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Au cours de leur mise en forme, les alliages de titane destinés à la fabrication de pièces tournantes pour moteurs d'avions suivent un schéma thermomécanique complexe constitué généralement de plusieurs opérations de forgeage. L'étape de forgeage dans le domaine monophasé β joue un rôle important, car elle conditionne la transformation de phases qui s'opère au cours du refroidissement ou du traitement thermique consécutif dans le domaine biphasé α-β, et donc les propriétés mécaniques finales de la pièce. La maîtrise de l'évolution de la microstructure et de la texture cristallographique au cours du forgeage dans le domaine β est donc indispensable pour optimiser l'ensemble du procédé de mise en forme. Pour cela, il est important de bien comprendre les mécanismes de recristallisation dynamique (continue et géométrique) se produisant dans ce type de matériau.Dans cette étude, les effets de la température, de la déformation et de la vitesse de déformation ont été étudiés au moyen d'essais de compression uniaxiale et de torsion à chaud. Les microstructures et textures de déformation ont été caractérisées par diffraction des électrons rétrodiffusés (EBSD). Ces approches expérimentales sont associées à un travail de modélisation, pour lequel le schéma de recristallisation dynamique "continue" (ou CDRX), consistant en une fragmentation progressive des grains β initiaux par la formation de nouveaux joints de grains, a été adapté à partir d'études antérieures. Les paramètres caractérisant les mécanismes élémentaires de la CDRX (écrouissage, restauration dynamique, migration des joints) ont été déterminés par ajustement sur les données expérimentales.Ces travaux ont pour objectif l'élaboration d'un post-processeur métallurgique dédié au forgeage des alliages de titane en phase β. / Titanium alloys used for the manufacture of rotating parts of aircraft engines undergo complex thermomechanical processing schedules, involving usually several forging operations. The forging step in the single -phase domain plays an important role, because it governs the phase transformation that occurs during cooling or subsequent heat treatment in the two-phase α+β range, and therefore the final mechanical properties of the workpiece. Controlling the microstructural evolutions during forging in the β-domain is of prime interest to optimize the entire process of disks manufacture. A full understanding of the (continuous and geometric) dynamic recrystallization mechanisms occurring in this type of material is therefore required.In this study, the effects of temperature, strain and strain rate were investigated by hot uniaxial compression and torsion tests. The microstructures and textures were characterized using backscattering electron diffraction (EBSD). These experimental approaches were associated with the development of a physical model. In the latter, the mechanism of "continuous" dynamic recrystallization (CDRX), consisting of a progressive initial fragmentation of the grains leading to the formation of new grain boundaries, was adapted from former studies. The parameters characterizing the elementary mechanisms of CDRX (strain hardening, dynamic recovery, grain boundary migration) were determined by fitting the experimental data.The final objective of this work is to develop a metallurgical post-processor dedicated to forging operations of titanium alloys in the -phase range.

Les alliages de titane sont largement utilisés dans le secteur de l’aéronautique. Cependant,ces alliages sont très sensibles aux gammes d’élaboration et de mises en forme. Cesdernières ont une grande influence sur l’intégrité de surface des produits finis, ce qui aun impact sur la durée de vie des pièces en service. Il est donc nécessaire de maîtriser lesprocédés afin de pouvoir définir la tenue en service des pièces.Dans ces travaux, les différentes intégrités de surface sont décrites en termes d’évolutionmicrostructurale, de contraintes résiduelles et de micro-géométrie. Des essais de fatigueen flexion 4 points sont réalisés pour tester les performances mécaniques de ces surfaces.Nous avons remarqué une bonne cohérence entre les modèles reliant l’intégrité de surfaceet la tenue en fatigue déterminée expérimentalement.De plus, étant donné que les alliages de titane ont des microstructures complexes etpeuvent être multiphasés, nous nous sommes aussi intéressés au caractère hétérogènede ces matériaux principalement dans l’analyse de contraintes résiduelles déterminées àl’aide de la diffraction des rayons X. Nous avons remarqué que ces hétérogénéités ontun impact sur la détermination des contraintes résiduelles. De fait une méthodologiede détermination de contraintes non standard a été mise en place et validée par dessimulations utilisant un modèle d’homogénéisation auto-cohérent.Ces modèles d’homogénéisation sont intéressants d’un point de vue de la déterminationdes contraintes résiduelles par diffraction des rayons X car ils permettent la prise encompte de l’aspect multiphasé des matériaux ainsi que leur caractère anisotrope. / Titanium alloys are widely used in aeronautics industries. However, these alloys are highlysensitive to the method for elaboration and transformation processes. These processesimpact on the surface integrity of products. Therefore, they have to be controlled inorder to predict life time of structures.In this work, the study of surface integrity is focused on the description of microstructalevolution, residual stresses and micro-geometry. Four points bending tests were performedin order to determined the fatigue limit of the different studied processes. We find outa good consistency between models used to determinate lifetime from surface integrityinvestigation and experimental results.Moreover, microstructures of titanium alloys are highly complex and they often are twophasedmaterials. Consequently, we studied the heterogeneous behaviour of such materialby X-ray diffraction investigation.We found out that these heterogeneities have an impacton residual stresses determination. Therefore, a non-standard methodology was definedand validate by simulation using a micro-mechanic model : a self-consistent model.Micro-mechanical models are interesting for the residual stresses determination using Xraydiffraction because they allow to take into account heterogeneous and anisotropicbehaviours through anisotropic elasticity and anisotropic texture.

As ligas Ti-??constituem um grupo promissor de ligas de titânio em termos de processamento, propriedades e aplicações potenciais. Além do projeto da liga em termos de composição, a obtenção de microestruturas adequadas por meio de tratamentos térmicos é necessária para otimizar o balanço entre resistência mecânica e tenacidade. O presente trabalho teve por objetivo estudar o comportamento em fadiga de alto ciclo da liga Ti-13V-11Cr-3Al, uma liga de alta resistência mecânica destinada ao emprego na indústria aeroespacial, sob duas condições de tratamento. O material foi recebido na forma de barras com 7,6 mm de diâmetro, apresentando microestrutura composta por grãos alongados e propriedades em tração combinando alto limite de resistência (1.479 MPa) e razoável ductilidade (deformação verdadeira de fratura igual a 0,217). O material foi tratado termicamente em duas condições: recozimento a vácuo a 750?C por 1 h com resfriamento lento, e tratamento flash em banho de sal a 650?C por 6 min, com posterior têmpera em água. O tratamento flash resultou em melhor combinação de resistência mecânica e ductilidade, avaliadas pelo ensaio de tração. O estudo da resistência à fadiga baseou-se na obtenção de curvas ?/N por meio de ensaios de flexão rotativa (R = -1) empregando-se corpos de prova do tipo viga em balanço. Para cada condição microestrutural, foram obtidas duas curvas ?/N, uma com amostras polidas e outra empregando-se corpos de prova com concentradores de tensão geométricos (entalhes), visando avaliar a sensibilidade ao entalhe desta liga nas duas condições microestruturais. O material submetido ao tratamento flash apresentou maior dispersão e menor resistência à fadiga, comparado ao material recozido; no entanto apresentou também menor sensibilidade ao entalhe em vidas longas (106 ciclos). O trabalho foi complementado por exames fractográficos para a identificação dos pontos de iniciação da fratura. / The Ti-??alloys are a promising group of titanium alloys in terms of processing, properties and potential applications. In order to achieve the optimum balance between strength and toughness it is necessary, besides determining a proper composition in the alloy design, to obtain suitable microstructures by means of thermo-mechanical process and heat treatment. This study aimed to assess the high cycle fatigue behavior of Ti-13V-11Cr- 3Al alloy, a high-strength alloy intended for use in the aerospace industry. The material was received in the form of bars with 7.6 mm in diameter, whose microstructure comprises elongated grains and whose mechanical properties combines high tensile strength (1,479 MPa), and reasonable ductility (true strain fracture equal to 0.217). The material was heat treated in two conditions: vacuum annealing at 750?C for 1 h with slow cooling, and flash 650?C salt bath for 6 min with subsequent water quenching. The study was based on obtaining ??/ N curves through by rotary bending (R = -1) fatigue tests employing cantilever beam specimens. For each microstructural condition, two ??/ N curves were obtained, the former with smooth samples and the latter employing specimens with geometric stress concentrators (notches), to evaluate the notch sensitivity of this alloy in both microstructural conditions. The material subjected to flash treatment showed greater dispersion and lower fatigue strength compared to annealed; however also showed lower notch sensitivity at long life (106 cycles). The study was complemented by fractographics analysis to identify the fracture initiation points.

Dentre as diversas aplicações onde o titânio e suas ligas são empregados destaca-se a indústria aeroespacial, pois suas excelentes propriedades de resistência à fratura, resistência à corrosão e razão resistência/peso, a qual se mantém a elevadas temperaturas, torna imprescindível o seu uso. Por outro lado, a usinabilidade de titânio e suas ligas é geralmente considerada como sendo ruim devido a algumas características, como por exemplo, baixa condutividade térmica, reatividade química com a maioria dos materiais ferramenta, alta resistência em altas temperaturas e baixo módulo de elasticidade, entre outras. Por esses motivos, foram realizadas operações de torneamento de ultraprecisão com ferramenta de diamante, a fim de determinar o comportamento do Ti comercialmente puro (CP) e sua liga mais comercialmente usada (Ti-6Al-4V). Os ensaios foram feitos com uso de fluido de corte aplicado em forma de névoa (~100ml/hora) em uma máquina-ferramenta com alta rigidez e precisão de posicionamento (resolução 10 nanômetros). As técnicas de caracterização de superfície usadas foram: microscopia eletrônica de varredura, óptica e de força atômica, microdureza Vickers, perfilometria mecânica e óptica. Os danos superficiais observados devido à interação ferramenta/material foram associados ao fenômeno conhecido como delaminação. Os resultados obtidos de microdureza superficial mostraram aumentos que chegaram à ordem de 85% para o Ti (CP) e de 77% para o Ti-6Al-4V. Os menores valores encontrados para Ra ficaram em torno de 30 nm para o Ti (CP) e de 46 nm para o Ti-6Al-4V enquanto os menores valores encontrados de Rt para o Ti(CP) foi de 590 nm e para o Ti-6Al-4V foi de 416 nm sob as mesmas condições de usinagem. Portanto, a usinagem de ultraprecisão se torna uma alternativa eficiente no uso de operações de acabamento melhorando a integridade superficial do Ti (CP) e suas ligas. / Among the various applications where titanium and its alloys are employed, the aeronautic industry is one of the most important, because of their excellent properties such as fracture strength, corrosion resistance and high strength to weight ratio, which is maintained at high temperatures. However, the machinability of titanium and its alloys is generally considered poor due to some of their characteristics as low thermal conductivity, chemical reactivity with most tool materials, high resistance to mechanical work at high temperature and low young\'s modulus, etc. Ultraprecision diamond turning has been carried out in order to determine the behaviour of commercially pure Ti and the most used alloy ((Ti-6Al-4V). Cutting fluid has been applied in the form of mist (~100 ml/hora) during the experiments. A high stiffness and high precision machine tool has been used (positioning precision of 10 nm). Scanning electron microscopy, optical microscopy, atomic force microscopy, Vickers microhardness test and contact and optical perfilometry have been used to characterize the machined surfaces. The surface damage observed due to the interaction tool-workpiece have been associated with the known phenomenon of delamination. The microhardness results presented an increase of the order of 85% for Ti (CP) and of 77% for the Ti-6Al-4V alloy after machining. The lowest values for Ra are around 30 nm for Ti (CP) and 46 nm for Ti-6Al-4V and the lowest values of Rt for Ti (CP) are about 590 nm and 416 nm for the Ti-6Al-4V, for the same conditions. The ultraprecision machining of titanium and its alloys may be an efficient alternative for finish operations resulting in better surface integrity.

A nitretação da liga NbTi 50% em peso de Ti, foi examinada em uma atmosfera de nitrogênio, na faixa de temperatura 800-1000&#176C, utilizando-se a técnica de difratometria de raios-X, microscopia eletrônica de varredura (MEV) e metalografia. Foi observada a formação de duas camadas contínuas de nitretos sobre a matriz: fase-&#948 (TiN) mais externa e a fase-&#958(Ti2N) mais interna. As duas camadas cresceram continuamente durante a reação, sendo que, a frente de saturação da matriz, pelo nitrogênio, cresceu mais rapidamente do que a frente de saturação do TiN. Foi proposto um método, utilizando-se dados de difratometria de raios-X, para se estudar o crescimento de camadas, sobre metais e ligas, formadas durante a reação com gases. A aplicação deste método permitiu fazer uma avaliação da cinética de crescimento da camada de TiN sobre a camada de Ti2N. A energia de ativação para o crescimento do TiN foi de 19 Kcal/mol, obtida a partir da lei temporal linear, e indicou um mecanismo não controlado pela difusão do nitrogênio através da camada de TiN. Os resultados obtidos indicam que o nitrogênio difunde através de túneis, ricos em Nb, onde sua difusão é mais rápida do que através do TiN. / The NbTi (50% wt. Ti) alloy nitretation under nitrogen atmosphere (p=760mm) at 800-1000&#176C was studied by X-ray diffractometry, TEM and optical metallograpy. During the reaction the two phases (Ti2N &#948 and TiN &#958) growed continuously, the (Ti2N, N2) reaction front growed faster than the (TiN, N2). A method for study the scale growing was proposed using X-ray diffractometry data. By using this method, the growth of TiN scale was analyzed and the activation energy of 19 Kcal/mol was determined using a linear timming Law indicating a mechanism not controlled by diffusion througth TiN layer. The present results suggest that the diffusion through the tight tunnels, rich in Nb, allow a fast transport of nitrogen through the TiN layer.

Dentre as diversas aplicações onde o titânio e suas ligas são empregados destaca-se a indústria aeroespacial, pois suas excelentes propriedades de resistência à fratura, resistência à corrosão e razão resistência/peso, a qual se mantém a elevadas temperaturas, torna imprescindível o seu uso. Por outro lado, a usinabilidade de titânio e suas ligas é geralmente considerada como sendo ruim devido a algumas características, como por exemplo, baixa condutividade térmica, reatividade química com a maioria dos materiais ferramenta, alta resistência em altas temperaturas e baixo módulo de elasticidade, entre outras. Por esses motivos, foram realizadas operações de torneamento de ultraprecisão com ferramenta de diamante, a fim de determinar o comportamento do Ti comercialmente puro (CP) e sua liga mais comercialmente usada (Ti-6Al-4V). Os ensaios foram feitos com uso de fluido de corte aplicado em forma de névoa (~100ml/hora) em uma máquina-ferramenta com alta rigidez e precisão de posicionamento (resolução 10 nanômetros). As técnicas de caracterização de superfície usadas foram: microscopia eletrônica de varredura, óptica e de força atômica, microdureza Vickers, perfilometria mecânica e óptica. Os danos superficiais observados devido à interação ferramenta/material foram associados ao fenômeno conhecido como delaminação. Os resultados obtidos de microdureza superficial mostraram aumentos que chegaram à ordem de 85% para o Ti (CP) e de 77% para o Ti-6Al-4V. Os menores valores encontrados para Ra ficaram em torno de 30 nm para o Ti (CP) e de 46 nm para o Ti-6Al-4V enquanto os menores valores encontrados de Rt para o Ti(CP) foi de 590 nm e para o Ti-6Al-4V foi de 416 nm sob as mesmas condições de usinagem. Portanto, a usinagem de ultraprecisão se torna uma alternativa eficiente no uso de operações de acabamento melhorando a integridade superficial do Ti (CP) e suas ligas. / Among the various applications where titanium and its alloys are employed, the aeronautic industry is one of the most important, because of their excellent properties such as fracture strength, corrosion resistance and high strength to weight ratio, which is maintained at high temperatures. However, the machinability of titanium and its alloys is generally considered poor due to some of their characteristics as low thermal conductivity, chemical reactivity with most tool materials, high resistance to mechanical work at high temperature and low young\'s modulus, etc. Ultraprecision diamond turning has been carried out in order to determine the behaviour of commercially pure Ti and the most used alloy ((Ti-6Al-4V). Cutting fluid has been applied in the form of mist (~100 ml/hora) during the experiments. A high stiffness and high precision machine tool has been used (positioning precision of 10 nm). Scanning electron microscopy, optical microscopy, atomic force microscopy, Vickers microhardness test and contact and optical perfilometry have been used to characterize the machined surfaces. The surface damage observed due to the interaction tool-workpiece have been associated with the known phenomenon of delamination. The microhardness results presented an increase of the order of 85% for Ti (CP) and of 77% for the Ti-6Al-4V alloy after machining. The lowest values for Ra are around 30 nm for Ti (CP) and 46 nm for Ti-6Al-4V and the lowest values of Rt for Ti (CP) are about 590 nm and 416 nm for the Ti-6Al-4V, for the same conditions. The ultraprecision machining of titanium and its alloys may be an efficient alternative for finish operations resulting in better surface integrity.

Using current technology, it is now possible to probe material at atomic length scales, increasing our fundamental understanding of material behavior and properties. Metastable β titanium alloys are a subset of titanium alloys with huge potential for the aerospace sector. However, they exhibit atomic transformations which, even after 60 years of research, are still disputed. For example, these alloys are strengthened using the ω phase, but the mechanism by which this phase forms and its stability are still in question. The aim of this PhD project was to investigate the strengthening of metastable Ti-15wt.%Mo by understanding the stability and transformation pathways which make the metastable β titanium alloy class unique. Athermal ω shares the same composition as the β matrix and is formed by rapid cooling from the β phase field. The classical theory of athermal ω formation is based upon a diffusion-less mechanism in which consecutive pairs of {111}β planes collapse together. However, latest high-resolution electron microscope observations have suggested chemical alterations occur as well, which give reason to challenge this classical formation mechanism. Two novel methods were explored to determine the nature of the ω phase: 1) electron imaging of thin material at different collection angles and 2) total X-ray scattering analysis of large volumes of material. Complementary techniques are invaluable since thin foil artefacts were identified. In particular, a new B2 structured phase in the Ti-15wt.%Mo alloy was observed only in thin electron transparent material. Experimental data from the two new methods were compared to simulations. It was found that a frozen phonon description of the ω structure provided a best fit in both scenarios. The results are therefore consistent with the classical theory of ω formation but the collapse of the {111}β planes towards the ω phase is not considered complete.

Le but de cette thèse est d'étudier la propagation des ondes ultrasonores dans les milieux présentant des diffuseurs allongés, et plus particulièrement les alliages de titane comportant des macrozones. Cette étude est menée à l'aide d'un dispositif expérimental constitué d'un transducteur multi-éléments linéaire en rotation autour de son axe, piloté par une électronique multivoie programmable. Le champ rétrodiffusé par un milieu à diffuseurs allongés présente une cohérence spatiale anisotrope et nous montrons que l'intensité rétrodiffusée par un tel milieu dépend de l'orientation du transducteur dans le plan perpendiculaire à la direction d'insonification. Cette dépendance permet de déterminer la direction d'élongation des macrozones, leur taille caractéristique, et de réduire le bruit de structure lors d'un contrôle ultrasonore. Une cartographie en trois dimensions de la direction locale des macrozones est ainsi réalisable en employant deux faisceaux ultrasonores focalisés en un même point selon deux directions différentes. Afin de déterminer le régime de diffusion aux fréquences usuelles de contrôle, nous avons développé, à partir des travaux de thèse d'Alexandre Aubry (UPMC 2008), un indicateur du niveau de diffusion multiple : l'estimateur de la proportion de diffusion simple. Il permet de démontrer que la diffusion multiple est importante dans les alliages de TA6V et Ti17. Nous avons de plus étendu cette méthode au cas d'interfaces eau/solide complexes suffisamment régulières. Enfin, nous proposons une nouvelle visualisation des résultats de simulations de forge permettant d'observer la direction réelle des macrozones et de mieux concevoir les plans de contrôle. / This manuscript is devoted to the study of ultrasonic wave propagation in complex media composed of elongated scatterers, and specifically titanium alloys comprising macrozones. This study is carried out using a linear phased array probe rotating around its axis and driven by a multi-channel programmable amplifier. The ultrasonic field backscattered by elongated scatterers presents an anisotropic spatial coherence and we show that the intensity backscattered by such a medium depends on the angle between scatterers elongation direction and the orientation of the probe in the plane perpendicular to the insonified direction. This dependence allows to determine the macrozones elongation direction and size, and to reduce the structural noise during a non-destructive evaluation. A three dimensional cartography of local elongation direction of macrozones is achievable by using two beams focused at the same point but with different directions of incidence. In order to assess the scattering regime at conventional testing frequencies, we developed a multiple scattering measure: the single scattering proportion estimator, based on Alexandre Aubry’s PhD thesis (UPMC 2008). Applied on TA6V and Ti17 alloys, it showed strong multiple scattering. We further extended this estimator to the case of water/solid complex interfaces. Lastly, we proposed a new visualization of forging simulation results, allowing to directly observe the real macrozones elongation direction and to better design inspection procedures.

The increasing use of alpha/beta Ti-Al alloys in critical aircraft gas turbine engine and airframe applications necessitates the further development of physically-based constitutive models that account for their complex microdeformation mechanisms. Alpha/beta Ti-Al alloys are dual-phase in nature consisting of a mixture of hcp (alpha) and bcc (beta) crystal structures, which through variation in alloying elements and/or processing techniques can be produced in a wide range of microstructural compositions and morphologies. A constitutive model for these materials should address the various sources of material anisotropy and heterogeneity at both the micro and macroscales. The main sources of anisotropy in these materials are the low symmetry of the hcp phase, the texture, the relative strengths of different slip systems, non-planar dislocation core structures, phase distributions, and dislocation substructure evolution. The focus of this work is the development of a 3-D crystal plasticity model for duplex Ti-6Al-4V (Ti-64), an (alpha+beta) alloy. The model is used to study the process of attachment fatigue. Attachment fatigue is a boundary layer phenomenon in which most of the plastic deformation and damage accumulation occurs at depths on the order of tens of microns and encompasses regions of only a few grains into the depth of the material. The use of computational micromechanics-based crystal plasticity models to study attachment fatigue is a relatively new approach. This approach has the potential to offer additional insight to classical homogeneous plasticity models, since the length scales over which relative slip and crack initiation occur during this process is on the order of microstructural dimensions. Emphasis is placed on understanding the effects that texture, slip strength anisotropy, and phase distribution have on the surface and subsurface deformation fields during attachment fatigue. The deformation fields are quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and plastic strain-based critical plane multiaxial fatigue parameters.