Afin de mieux appréhender la plasticité des alliages métalliques, il est important de pouvoir décrire à l'échelle atomique les interactions entre dislocations et solutés et d’en déduire l’effet sur la mobilité des dislocations. Au cours de cette thèse, nous nous sommes intéressés aux métaux de transition cubiques centrés (CC), et en particulier au fer, en présence de solutés interstitiels. A l’aide de calculs en Théorie de la Fonctionnelle de la Densité (DFT), la structure de cœur de la dislocation vis de vecteur de Burgers b=½<111> a été étudiée dans le fer en présence de solutés de bore, carbone, azote et oxygène et dans les métaux CC des groupes 5 (V, Nb et Ta) et 6 (Mo, W) en présence de carbone. Nous avons mis en évidence dans le fer et les métaux du groupe 6 une reconstruction du cœur de la dislocation en présence de solutés, associée à une très forte énergie d’attraction dislocation/soluté. Un comportement différent a été observé pour le groupe 5, la configuration la plus stable pour le carbone étant un site octaédrique proche de la dislocation, sans reconstruction de cœur. Cette tendance de groupe a été reliée à la structure des mono-carbures. Les conséquences des interactions fortement attractives dans le fer en présence de carbone ont ensuite été développées. D’une part la ségrégation d’équilibre du carbone proche du cœur de la dislocation a été étudiée à l’aide de modèles en champ moyen et de simulations Monte Carlo. D’autre part, la mobilité de la dislocation décorée a été étudiée en modélisant le mécanisme de double décrochement, en lien avec des observations expérimentales en microscopie électronique à transmission / In order to improve our understanding of alloy plasticity, it is important to describe at the atomic scale the dislocation-solute interactions and their effect on the dislocation mobility. This work focuses on the body-centered cubic (BCC) transition metals, in particular Fe, in presence of interstitial solute atoms. Using Density Functional Theory (DFT) calculations, the core structure of the screw dislocation of Burgers vector b=½<111> was investigated in iron in presence of boron, carbon, nitrogen and oxygen solute atoms, and in BCC metals from group 5 (V, Nb, Ta) and 6 (Mo, W) in presence of carbon solutes. A core reconstruction was evidenced in iron and group 6 metals, along with a strong attractive dislocation-solute interaction energy. A different behavior was observed in group 5 metals, for which the most stable configuration for the carbon atom is an octahedral site in the vicinity of the dislocation, without any core reconstruction. This group tendency was linked to the structure of mono-carbides. Consequences of the strongly attractive dislocation-solute interactions in Fe(C) were then investigated. First the equilibrium segregation close to the dislocation core was studied using a mean-field model and Monte Carlo simulations. Then, the mobility of the dislocation in presence of carbon atoms was investigated by modeling the double-kink mechanism with DFT, in relation with experimental data obtained with transmission electron microscopy

L'objectif principal de la présente dissertation est d'étudier le rôle des fissures pointues sur le comportement mécanique des cristaux sous charge à l'échelle atomique. La question d'intérêt est la façon dont un cristal pur, qui contient une seule fissure en équilibre mécanique, se déforme. Deux métaux ont été considérés: l'aluminium, qui est ductile à toute température, et le fer, transformé de ductile en fragile à une température décroissante inférieure à T=77K. Les forces de cohésion dans les deux métaux ont été modélisées via les potentiels phénoménologiques "n-body". A (010)[001] mode I nano-crack a été introduit dans le réseau cristallin parfait de chacun des métaux étudiés en utilisant des déplacements appropriés attribués par l'élasticité anisotrope. A T=0K, des configurations de fissures à l'équilibre ont été obtenues par minimisation d'énergie avec un type mixte de conditions aux limites. Les deux modèles ont révélé que les configurations de fissures restaient stables sous une gamme finie de contraintes appliquées en raison de l'effet de piégeage en treillis. La présente thèse propose une nouvelle approche pour interpréter le comportement mécanique intrinsèque des deux systèmes métalliques sous le chargement. En particulier, la réponse ductile ou fragile d'un système cristallin peut être déterminée en examinant si la barrière de piégeage en treillis d'une fissure préexistante est suffisante pour provoquer le glissement de dislocations statiques préexistantes. Les résultats des simulations ainsi que les données expérimentales démontrent que, selon le modèle proposé, l'aluminium et le fer sont ductiles et fragiles à T=0K, respectivement. / The principal aim of the present dissertation is to investigate the role of sharp cracks on the mechanical behaviour of crystals under load at the atomic scale. The question of interest is how a pure crystal, which contains a single crack in mechanical equilibrium, deforms. Two metals were considered: aluminium, ductile at any temperature below its melting point, and iron, being transformed from ductile to brittle upon decreasing temperature below T=77K. Cohesive forces in both metals were modeled via phenomenological n-body potentials. A (010)[001] mode I nano-crack was introduced in the perfect crystalline lattice of each of the studied metals by using appropriate displacements ascribed by anisotropic elasticity. At T=0K, equilibrium crack configurations were obtained via energy minimization with a mixed type of boundary conditions. Both models revealed that the crack configurations remained stable under a finite range of applied stresses due to the lattice trapping effect. The present thesis proposes a novel approach to interpret the intrinsic mechanical behaviour of the two metallic systems under loading. In particular, the ductile or brittle response of a crystalline system can be determined by examining whether the lattice trapping barrier of a pre-existing crack is sufficient to cause the glide of pre-existing static dislocations on the available slip systems. Simulation results along with experimental data demonstrate that, according to the model proposed, aluminium and iron are ductile and brittle at T=0K, respectively.

The dependence of flow stress of body centered cubic metals on variables such as strain rate, temperature, strain and microstructural is a research area of continued interest. Recently, there has been renewed interest in deformation of fine grained BCC metals, which display characteristics that are different from their coarse-grained counterparts. Deformation mechanisms, strain-rate and temperature dependence, and strain hardening characteristics of fine-grained BCC metals are not well understood. The aim of this thesis is to understand the effect of strain-rate, temperature, strain and microstructure (i.e., grain size) on the mechanical response of poly¬crystalline tantalum. Among the topics addressed were constitutive modeling of flow stress, understanding the microstructural origins of strain hardening, and characterizing the effect of severe plastic deformation (SPD) on microstructure and mechanical properties. Rolling and equal-channel angular pressing (ECAP) were among the processing techniques employed. Mechanical testing was conducted over a range of temperatures and strain rates, and this was supported by a slew of microscopic characterization methods. It was found that the strain hardening response depends on microstructural evolution at different strain rates. Results indicate that the same thermally activated mechanisms operate in both as-received and processed material and this was found to be the overcoming of Peierls barriers via a double-kink mechanism. Lastly, it was found that the low strain rate sensitivity of SPD processed material was not due to fine grain size, but instead due to high internals stresses.

No description available.

An experimental study of several variants of radius and strut angle of the body centered cubic unit cell was performed to determine the mechanical properties and failure mechanisms of the mesostructure. Quasi static compression tests were performed on an Instron® universal testing machine with a 50kN load cell at 0.2mm/min. The test samples were built using a SLM Solutions 125 selective laser melting machine with 316L stainless steel. Test specimens were based on 5mm cubic unit cells, with a strut diameter 10% of the unit cell size, with skins on top and bottom to provide a cantilever boundary constraint. Specimens were inspected for dimensional accuracy using precision calipers and inspected for morphology using a MicroVu® macroscope. The compressive properties of the mesostructure was compared to the compressive properties of macrostructure. The BCC unit cell behaves significantly different at the boundary layer of a constrained lattice. The failure mode at the boundary is characterized by plastic bending within the microstruts while the non boundary layer cells fail via plastic bending at the node. Manufacturing compensation parameters were determined for part shrinkage and droop. Two predictive numerical models were developed, based on the Gibson-Ashby model of cellular solids, as well as a finite element model. Numerical results did not agree well with the experimental results, indicating that the droop observed on the structures significantly affects the mechanical properties of the overall structure. The 25% radius cubic unit cell and 3^3 lattice withstood the greatest stress of all specimens tested and exhibited nearly ideal plastic deformation behavior.

No description available.

This master thesis presents a theoretical investigation of the energy and stability of interfaces in iron-carbide compounds, specifically focusing on the α-Fe/Cr7C3 system. The study aims to fill the gap in knowledge regarding the surface energetics of these interfaces using Density Functional Theory (DFT). Six different α-Fe/Cr7C3 interfaceswere constructed α-Fe(001)/Cr7C3(024), α-Fe(001)/Cr7C3(202), α-Fe(001)/Cr7C3(040),α-Fe(110)/Cr7C3(024), α-Fe(110)/Cr7C3(202) and α-Fe(110)/Cr7C3(040). Due to limited computational resources, only one of them was computationally analyzed to determine its interfacial energy value. The results revealed that the interfacial energy of the α-Fe(001)/Cr7C3(040) interface falls within the range of incoherent interfaces, indicating its stability. The computed interfacial energy values ranged from 0.94 to 3.39 J/m2, consistent with similar studies on other iron interfaces. The simulations also identified minimum and local minimum points in the interface energy curve, representing stable configurations at specific interface separation distances. The presence of a minimum point at an interface separation value of d = 1.3551 Å with an interfacial energy of 0.94 J/m2 indicates the most stable configuration, while a local minimum point at d = 2.27 Å with an interfacial energy of 2.12 J/m2 suggests another stable configuration for the interface. The conclusion that the computations were correctly performed with an interfacial energy value of 0.94 J/m2 for the most stable configuration at a supercell length (aSupercell ) of 22.23 Å is drawn. The findings of this research have significant implications for future investigations and applications. Firstly, this study fills the gap of the unresearched ferrite-carbide interfaces with theoretical data. Secondly, the knowledge gained from studying these interfaces contributes to understanding hydrogen interactions, which is fundamental for the transition towards a hydrogen economy. Additionally, the incoherent nature of the interface introduces challenges in understanding material behavior and properties, necessitating further investigations for designing efficient systems. Future work includes experimental validation of the α-Fe/Cr7C3 interface to compare the theoretical and experimental energies and stability. Investigating the remaining interfaces and examining the effects of introducing hydrogen atoms in these interfaces, along with calculating the corresponding hydrogen trapping energies, are important research areas. Further advancements in understanding these interfaces can be achieved through interface engineering, multiscale modeling, and studying other iron-carbide systems. / Detta examensarbete presenterar en teoretisk undersökning av energin och stabiliteten hos gränssnitt i järnkarbidföreningar och fokuserar specifikt på α-Fe/Cr7C3-systemet. Studien syftar till att fylla kunskaps tomrummet gällande ytegenskaperna hos dessa gränssnitt genom användning av densitetsfunktionalteori (DFT). Sex olika α-Fe/Cr7C3-gränssnitt konstruerades α-Fe(001)/Cr7C3(024), α-Fe(001)/Cr7C3(202), α-Fe(001)/Cr7C3(040), α-Fe(110)/Cr7C3(024), α-Fe(110)/Cr7C3(202) och α-Fe(110)/Cr7C3(040). På grund av begränsade beräkningsresurser analyserades endast ett av dem för att bestämma dess gränssnittsenergivärde. Resultaten visade att gränssnittsenergin för α-Fe(001)/Cr7C3(040)- gränssnittet ligger inom intervallet för inkoherenta gränssnitt, vilket indikerar dess stabilitet. De beräknade gränssnittsenergivärdena varierade mellan 0,94 och 3,39 J/m2 , vilket är i linje med liknande studier där järngränssnitt studeras. Minimi och lokala minimipunkter i gränssnittets energikurva, vilket representerar stabila konfigurationer vid specifika avstånd mellan gränssnittet. Förekomsten av en minimipunkt vid ett gränssnittsavstånd på d = 1,35 Å med en gränssnittsenergi på 0,94 J/m2 indikerar den mest stabila konfigurationen, medan en lokal minimipunkt vid d = 2,27 Å med en gränssnittsenergi på 2,12 J/m2 antyder en annan stabil konfiguration för gränssnittet. Slutsatsen dras att beräkningarna utfördes korrekt med ett gränssnittsenergivärde på 0,94 J/m2 för den mest stabila konfigurationen vid en supercellslängd (aSupercell) på 22,23 Å. Fynden från denna forskning har betydande implikationer för framtida undersökningar och tillämpningar. För det första fyller denna studie kunskapsgapet gällande de otillräckligt utforskade ferrit-karbidgränssnitten med teoretisk data. För det andra bidrar den erhållna kunskapen från studiet av dessa gränssnitt till förståelsen av väteinteraktioner, vilket är grundläggande för övergången till en väteekonomi. Dessutom innebär gränssnittets inkoherenta natur utmaningar när det gäller att förstå materialbeteende och egenskaper, vilket kräver ytterligare undersökningar för att utforma effektiva system. Framtida arbete inkluderar experimentell validering av gränssnittet mellan α-Fe/Cr7C3 för att jämföra teoretiska och experimentella energier och stabilitet. Att undersöka återstående gränssnitt och undersöka effekterna av att introducera väteatomer i dessa gränssnitt och beräkna motsvarande vätefällningsenergier är viktiga forskningsområden. Gränssnittsdesign, flerskalig modellering och studier av andra järnkarbid-system kan ytterligare främja förståelsen av dessa gränssnitt.