Free machining alloys containing soft immiscible phases in the aluminium (Al) matrix, like lead (Pb) and bismuth (Bi), are of great industrial interest. Typical applications in automotive industry are components requiring very high machinability, such as braking pistons and antiblocking system (ABS) housings. Presence of soft immiscible phases is giving their machining properties to this class of alloys. These phases melt due to localised heat build-up generated by machining process and induce chips breaking. Such type of alloys offers best in class performance when the soft phase is uniformly distributed in the Al matrix. The main objective of this work was to develop a method to tailor the distribution of the immiscible phase particles in the final solidified structure of DC cast billets in order to provide enhanced machinability while keeping low levels of Pb and/or Bi additions. As a consequence, another objective of this study was to improve recyclability of such alloys as well as to reduce their environmental impact. Three categories of Al-Pb alloys and different solidification paths were studied: hypermonotectic Al-3Pb, monotectic Al-1.2Pb and industrial hypo-monotectic free machining alloy containing both Pb and Bi. A newly developed melt conditioning combines mechanical, thermal and chemical treatments to obtain a very fine and uniform distribution of the immiscible phase droplets and eliminate compositional heterogeneities. The effect of these new melt treatments on microstructure was evaluated. For the soft phase droplets size was reduced and distribution becomes finer and more homogeneous under the individual effect of each of the treatments and optimum results obtained with the combination of them. These new melt treatments affect not only the nucleation of the Pb/Bi droplets, enhancing their heterogeneous nucleation but reduces considerably the Marangoni motion and Stokes sedimentation reducing therefore the droplet coalescence and restricting their growth. As a consequence of this improved microstructure, mechanical properties and machining performance were enhanced considerably. The results from this study provide a promising new microstructure with a fine and uniform distribution of droplets.

Os Ferros Fundidos de Alto Cromo (FFAC), por apresentarem excelentes propriedades tribológicas, têm sido amplamente utilizados em aplicações específicas envolvendo elevadas perdas de material por abrasão, especialmente no setor da mineração. Entretanto, a demanda por materiais com maior resistência ao desgaste aumenta continuamente, sendo necessárias novas pesquisas nesta área. Portanto, o presente trabalho objetiva avaliar a utilização do nióbio para aumentar, ainda mais, a resistência à abrasão dos FFAC\'s. Por outro lado, quando o FFAC é utilizado na fabricação de peças com geometrias irregulares (por exemplo, rotores de bombas), o componente pode apresentar diferentes níveis de refinamento da microestrutura, entre as regiões finas e espessas, devido às variações na taxa de resfriamento. No presente trabalho foi avaliado, o efeito do grau de refinamento da microestrutura, e a interação do refinamento com a adição de nióbio, na resistência ao desgaste abrasivo dos FFAC\'s. Para tanto, foram desenvolvidos quatro estudos principais: no primeiro estudo foram fabricados blocos de FFAC variando o grau de refinamento da microestrutura e foi mostrado que: grandes incrementos no grau de refinamento resultam em maiores perdas de massa por abrasão. Nas microestruturas menos refinadas, os carbonetos de cromo M7C3, de maior tamanho, são menos susceptíveis ao micro trincamento e podem, ocasionalmente, atuar como barreiras ante os eventos abrasivos. Em uma segunda série de experimentos, foi avaliada a interação do efeito do grau de refinamento da microestrutura com a adição de nióbio em teores baixos (1 %); mostrando que, para microestruturas com alto grau de refinamento, adições de nióbio reduzem as perdas de massa por abrasão em até 50 %. Em uma terceira série de experimentos foi avaliada a interação dos efeitos da adição de nióbio e de molibdênio. Quando comparado com a liga isenta de molibdênio, adições simultâneas de nióbio e molibdênio resultaram em microestruturas mais refinadas, com maior microdureza da matriz, e com carbonetos de nióbio (NbC) de maior dureza. Para condições de desgaste abrasivo por baixos esforços, onde o desgaste foi mais acentuado na matriz, adições simultâneas de nióbio e molibdênio resultaram em aumentos da resistência á abrasão dos FFAC estudados. Na última etapa do trabalho foi adicionado 3 % de nióbio em um liga de FFAC com composição química inicial hipereutética (25%Cr/3%C), a qual apresentaria carbonetos primários de cromo M7C3 de grande tamanho que induziriam comportamento frágil do material quando exposto ao desgaste. Porém, a adição de nióbio resultou em um FFAC com microestrutura mais refinada (eutética), contendo NbC\'s compactos e por conseguinte, mais resistente ao desgaste abrasivo. / High Chromium Cast Irons (HCCI\'s), because of their excellent tribological properties, have been widely used for specific applications involving high wear rates by abrasion, especially in the mining sector. However, the demand for materials with higher wear resistance is continuously growing and thus further research is needed in this area. For that reason, the current work purposes to assess the use of niobium to further increase the wear resistance of HCCI\'s. On the other hand, when HCCI is used for manufacturing components with irregular geometries (e.g. pump impellers), the components thin and thick regions can contain different levels of structure refinement due to variation in their cooling rates. In this work, the effect of structure refinement and the interaction between structure refinement and niobium addition on the abrasion resistance of HCCI\'s were evaluated. For that purpose, four systematic main studies were developed: in the first study, blocks of HCCI were manufactured varying the structure refinement and it was shown that large increases in the degree of structure refinement result in higher wear mass losses by abrasion. In less refined microstructures, the larger M7C3 chromium carbides are less susceptible to microcracking and can occasionally act as a barrier to abrasive particles. In the second series of experiments, the interaction between structure refinement and niobium addition in low concentrations (1 %) was evaluated; showing that for more refined microstructures, niobium additions reduce the mass losses by abrasion up to 50 %. In the third series of experiments, the interaction between niobium and molybdenum additions was evaluated. Compared to molybdenum-free alloy, simultaneous additions of niobium and molybdenum resulted in a more refined microstructure, higher hardness of the matrix and harder niobium carbides (NbC). For Low Stress Sliding Abrasion (LSSA) wear configuration, where wear was more pronounced in the matrix, simultaneous addition of niobium and molybdenum resulted in increase of abrasion resistance in the studied HCCI. In the last stage of this work, 3 % of niobium were added in an HCCI alloy with hypereutectic initial chemical composition (25%Cr/3%C), which presents primary large sized chromium carbides that induce a brittle behavior of the HCCI when subjected to wear. However, the niobium addition resulted in a more refined microstructure (eutectic) HCCI containing compact-shaped NbC carbides, and consequently in more resistance to abrasive wear.

Os Ferros Fundidos de Alto Cromo (FFAC), por apresentarem excelentes propriedades tribológicas, têm sido amplamente utilizados em aplicações específicas envolvendo elevadas perdas de material por abrasão, especialmente no setor da mineração. Entretanto, a demanda por materiais com maior resistência ao desgaste aumenta continuamente, sendo necessárias novas pesquisas nesta área. Portanto, o presente trabalho objetiva avaliar a utilização do nióbio para aumentar, ainda mais, a resistência à abrasão dos FFAC\'s. Por outro lado, quando o FFAC é utilizado na fabricação de peças com geometrias irregulares (por exemplo, rotores de bombas), o componente pode apresentar diferentes níveis de refinamento da microestrutura, entre as regiões finas e espessas, devido às variações na taxa de resfriamento. No presente trabalho foi avaliado, o efeito do grau de refinamento da microestrutura, e a interação do refinamento com a adição de nióbio, na resistência ao desgaste abrasivo dos FFAC\'s. Para tanto, foram desenvolvidos quatro estudos principais: no primeiro estudo foram fabricados blocos de FFAC variando o grau de refinamento da microestrutura e foi mostrado que: grandes incrementos no grau de refinamento resultam em maiores perdas de massa por abrasão. Nas microestruturas menos refinadas, os carbonetos de cromo M7C3, de maior tamanho, são menos susceptíveis ao micro trincamento e podem, ocasionalmente, atuar como barreiras ante os eventos abrasivos. Em uma segunda série de experimentos, foi avaliada a interação do efeito do grau de refinamento da microestrutura com a adição de nióbio em teores baixos (1 %); mostrando que, para microestruturas com alto grau de refinamento, adições de nióbio reduzem as perdas de massa por abrasão em até 50 %. Em uma terceira série de experimentos foi avaliada a interação dos efeitos da adição de nióbio e de molibdênio. Quando comparado com a liga isenta de molibdênio, adições simultâneas de nióbio e molibdênio resultaram em microestruturas mais refinadas, com maior microdureza da matriz, e com carbonetos de nióbio (NbC) de maior dureza. Para condições de desgaste abrasivo por baixos esforços, onde o desgaste foi mais acentuado na matriz, adições simultâneas de nióbio e molibdênio resultaram em aumentos da resistência á abrasão dos FFAC estudados. Na última etapa do trabalho foi adicionado 3 % de nióbio em um liga de FFAC com composição química inicial hipereutética (25%Cr/3%C), a qual apresentaria carbonetos primários de cromo M7C3 de grande tamanho que induziriam comportamento frágil do material quando exposto ao desgaste. Porém, a adição de nióbio resultou em um FFAC com microestrutura mais refinada (eutética), contendo NbC\'s compactos e por conseguinte, mais resistente ao desgaste abrasivo. / High Chromium Cast Irons (HCCI\'s), because of their excellent tribological properties, have been widely used for specific applications involving high wear rates by abrasion, especially in the mining sector. However, the demand for materials with higher wear resistance is continuously growing and thus further research is needed in this area. For that reason, the current work purposes to assess the use of niobium to further increase the wear resistance of HCCI\'s. On the other hand, when HCCI is used for manufacturing components with irregular geometries (e.g. pump impellers), the components thin and thick regions can contain different levels of structure refinement due to variation in their cooling rates. In this work, the effect of structure refinement and the interaction between structure refinement and niobium addition on the abrasion resistance of HCCI\'s were evaluated. For that purpose, four systematic main studies were developed: in the first study, blocks of HCCI were manufactured varying the structure refinement and it was shown that large increases in the degree of structure refinement result in higher wear mass losses by abrasion. In less refined microstructures, the larger M7C3 chromium carbides are less susceptible to microcracking and can occasionally act as a barrier to abrasive particles. In the second series of experiments, the interaction between structure refinement and niobium addition in low concentrations (1 %) was evaluated; showing that for more refined microstructures, niobium additions reduce the mass losses by abrasion up to 50 %. In the third series of experiments, the interaction between niobium and molybdenum additions was evaluated. Compared to molybdenum-free alloy, simultaneous additions of niobium and molybdenum resulted in a more refined microstructure, higher hardness of the matrix and harder niobium carbides (NbC). For Low Stress Sliding Abrasion (LSSA) wear configuration, where wear was more pronounced in the matrix, simultaneous addition of niobium and molybdenum resulted in increase of abrasion resistance in the studied HCCI. In the last stage of this work, 3 % of niobium were added in an HCCI alloy with hypereutectic initial chemical composition (25%Cr/3%C), which presents primary large sized chromium carbides that induce a brittle behavior of the HCCI when subjected to wear. However, the niobium addition resulted in a more refined microstructure (eutectic) HCCI containing compact-shaped NbC carbides, and consequently in more resistance to abrasive wear.

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

L'hyperdéformation est une technique efficace pour transformer la microstructure des métaux en une structure de grain de taille inférieure au micron ou même en nanostructure (<100 nm). Cette très petite taille de grain confère d'excellentes propriétés mécaniques au matériau. Dans ce travail de thèse, deux techniques d'hyperdéformation récemment développées, appelées High Pressure Tube Twisting (HPTT) and Cyclic Expansion and Extrusion (CEE) ont été appliquées à température ambiante sur différents matériaux métalliques. La fragmentation de la microstructure ainsi que le développement de la texture cristallographique ont été analysés en détails par la diffraction d'électrons rétrodiffusés (EBSD), par microscopie électronique en transmission (TEM), par transmission Kikuchi diffraction (TKD) ainsi que par diffraction des rayons X (XRD). Le gradient de déformation de cisaillement dans l'épaisseur des tubes d'aluminium déformés par HPTT a été déterminé par une méthode de mesure locale du cisaillement. Ce gradient de cisaillement induit une hétérogénéité aussi bien de microstructure que de texture dans les échantillons d'aluminium et de magnésium purs ainsi que dans l'alliage Al-4%Mg en solution solide. La micro-dureté et la taille de grain dans différentes zones ont été mesurées et analysées en fonction du taux cisaillement local. Les tailles de grain limites atteintes de façon stationnaire pour ces différents matériaux produit par HPTT sont respectivement de 700 nm, 900 nm et 100 nm. L'évolution de texture du magnésium pur après HPTT jusqu'à un cisaillement de 16 a été simulée par cisaillement simple par le model auto-cohérent (VPSC), le résultat de simulation a montré de bons accords avec les mesures de texture obtenues par XRD. Sur la base des mesures de distribution de désorientation dans l'aluminium déformé par HPTT, une nouvelle technique de détermination du taux de cisaillement local dans les procédés d'hyper déformation a été proposée. Cette nouvelle technique a été appliquée sur deux échantillons d'aluminium produit par twist extrusion (TE) et par torsion à extrémités libres. Les échantillons d'aluminium et de cuivre ont été déformés intensément par CEE. Les évolutions de texture et de microstructures ont été mesurées par EBSD, montrant un gradient du centre à la périphérie des échantillons cylindriques. L'évolution de texture dans le cuivre déformé par CEE a été simulée par le modèle VPSC en utilisant un modèle de ligne de courant pour décrire la déformation dans le procédé. Les résultats de simulation confirment les caractéristiques de la texture expérimentale observées après CEE. Le comportement en traction du cuivre pré-déformé par grande déformation en torsion a ensuite été testé. En dépit du gradient de cisaillement existant dans la barre, une technique a été proposée pour obtenir la courbe contrainte-déformation pour ce type de matériau. / Severe plastic deformation (SPD) is an efficient technique to transform the microstructure of bulk metals into ultra fine grained structure with grain sizes less than 1 µm or even into nanostructure with nano-grains of less than 100 nm in diameter. The very small grain size attributes excellent mechanical properties to the material. In present thesis work, two recently developed SPD techniques, namely, High Pressure Tube Twisting (HPTT) and Cyclic Expansion and Extrusion (CEE) were performed on different metallic materials at room temperature. Details of fragmentation of microstructure and metallographic texture evolution were investigated by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), transmission kikuchi diffraction (TKD) and X-ray diffraction (XRD). Shear strain gradient across the thickness of the HPTT deformed Al tube sample was found by a local shear measurement method. This shear strain gradient induced the inhomogeneity of microstructure and texture in HPTT deformed pure Al, solid solution alloy Al-4%Mg and pure Mg. The microhardness and average grain size in different zones as a function of shear strain were measured. The limiting steady grain sizes in the steady state for these different materials produced by HPTT were 700 nm, 100 nm and 900 nm, respectively. The texture evolution of pure Mg in HPTT up to a shear strain of 16 was simulated in simple shear using the self-consistent (VPSC) polycrystal model and showed good agreements with the experimental results measured by XRD. Based on the measured disorientation distribution function in HPTT deformed Al, a new technique for the magnitude of local shear strain in SPD was proposed. This new technique was applied to a protrusion produced in twist extrusion (TE) and to an Al sample deformed in free-end torsion. Cu and pure Al samples were intensively deformed by the CEE SPD technique. The microstructure and texture evolutions were measured by EBSD, showing a gradient from the center-zone to the edge part of the rod sample. The texture evolution of CEE deformed Cu was simulated by the VPSC polycrystal model using a flow line function. The simulation results confirmed the experimental texture features observed in the CEE process. The tensile testing behavior of large strain torsion pre-processed Cu was examined. In spite of the shear strain gradient existing in the bar, a technique was proposed to obtain the tensile stress-strain curve of such gradient material.

Des travaux scientifiques récents ont ouvert de nouveaux champs d’application aux traitements mécaniques tels que le grenaillage. Il a été montré que de tels traitements, réalisés avant un traitement de nitruration à la surface d’alliage ferreux, permettaient d’abaisser les températures de traitement et d’augmenter significativement les cinétiques de diffusion. Nous avons entrepris de tester les performances de cette combinaison de traitements mécanique et thermochimique sur des aciers inoxydables et des alliages à base nickel austénitiques. Des essais ont été réalisés à partir d’une technique de grenaillage mécanique appelée « Surface Mechanical Attrition Treatment » (SMAT). Des billes en métal ou en céramique sont introduites dans l’enceinte et mises en mouvement par la sonotrode. Les billes percutent et introduisent donc une déformation plastique à la surface des échantillons. Après cette étape, les échantillons subissent un traitement thermochimique de nitruration assisté plasma. La comparaison des résultats obtenus après nitruration sur des échantillons traités mécaniquement avec ceux n’ayant pas été pré-traités mécaniquement a permis de quantifier les effets des traitements combinés. Les analyses par diffraction des rayons X, les mesures de microdureté, les observations au microscope optique/électronique à balayage/électronique en transmission, les analyses de texture par EBSD (Electron BackScatered Diffraction) et la mesure des profils de concentration en azote par SIMS (Secondary Ion Mass Spectrometry) et SDL (Spectroscopie à Décharge Luminescente) ont montré l’importance de la nature de la couche transformée mécaniquement sur la diffusion de l’azote / Recent scientific work has opened new fields of application to mechanical treatments such as shot blasting or peening. Indeed, it has been shown that this treatment, performed before a nitriding treatment on the surface of ferrous alloy, lowers processing temperatures and significantly increases the diffusion kinetics. We undertook to test this combination of mechanical and thermochemical treatments on stainless steels and nickel-based alloys. The mechanical treatments were done by Surface Mechanical Attrition Treatment (SMAT). This method is implemented in a box where metal or ceramic balls were introduced and set in motion by an ultrasound system in order to impact the surface of the pieces. The treated samples were then nitrided at low temperature by using a remote plasma. The comparison of the results obtained after nitriding treatments on mechanically treated samples and those not mechanically treated allows quantifying the effects of the combined treatments. Analyses by X-ray diffraction, microhardness measurement, observations by optical and scanning and transmission electron microscopy, texture analysis by EBSD (Electron Diffraction BackScatered) and measurement of nitrogen concentration profiles by SIMS (Secondary Ion Mass Spectrometry) show the importance of the nature of the deformed layer