The effect of deformation on microstructural changes in metals and alloys is the subject of considerable practical interest. The ultimate goal is to control, improve and optimize the microstructure and texture of the finished products produced by metal forming operations. The development in the subject field is remarkable but a more in-depth study could lead us to the better understanding of the phenomena.   In the present work microstructural evolution during the plastic deformation of as-cast pure metals and alloys is studied. An experimental method was developed to study the material behavior under the hot compression testing. This method was applied on the as-cast structure of copper, bearing steel, Incoloy 825 and β brass at different temperatures and strain rates. The temperature of the samples was measured during and after the deformation process. The microstructure of the samples was examined by optical microscopy and scanning electron microscopy (SEM). The microstructural evolution during deformation process was investigated by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The samples were subsequently subjected to electron microprobe analysis (EMPA) to investigate the effect of the deformation on the microsegregation of Mo, Cr, Si, and Mn.   It was observed that the temperature of the samples deformed at strain rates of 5 and 10 s-1 increases abruptly after the deformation stops. However, compression test at the lower strain rates of 1 and 0.5 s-1 revealed that a constant temperature was maintained in the early stage of deformation, followed by an increase until the maximum temperature was obtained. This temperature behavior can be explained by the microstructural evolution during the deformation process. Micrograph analysis revealed the formation of deformation bands (DBs) in highly strained regions. The DBs are highly effective sites for recrystallization. The interdendritic regions are suitable sites for the formation of DBs due to the high internal energy in these regions. EMPA indicated a tendency towards uphill diffusion of Mo in the DBs with increasing strain. The effect of strain on the dissolution of carbides in the band structure of bearing steel was investigated by measuring the volume fraction of carbides inside the band structure at different strain levels. The results indicate that carbide dissolution is influenced by strain.    The microstructural evolution inside the DBs was studied as a function of several properties: temperature, internal energy, and microsegregation. Compression of β brass revealed that twinning is the most prominent feature in the microstructure. EBSD analysis and energy calculations demonstrated that the twinning is not due to a martensitic process but rather the order/disorder transition during the deformation process. The effect of heat treatment at Tc (650°C) prior to deformation on the microstructure of β brass was also investigated, which revealed a relationship between twin formation and the anti-phase domain boundaries / <p>QC 20131104</p>

Da in der Natur die Festigkeit der Stoffe bzw. Werkstoffe mit deren Massendichte korreliert [1], bieten sich dem Werkstoffingenieur zwei Möglichkeiten das genannte Ziel zu erreichen: Entweder er reduziert die effektive Dichte bereits sehr fester Werkstoffe durch konstruktive bzw. geometrische Optimierungen, oder es gelingt sehr leichte Werkstoffe mit deutlich gesteigerter Festigkeit herzustellen. Die erstgenannte Verfahrensweise stellt zu großen Teilen ein konstruktives bzw. fertigungstechnisches Problem dar. Von werkstoffwissenschaftlichem Interesse ist deshalb nur die zweite Möglichkeit. Dabei sollen sämtliche derzeit bekannte festigkeitssteigernde Faktoren und möglicherweise auch deren Synergien genutzt werden um einen hochfesten Leichtbauwerkstoff herzustellen. Dazu muss gleichzeitig ein neuartiges Hochumformverfahren für Leichtmetallverbundwerkstoffe erarbeitet werden, das diesen Anforderungen entspricht und eine dafür geeignete Werkstoffkombination gefunden werden. Konventionelle Verfahren zur Hochumformung erlauben häufig nur unter erheblichem Mehraufwand die Verarbeitung von Verbundwerkstoffen, weshalb die Hochumformung von Leichtmetallverbundwerkstoffen zur Festigkeitssteigerung in der Literatur praktisch keine Rolle spielt. Deshalb soll in dieser Arbeit das Umformverfahren Rundkneten zur Anwendung kommen, das die Hochumformung auch sehr heterogener Werkstoffe erlaubt. Darüber hinaus wird eine zusätzliche positive Wirkung auf die Festigkeit durch eingebaute Grenzflächen auf den Gesamtverbund erwartet. Wie sich im Laufe der Arbeit heraus stellte, eignet sich das verwendete Verfahren nicht ausschließlich zur Festigkeitssteigerung von Verbundwerkstoffen. Durch die sehr regelmäßige und fraktale Anordnung der Komponenten im Gesamtverbund ergaben sich auch einige Anknüpfungspunkte, die weit über die Eignung im Sinne eines Leichtbauwerkstoffes hinaus gehen. Aus diesem Grund liegt der Schwerpunkt der Arbeit zwar auf der mechanischen Charakterisierung der hergestellten Verbunde, in Kapitel 6 werden aber auch weitere Nutzungsmöglichkeiten diskutiert. Die gewählte Materialkombination Titan-Aluminium ist als Beispiel zu verstehen. Prinzipiell ist das vorgestellte Verfahren auf viele weitere Materialkombinationen anwendbar, solange grundlegende umformtechnische Regeln beachtet werden. [1] Ashby, M. F.: Materials Selection in Mechanical Design. Heidelberg: Spektrum Akademischer Verlag, 2006. 648 S.

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-08-16T19:31:06Z No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-01-30T16:50:04Z (GMT) No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-01-30T16:50:13Z (GMT) No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Made available in DSpace on 2018-01-30T16:57:24Z (GMT). No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) Previous issue date: 2017-03-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Biomedical devices currently in use (prostheses, implants) have satisfactory performance in many cases. However, sometimes the body reacts to the device insertion and may lead to its rapid replacement. Some of these disadvantages can be solved by the use of titanium and its alloys, due to their excellent combination of corrosion resistance, wear resistance and biocompatibility compared to other competing biomaterials. This work presents the possibility of obtaining near β titanium alloy with ultrafine grains produced by severe plastic deformation. For this, the Ti-13Nb-13Zr alloy was processed by high-pressure torsion processing method. Samples were processed with different loads and number of turns. The samples were evaluated by Vickers microhardness. As-received and deformed samples were analyzed through X-Ray diffraction. The microstructures were observed by optical microscopy and scanning electron microscope and the microtexture and phase mappings of the material evaluated through the ASTAR equipment in the transmission electron microscope. After characterization, it was observed that there is a refinement of the microstructure and increase of the microhardness of the Ti-13Nb-13Zr alloy deformed by HPT. Due to the superior microhardness of the deformed material and the results of phase transformations indicate a potential application as nanostructured biomaterial. / Os dispositivos biomédicos utilizados atualmente (próteses, implantes) possuem desempenho satisfatório em muitos casos. No entanto, às vezes, o corpo reage à inserção destes dispositivos exigindo a sua rápida substituição. Algumas destas desvantagens podem ser resolvidas pelo uso de titânio e suas ligas, devido à sua excelente combinação de resistência à corrosão, resistência ao desgaste e biocompatibilidade em comparação com outros biomateriais concorrentes. Este trabalho apresenta a possibilidade de obtenção de liga de titânio quase β com grãos ultrafinos produzidos por deformação plástica severa para três diferentes condições iniciais de microestrutura. Para isso, a liga Ti-13Nb-13Zr foi processada pelo método de processamento de torção sob alta pressão (High-Pressure Torsion - HPT). As amostras foram processadas com diferentes cargas e número de voltas e avaliadas por meio de microdureza Vickers. Amostras como recebidas e deformadas foram analisadas através de difração de raios X. As microestruturas foram observadas por meio de microscopia óptica e eletrônica de varredura. A microtextura e mapeamentos de fase do material foram avaliados através do equipamento ASTAR no microscópio eletrônico de transmissão. Após a caracterização, foi observado que existe um refinamento da microestrutura e aumento da microdureza da liga Ti-13Nb-13Zr deformada por HPT. A microdureza superior do material deformado e os resultados de transformações de fase apontam para uma potencial aplicação como biomaterial nanoestruturado.

Les composites à matrice Titane avec des renforts nanométriques présente des améliorations intéressantes quant aux propriétés mécaniques, sans affecter la ductilitédu matériau. Ce travail de thèse se concentre sur l’élaboration et la caractérisation dematériaux composites de Titane obtenus par deux différents procédés de métallurgie despoudres : La densification par déformation plastique sévère utilisant l’ECAP (Equal ChannelAngular Pressing) et les procédés de frittage par hydrogénation/déshydrogénation (HDH).L’ECAP est un procédé de densification rapide utilisant la déformation plastique desmatériaux, se faisant à relativement basse température. Les procédés HDH utilisent ladéshydrogénation du titane comme un levier sur les mécanismes de frittage. Les différentsnano-renforts utilisés dans ces travaux sont les particules sphériques de TiC et les aiguillesde TiB. Cette étude montre l’influence de la nature et de la forme des renforts, ainsi que duprocédé de métallurgie des poudres utilisé, sur les propriétés et la microstructure final desmatériaux denses. / Titanium based composites using nano-sized reinforcements are goodcandidates for the improvement in mechanical properties without affecting ductility. Thisstudy is dedicated to fabrication and characterisation of Ti-based composites using twodifferent powder metallurgy processes: Densification using severe plastic deformation viaEqual Channel Angular Pressing (ECAP) and Hydrogenation/Dehydrogenation (HDH)sintering processes (pressureless sintering and hot pressing).ECAP is a fast process basedon a severe plastic deformation of material at relatively low temperature. HDH processes usethe dehydrogenation of Ti as a leverage of the sintering. The different nanosizedreinforcements used in this study are the TiC spherical particles and the whisker shaped TiB.This study shows the influence of either the reinforcement nature and type, and the powdermetallurgy processes used, on the final microstructure and properties of the dense materials.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This work investigated the effect of different types of homogenization on the microstructure and the cold deformation behavior of Ti-35Nb-0,15Si alloy. Homogenization was performed under two conditions. Condition 1: treatment in ambient air at 1000 °C for 8, 24, 48, 72, 96 and 120 hours. Condition 2: treatment in argon atmosphere at 1000 °C for 8 hours. For the cold deformation study samples of the Ti-35Nb-0,15Si alloy and samples of Ti CP were deformed by ECAP with up to 8 passes, using the routes A and BC in a matrix with angle of intersection between the channels of Φ = 120º. The microstructural characterization was performed in the internal and external regions with the aid of optical microscopy, scanning electron microscopy and X-ray diffractograms. Vickers microhardness measurements were performed to evaluate the changes caused by deformation. In the ambient air atmosphere the samples showed the presence of the oxide layer influencing the hardness and the level of cold deformation. With the deformation of Ti-35Nb-0,15Si by ECAP it was possible to refine the structure and increase the hardness with increasing the number of passes. XRD analysis showed the presence of the α” phase induced by deformation. With the Ti CP deformed by ECAP, it was possible to analyze the microstructural evolution throughout the sample. / Este trabalho investigou o efeito de diferentes tipos de homogeneização sobre a microestrutura e o comportamento em deformação a frio da liga Ti-35Nb-0,15Si. A homogeneização foi realizada em duas condições. Condição 1: tratamento em atmosfera de ar ambiente na temperatura 1000 °C por 8, 24, 48, 72, 96 e 120 horas. Condição 2: tratamento em atmosfera inerte de argônio de 1000 °C por 8 horas. Para o estudo de deformação a frio, amostras da liga Ti-35Nb-0,15Si e amostras de Ti CP foram deformadas via ECAP com até 8 passes, utilizando as rotas A e BC numa matriz com ângulo de intersecção entre os canais de Φ = 120º. A caracterização microestrutural foi realizada nas regiões internas e externas com o auxílio de microscopia óptica, microscopia eletrônica de varredura e difratogramas de raios-X. Medidas de microdureza Vickers foram realizadas para avaliar as mudanças ocasionadas pela deformação. Em atmosfera de ar ambiente as amostras apresentaram a presença da camada de óxido influenciando na dureza e no nível de deformação a frio. Com a deformação de Ti-35Nb-0,15Si por ECAP foi possível refinar a estrutura e aumentar a dureza com o aumento do número de passes. A análise de DRX mostrou a presença da fase α’’ induzida por deformação. Com o Ti CP deformado por ECAP foi possível analisar a evolução microestrutural ao longo da amostra. / São Cristóvão, SE

Neste trabalho se estuda a aplicação de um método de Ensaio Não Destrutivo, baseado no Ruído Magnético de Barkhausen (RMB), na medição de falhas superficiais em aços. O RMB é gerado por abruptas mudanças na magnetização de materiais quando submetidos a campos magnéticos variáveis. Essas mudanças são afetadas pela microestrutura e a presença e distribuição de tensões elásticas (compressão e tração). Inicialmente apresenta-se um estudo de medições de tensões produzidas por flexão, e perfis de dureza, via RMB. Analisa-se a influência de parâmetros de medição e analise dos sinais, na qualidade do diagnostico. Analisou-se a sensibilidade dos sinais de RMB na medição de tensão a parâmetros tais como direção de laminação da chapa, intensidade e frequência de campo magnético de excitação. Nos estudos da aplicação do RMB em medições de dureza, se usaram amostras de ensaio Jominy, com variação contínua de dureza. Utilizou-se dois tipos de materiais e foram feitas medidas de dureza HRC, RMB e análise metalográfica. O comportamento de diferentes parâmetros dos sinais de RMB se correlacionou com as medições de Dureza e as micrografias obtidas. As amplitudes do RMB mostraram ser inversamente proporcionais aos níveis de dureza. Também se apresenta uma nova tecnologia para inspeção de superfícies, baseado no conceito do Barkhausen Contínuo. São apresentadas análises de parâmetros de medição e de configuração de sonda, na inspeção sob três situações: presença de defeitos volumétricos, deformação plástica, e tensões mecânicas aplicadas. Estudou-se a influência de parâmetros como, amplitude do campo aplicado, velocidade de varredura, posicionamento e características do sensor, no nível de detecção do dano. Métodos de processamento de sinais específicos foram desenvolvidos. Os resultados mostraram, que para cada um dos casos estudados, é possível detectar a posição e nível do dano produzido. Essa nova tecnologia aumenta o espectro de soluções de Ensaios Não Destrutivos para problemas não contemplados pelos métodos existentes. / This work studies the application of a non-destructive testing method, based on the Magnetic Barkhausen Noise (MBN). MBN is generated by abrupt changes in magnetization of ferromagnetic materials subjected to variable magnetic fields. These changes are affected by the presence and distribution of elastic stresses (compression and traction) in the material and by its microstructure. At first, the present study describes MBN measurements of stresses produced by bending and MBN measurements of hardness profiles, in steel samples. The influence of both, measurement parameters and signal analysis, in the quality of fault diagnosis is also analysed. In this context, regarding stress measurements, the MBN signal sensitivity to parameters like surface lamination direction, intensity and frequency of magnetic excitation field is studied. On the other hand, application of MBN to hardness measurements used Jominy essay samples presenting continuous hardness variation. In this case, samples of two different materials were employed and hardness measurements were obtained by HRC, MBN as well as by metallographic analysis techniques. Behaviour of different MBN signal parameters were correlated with obtained hardness measurements and micrography. MBN amplitudes were shown to be inversely proportional to hardness levels. Finally, the work describes a new surface inspection technology, based on the concept of Continuous Barkhausen. In this context, the study analyses probe configuration and measurement parameters, concerning inspection of surface fault from three different origins: volumetric, plastic deformation, applied mechanical stress. The influence of parameters like applied field amplitude, probe velocity, sensor position and characteristics, in detecting damage level was also evaluated. In particular, specific signal processing methods were developed. Results showed that, for each studied case, it is possible to detect damage position and level. This new technology increases the range of NDT essay solutions for problems not yet contemplated by existing methods.

Este trabalho apresenta um método para o reconhecimento e a detecção automática dos diferentes valores ou graus de deformação plástica em Ensaios Não Destrutivos empregando o Ruído Magnético de Barkhausen. O método é baseado no uso de uma Rede Neural Probabilística que permite o diagnóstico automático dos diferentes valores de deformação plástica, conteúdo de carbono, estas medidas são procedentes das medições das amostras de placas de aço AISI 1006, 1050 e 1070, esta base de dados foi feita pelo grupo de pesquisadores do Laboratório de Dinâmica e Instrumentação LADIN da Escola Politécnica da USP, departamento da Mecânica. Os excelentes resultados da rede neural probabilística de detectar automaticamente os valores de deformação mostram a efetividade do desempenho da rede neural probabilística que tem um desempenho superior aos métodos não destrutivos tradicionais e que realmente esta nova tecnologia é uma excelente solução para o diagnóstico. / This work presents a method for automatic detection and recognition of different levels or degrees of plastic deformation in Non-Destructive Testing using the Magnetic Barkhausen Noise. The method is based on using a Probabilistic Neural Network that allows the automatic diagnosis of the different values of plastic deformation and carbon content. The measurements corresponds to samples of steel plates AISI 1006, 1050 and 1070, this database was made by the group of researchers from the Laboratory of Dynamics and Instrumentation LADIN the Polytechnic School of USP, Department of Mechanical Engineering. The results show the effectiveness of the probabilistic neural network to automatically detect plastic deformation levels as well as carbon content level. This method has a superior performance in comparison to traditional nondestructive methods.

Les procédés d’hyper-déformations (SPD) peuvent imposer de très grandes déformations à un métal et en transformer les propriétés métallurgiques de la matière en introduisant une forte densité de dislocations et un important affinement de la microstructure. Dans ce travail de thèse présenté, des expériences en torsion à haute pression (HPT) ont été réalisées pour la consolidation des différentes poudres de fer de taille à l’échelle nano et micrométrique. Ces expériences ont été effectuées avec succès à la température ambiante aboutissant à la fois à un faible niveau de porosité résiduelle et l'affinement significatif de la taille de grain, grâce à une importante déformation en cisaillement et à de la pression hydrostatique appliquée au procédé HPT. La compression a été faite en deux étapes: d'abord une compression axiale, puis déformation en cisaillement en tournant la partie inférieure de la filière HPT tout en maintenant constante la force axiale. L'homogénéité de la déformation en cisaillement à travers l'épaisseur du disque a été examinée par une mesure de déformation locale, qui montre une distribution du gradient. L'analyse par diffraction à rayons X a été réalisée sur des échantillons consolidés qui ont révélé une proportion peu importante d’oxydes. L'effet de la déformation en cisaillement sur la microstructure et la texture a été étudié par microscopie électronique à balayage et EBSD. La micro-dureté et la porosité moyenne des échantillons en fonction de la déformation en cisaillement, à pression hydrostatique constante, ont également été mesurées. Une trame de modélisation mise en œuvre dans le modèle de Taylor a été développée pour simuler l'effet du glissement aux joints de grains pour l'évolution de la texture cristallographique. Le principal effet constaté est un décalage des orientations idéales dans les conditions de cisaillement simple, ce qui a été vérifié expérimentalement. Le procédé de consolidation par HPT a été simulé numériquement en utilisant la méthode des éléments finis pour un modèle de plasticité des poudres. La simulation de ce dernier a permis de confirmer la porosité résiduelle moyenne observée expérimentalement et les différents gradients de la déformation plastique. La distribution de la densité locale a également été modélisée / Severe plastic deformation (SPD) processes can impose extremely large strains to a metal and transforming the metallurgical state of the material by introducing high dislocation density and high level of microstructure refinement. In the present thesis work High Pressure Torsion (HPT) experiments were performed for consolidation of different powders including Nano- and Micro- scaled iron powders. The experiments were carried out successfully at room temperature, achieving both low level of residual porosity and significant grain refinement, thanks to the intense shear strain and hydrostatic pressure applied in HPT. The compaction was done in two steps: first axial compaction, then shear deformation by rotating the bottom part of the HPT die while maintaining the axial force constant. The homogeneity of shear strain across the thickness of the disk was examined by local strain measurement, showing a gradient distribution. X-ray diffraction analysis was carried out on the consolidated samples which revealed no significant proportion of oxides. The effect of shear deformation on the microstructure and texture was investigated by metallographic scanning electron microscopy and electron backscattered diffraction (EBSD). The micro-hardness and average porosity of the samples as a function of shear strain at constant hydrostatic pressure were also measured. A modeling frame implemented into the Taylor model was developed to simulate the effect of Grain Boundary Sliding (GBS) on the evolution of crystallographic texture. The main effect found is a shift of the ideal orientations under simple shear conditions, which was verified experimentally. The consolidation process by HPT was simulated numerically using the finite element method together with a powder plasticity model. The simulation of the consolidation process permitted to confirm the experimentally observed average residual porosity and the different gradients in the plastic strain. The local density distribution was also modeled

Indentation hardness, H, has been widely used to characterize the mechanical properties of materials for more than a century because of the following advantages of this technique; (1) it requires small sample and (2) the test is non destructive in nature. Recent technological advances helped in the development of instrumented indentation machines which can record the load, P, vs. displacement, h, data continuously during indentation with excellent load and displacement resolutions. From these, H and the elastic modulus, E, of the indented material can be obtained on the basis of the ‘contact area’ of the indentation at the maximum load. The estimation of true contact area becomes difficult during ‘pile-up’ and ‘sink-in’, commonly observed phenomena while indentation of a low and high strain hardened materials. In order for the better understanding of these phenomena it is important to understand the plastic flow distribution under indenters. It is also important for the prediction of elastic-plastic properties from the P-h data. Recently, there have been considerable theoretical and simulation efforts on this front with a combination of dimensional analysis and finite element simulations. One of the important input parameter for the dimensional analysis is the ‘representative strain’ under the indenter, which is a strong function of the indenter geometry. However there is no comprehensive understanding of the representative strain under the indenter despite several studies till date. One objective of the present thesis is to conduct an experimental analysis of the plastic flow during the sharp indentation. The plastic zone size and shape under conical indenters of different apex angles in a pure and annealed copper were examined by employing the subsurface indentation technique to generate the hardness map. From these isostrain contours are constructed joining the data having similar strain values. The following are the key observations. (1) The plastic strain contours are elliptical in nature, spreading more along the direction of the indenter axis than the lateral direction. (2) The magnitude of the plastic strain in the contact region decreases with increasing the indenter angle. (3) The strain decay in the indentation direction follow a power-law relation with the distance. The estimated representative strains under the indenters, computed as the volume average strain within the elastic-plastic boundary, decreases with increasing indenter angle. We also performed finite element simulations to generate plastic flow distribution under the indenter geometries and compared with the experimental results. The results suggest that the experimental and computed average strains match well. However, the plastic strain contours do not, suggesting that further detailed understanding of the elasto-plastic deformation underneath the sharp indenter is essential before reliable estimates of plastic properties from the P-h curves can be made routinely. The second objective of this thesis is to understand plastic flow in amorphous alloys. It is now well established that plastic deformation in metallic glasses is pressure sensitive, owing to the fundamentally different mechanisms vis-à-vis the dislocation mediated plastic flow in crystalline metals alloys. Early work has shown that the pressure sensitivity of amorphous alloys gets reflected as high constraint factor, C (hardness to yield stress ratio), which sometimes exceed 3.0. In this thesis, we study the temperature dependence of pressure sensitive plastic flow in bulk metallic glasses (BMGs) using C as the proxy for the pressure sensitivity. Experiments on three different BMGs show that C increases with temperature hence the pressure sensitivity. In addition we have carried out finite element simulations to generate P-h curves for different levels of pressure sensitivities and match them with the experimental curves that are obtained at different temperatures. Simulations predict that higher pressure sensitivity index values are required to match the experimental curves at high temperatures confirming that the pressure sensitivity increases with increasing temperature. The fundamental mechanisms responsible for the increase in pressure sensitivity are discussed in detail. Finally we pose a question, is the increase in pressure sensitivity with temperature is common to other amorphous materials such as strong amorphous polymers? In order to answer this question we have chosen PMMA, a strong amorphous polymer. In this study also we have taken C as a proxy to index the pressure sensitivity. Indentation stress-strain curves are constructed at different temperature using spherical indentation experiments. The C values corresponding to different temperatures are determined and plotted as a function of temperature. It is found that C increases with temperature implying that the pressure sensitivity of amorphous polymers also increases with temperature. The micro-mechanisms responsible for the increase in pressure sensitivity are sought.

Le titane et ses alliages sont devenus des matériaux incontournables dans l'aéronautique, le domaine biomédical et l'industrie chimique et ce depuis le début des années 1950 en raison de haut rapport résistance/poids, une excellente biocompatibilité et une bonne résistance à la corrosion. La structure hexagonale du titane conduit à une anisotropie intrinsèque, due à la configuration atomique particulière et à une anisotropie extrinsèque, due à une texture marquée produite lors de leur élaboration. Le but de cette étude est d'améliorer la compréhension des mécanismes de déformation (glissement et maclage) dans le titane conduisant à cette anisotropie extrinsèque. Une technique d’essais interrompus in situ en MEB / EBSD a été utilisée lors la déformation plastique d’un alliage de titane T40 commercialement pur afin de suivre l'évolution de l'orientation cristalline et de la morphologie des grains au cours de la déformation sous différents types de chargement (traction, laminage, cisaillement). Cette technique permet de suivre l’apparition du maclage, la croissance et la forme des macles, l’interaction des variants de macles avec les grains voisins. Le Facteur de Schmid, l’orientation cristallographique, l'énergie de déformation plastique, et la déformation localisée liée à l’accommodation ont été calculés pour analyser l'activation du maclage, la croissance des macles, et la sélection de variant de macle. Des variants de macles avec un faible facteur Schmid, facteur qui exprime la force appliquée externe sur le plan de maclage et le long de la direction de maclage, ont été observés. Cela indique le facteur Schmid n'est pas très approprié pour prédire le maclage. Une règle de sélection de variant de maclage a été proposée sur la base de la consommation maximale d'énergie plastique. L'énergie plastique (on suppose le matériau comme étant rigide parfaitement plastique) consommée est calculée de la façon suivante : la déformation que produit chaque variant est exprimée du repère cristal au repère macroscopique ; la contrainte suit une loi de type Hall Petch, le diamètre des grains est alors défini comme la longueur maximale que peut prendre chaque variant dans le grain. Le variant qui consomme le plus d’énergie est activé le premier. Ces calculs sont en bon accord avec l'observation expérimentale : Les variants de macles sélectionnés étaient ceux conduisant à une consommation maximale de l'énergie plastique. Le libre parcours que peut prendre chaque variant est un critère pertinent puisque cela permet d’expliquer les différences observées dans un grain équiaxe ou un grain allongé en terme de nombre de variants présents et croissance des macles. Les essais interrompus in – situ montrent des chaines de macles ou paires de macles (propagation dans 2 ou plusieurs grains voisins). Une macle est activée en premier dans un grain, avec sa croissance, elle impose un champ de contrainte et un champ de déformation dans les grains voisins ce qui peut activer une macle dans les grains voisins. Les paires de macles ont un paramètre de Luster - Morris relativement élevé, ce qui traduit une bonne compatibilité entre les macles et une propagation facile de macle dans les grains voisins. Par une simple transformation du tenseur gradient de déplacement du variant utilisé dans le grain considéré on obtient la déformation imposée dans le grain voisin. Le variant de macle dans ce grain voisin pouvant accommoder le plus facilement la déformation imposée sera activé. Ceci montre que la déformation locale peut influencer l’activation et la sélection de variant de macle / Titanium and its alloys have become backbone materials for aerospace, biomedical field and chemical industries since the early 1950s because of the high strength-weight ratio, excellent biocompatibility, and good corrosion resistance. The hexagonal structure of titanium leads to an intrinsic anisotropy of the particular atomic configuration and extrinsic anisotropy of the texture to a product marked in their elaboration. The purpose of this study is to improve understanding of the deformation mechanisms (slip and twinning) in the titanium leading to the extrinsic anisotropy. Technical testing interrupted in situ SEM / EBSD was used in the plastic deformation of a commercially pure titanium alloy T40 to follow the crystal orientation and morphology of the grains during deformation under different types of loading (tension, rolling). This technique allows following the occurrence of twinning, growth and shape of the twins, the interaction of the twin variants with neighboring grains. Schmid factor, crystallographic orientation, plastic deformation energy, and localized strain accommodation were calculated to analyze the twin activation, the twin growth, and twin variant selection. Twinning variant with relatively low global Schmid factor, which resolves the externally applied force onto the twinning plane and along the twinning direction, were observed. That indicates the global Schmid factor is not very appropriate to predict twinning. A twinning variant selection rule was proposed based on the maximum plastic energy consumption. The plastic energy ( the material is assumed to be perfectly rigid plastic) consumed is calculated as follows: the deformation produced by each variant is expressed from crystal frame to macro frame; stress follows a Hall Petch type rule, the grain diameter is then defined as the maximum length that can take each variant in the grain. The variant which can consume the maximum of the externally imposed energy is expected to twin first. The calculation results show good agreement with the experimental observation: The selected twin variants were the one leading to maximum plastic energy consumption. The variant free path length relevant criterion so indicates the reason why the equiaxed grains and elongated grains have the difference in twin variant number and twinning variant growth. The interrupted in-situ investigation shows that the twin chains or twin pairs observed in this work are a sequential twinning behavior. A twin activated first, with its growth, it imposes a strain field and a stress field into its neighboring grains and possibly stimulate a twin in the neighboring grain. The twin pairs show a good alignment with a relatively high Luster-Morris parameter, which is a measure of the compatibility of twinning systems through a grain boundary. By the displacement gradient tensor transformation, the imposed strain into the neighboring grain was obtained. The twinning variant which can better accommodate the imposed strain can be activated. It indicates that the local strain can influence the twinning activation and variant selection