadas uniformes mesmo em pe?as com geometria complexa. Neste trabalho, investigou-se a efici?ncia desta t?cnica para obten??o de superf?cie duplex, quando utilizada, simultaneamente, para tratamento de nitreta??o e deposi??o de filmes finos em temperaturas inferiores a 500?C. Para tal, foram utilizadas amostras do a?o inoxid?vel martens?tico AISI 410 e realizados tratamentos a plasma, combinando a nitreta??o e deposi??o de filmes finos de Ti e/ou TiN, em uma atmosfera contendo N2-H2. Foi utilizada uma gaiola cat?dica de tit?nio puro grau II em forma cil?ndrica, com 70 mm de di?metro e 34 mm de altura. As amostras foram tratadas numa temperatura de 420?C, com dura??o de 2 e 12 horas e em diferentes press?es de trabalho. Microscopia ?ptica (MO), Microscopia Eletr?nica de Varredura (MEV) com microan?lise por Espectroscopia de Energia Dispersiva (EDS), Difra??o de Raios-X (DRX), Microscopia de For?a At?mica (MFA) e ensaio de Microdureza Vickers foram empregados para investigar as propriedades do revestimento, tais como homogeneidade e topografia superficial, composi??o qu?mica, espessura da camada, fases cristalinas, rugosidade e microdureza superficial. Os resultados mostraram existir uma proporcionalidade direta entre a presen?a de H2 na atmosfera do plasma e a quantidade do elemento qu?mico tit?nio na composi??o qu?mica superficial. Observou-se, tamb?m, que o tratamento a plasma em baixa press?o ? mais eficaz na forma??o do filme fino de nitreto de tit?nio

Lors de l’usinage, les conditions de pression et de température à la surface usinée sont trèsélevées. La microstructure et l’état mécanique du matériau sont impactés, ce qui a desconséquences sur la réactivité chimique de la surface usinée. Dans cette thèse, ons’intéresse au tournage de superfinition de l’acier inoxydable martensitique X4CrNiMo16-5-1traité thermiquement (dénommé APX4 optimisé). Il présente la particularité d’êtremultiphasé à la température ambiante (martensite, ferrite et austénite). L’objectif est dequantifier les relations entre les conditions de coupe, les propriétés d’usage et ladégradation par corrosion des surfaces usinées. Des essais d’usinage en coupe orthogonale(type QST) ont été réalisés afin de comparer directement les résultats expérimentaux auxprédictions numériques 2D des surfaces usinées.Les changements de phase du matériau ont été étudiés par des essais de calorimétriejusqu’à 1200°C. Les cycles thermiques réalisés ont mis en évidence différentestransformations métallurgiques. L’étude bibliographique complémentaire permet deconclure quant aux possibilités de transformation de phase en usinage, qui sont quasiinexistantes du fait des cinétiques de chauffage extrêmement élevées en tournage.Les échantillons usinés ont été caractérisés avec une étude microstructurale parmicrographie optique et par MEB-EBSD. Ces deux techniques mettent en évidence lesdéformations importantes en extrême surface indiquées par l’étirement des îlots de ferriteparallèlement à la surface usinée. Ces résultats microstructuraux ont été mis en relationavec les résultats de microdureté Vickers. Une relation de la dureté superficielle a étéexprimée en fonction des conditions de coupe. La technique EBSD a permis de montrer pourles échantillons usinés dans les conditions les plus sévères la déformation de la ferrite avecapparition de sous-joints de grain, un affinement microstructural de la martensite et uneaccentuation de l’affinement de la matrice martensitique à proximité de la ferrite déformée.Une simulation numérique de prédiction des déformations à l’échelle macroscopiquea été réalisée. Ces résultats numériques ont été comparés aux résultats de déformation etde microdureté issus d’une campagne d’essais. Des essais numériques complémentairesappliqués à l’échelle de la microstructure (matériau considéré biphasé) permettent ded’appréhender la déformation de la phase ferritique et son influence locale sur ladéformation de la matrice martensitique. Ces résultats sont cohérents avec les résultatsexpérimentaux.Les phénomènes de corrosion par piqûres ont été étudiés par le biais d’essais depolarisation à l’aide de la microcellule électrochimique et d’essais spécifiques pulsés. Cesderniers essais ont permis l’analyse des piqûres générées (en diamètre et en densité). Larésistance à la corrosion localisée des surfaces d’acier inoxydable martensitique reste bonneaprès usinage, même améliorée (densité de piqûres plusieurs fois plus faible que l’état deréférence). Ce comportement est corroboré à l’état de compression de la surface.La microdureté de surface, qui est contrôlée majoritairement par l’avance, conditionne lepotentiel de piqûre. L’augmentation du diamètre de piqûre a été reliée à l’affinementmicrostructural (observé à partir d’un certain seuil de microdureté superficielle). / During machining, pressure and temperature conditions at the machined surface are veryintensive. Machined material microstructure and mechanical state are changed, whichimpacts the chemical reactivity of the machined surface. In this PhD study, we focus onsuperfinish turning of heat treated martensitic stainless steel X4CrNiMo16-5-1 (namedoptimized APX4). It has to be mentioned that this material has a multiphased microstructureat ambient temperature (martensite, ferrite and austenite). The aim is then to quantifyrelationships between cutting conditions, properties and the deterioration (corrosion) ofmachined surfaces. Machining trials in orthogonal cutting configuration (QST) have beenrealized in order to compare experimental results directly to 2D numerical forecast ofmachined surfaces.Material phase transformations have been studied by calorimetry tests until 1200°C. Testshave evidenced different metallurgical transformations. The complementary bibliographystudy allows to conclude that phase transformation possibilities during machining are veryquasi nonexistent due to extremely high heating kinetics in turning.Machined samples have been characterized by a microstructural study by opticalmicrography and SEM-EBSD. Both techniques highlight high strains in extreme machinedsurface as indicated by the stretching of ferrite islands in parallel to the machined surface.Microstructural results have been linked to the Vickers microhardness results. Surfacehardness has been expressed as a function of the cutting conditions.EBSD measurements have shown for machined samples in the most severe conditionsstraining of the ferrite with low angle grain boundaries, microstructural refining ofmartensite and heightening of martensitic matrix close to the strained ferrite.A numerical simulation predicting strains at the macroscopic scale has been carried out.These numerical results have been compared to strain and microhardness results arise fromone trials campaign. Further numerical simulations applied at the microstructure level(considered as biphased material) allow understanding of ferritic phase strain and its localinfluence on martensitic matrix strain. These results are consistent with experimentalresults.Pitting corrosion phenomenon has been studied by polarization testing using theelectrochemical microcell and specific pulsed testing. Last used method has been conductedto analyze the generated pits (diameter and density). Localized corrosion resistance ofmachined martensitic stainless steel surfaces remains good, even improved (few times lowerpitting density than the reference state). This behaviour is corroborated to the compressivestate of the surface.Surface microhardness, which is mainly controlled by the feed rate, conditions the criticalpitting potential. Increased pitting diameter has been linked to microstructural refining(observed above a surface microhardness level).

Цель работы – установление влияния нейтронного облучения на стабильность структуры и фазового состояния ферритно-мартенситной стали ЭП823. Данная работа посвящена анализу влияния нейтронного облучения на фазовые и структурные составляющие стали. Исследования проводились на образцах оболочек твэлов, облученных в составе материаловедческой сборки в реакторе БН-600 при температурах 570, 600, 660 ˚С до повреждающих доз 51,3, 75,1 и 81,1 сна. Определялись методами металлографии и стереометрического анализа характеристики структурных составляющих, вторичных фаз, образовавшихся в процессе облучения, оценивались характеристики радиационной пористости, плотность дислокаций. Исследования были выполнены на сканирующем электронном микроскопе MIRA3 FEG-SEM, оснащенном детекторами вторичных и отраженных электронов и приставкой энергодисперсионного анализа характеристического рентгеновского излучения x-Act 6 фирмы Oxford Instruments. Так же проводились исследования на просвечивающем электронном микроскопе фирмы JEM-2000EX при ускоряющем напряжении 100 кВ в просвечивающем режиме. Количественная обработка изображений для получения размерных характеристик выделений проводилась с использованием программного обеспечения «Цифровая фотолаборатория SIAMS Photolab», а также при помощи программного обеспечения сканирующего электронного микроскопа MIRE3 FEG-SEM. Проведенное исследование стали Х12НМВБФСР в исходном состоянии показало, что ее структура состоит из мартенсита, остаточного феррита, крупных карбидов на основе Nb и по границам ферритных зерен и мартенситных реек, образуются карбиды типа М23С6. После нейтронного облучения содержание карбидов типа М23С6 увеличивается, в зернах феррита образуется χ-фаза, у которой с повышением температуры облучения понижается концентрация и увеличивается в размерах, по границам обнаружена мелкодисперсная α-фаза, обогащенная хромом, которая при повышенных температурах исчезает, так же по границам зерен мартенсита наблюдается небольшое количество радиационных пор. Прошедшие изменения микроструктуры под воздействием нейтронного облучения не привели к существенным изменениям механических свойств. / The aim of the work is to establish the effect of neutron irradiation on the stability of the structure and phase state of the EP823 ferrite-martensitic steel. This work is devoted to the analysis of the effect of neutron irradiation on the phase and structural components of steel. The studies were carried out on samples of fuel rod shells irradiated as part of a materials science assembly in the BN-600 reactor at temperatures of 570, 600, 660 ℃ to damaging doses of 51.3, 75.1, and 81.1 dpa. The characteristics of structural components and secondary phases formed during irradiation were determined by metallography and stereometric analysis, and the characteristics of radiation porosity and dislocation density were evaluated. The studies were performed using a MIRA3 FEG-SEM scanning electron microscope equipped with secondary and reflected electron detectors and an x-Act 6 energy dispersive analysis of characteristic X-ray radiation from Oxford Instruments. Studies were also carried out on a transmission electron microscope of the JEM-2000EX company at an accelerating voltage of 100 kV in the transmission mode. Quantitative image processing to obtain the dimensional characteristics of the secretions was carried out using the software "Digital Photo Laboratory SIAMS Photolab", as well as using the software of the scanning electron microscope MIRE3 FEG-SEM. The study of steel X12NMVBFSR in the initial state showed that its structure consists of martensite, residual ferrite, large carbides based on Nb and along the boundaries of ferritic grains and martensitic rails, carbides of the M23C6 type are formed. After neutron irradiation, the content of carbides of the M23C6 type increases, a χ-phase is formed in the ferrite grains, in which the concentration decreases and increases in size with an increase in the irradiation temperature, a fine alpha-phase enriched in chromium is found along the boundaries, which disappears at elevated temperatures, as well as a small number of radiation pores are observed along the boundaries of martensite grains. The past changes in the microstructure under the influence of neutron irradiation did not lead to significant changes in the mechanical properties.