Ultra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.

Magnesium, with a Hexagonal Close-Packed (HCP) structure, is the eighth most abundant element in the earth’s crust and the third most plentiful element dissolved in the seawater. Magnesium alloys exhibit the attractive characteristics of low densities and high strength-to-weight ratios along with good castability, recyclability, and machinability. Replacing the steel and/or aluminum sheet parts with magnesium sheet parts in vehicles is a great way of reducing the vehicles weight, which results in great savings on fuel consumption. The lack of magnesium sheet components in vehicle assemblies is due to magnesium’s poor room-temperature formability. In order to successfully form the sheets of magnesium at room temperature, it is necessary to understand the formability of magnesium at room temperature controlled by various plastic deformation mechanisms. The plastic deformation mechanisms in pure magnesium and some of its alloys at room temperature are crystallographic slip and deformation twinning. The slip systems in magnesium at room temperature are classified into primary (first generation), secondary (second generation), and tertiary (third generation) slip systems. The twinning systems in magnesium at room temperature are classified into primary (first generation) and secondary (second generation, or double) twinning systems. A new comprehensive rate-dependent elastic-viscoplastic Crystal Plasticity Constitutive Model (CPCM) that accounts for all these plastic deformation mechanisms in magnesium was proposed. The proposed model individually simulates slip-induced shear in the parent as well as in the primary and secondary twinned regions, and twinning-induced shear in the primary and secondary twinned regions. The model also tracks the texture evolution in the parent, primary and secondary twinned regions. Separate resistance evolution functions for the primary, secondary, and tertiary slip systems, as well as primary and secondary twinning systems were considered in the formulation. In the resistance evolution functions, the interactions between various slip and twinning systems were accounted for. The CPCM was calibrated using the experimental data reported in the literature for pure magnesium single crystals at room temperature, but needs further experimental data for full calibration. The partially calibrated model was used to assess the contributions of various plastic deformation mechanisms in the material stress-strain response. The results showed that neglecting secondary slip and secondary twinning while simulating plastic deformation of magnesium alloys by crystal plasticity approach can lead to erroneous results. This indicates that all the plastic deformation mechanisms have to be accounted for when modelling the plastic deformation in magnesium alloys. Also, the CPCM in conjunction with the Marciniak–Kuczynski (M–K) framework were used to assess the formability of a magnesium single crystal sheet at room temperature by predicting the Forming Limit Diagrams (FLDs). Sheet necking was initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field was assumed inside and outside the band, and conditions of compatibility and equilibrium were enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs were simulated under two conditions: a) the plastic deformation mechanisms are primary slip systems alone, and b) the plastic deformation mechanisms are primary slip and primary twinning systems. The FLDs were computed for two grain orientations. In the first orientation, primary extension twinning systems had favourable orientation for activation. In the second orientation, primary contraction twinning systems had favourable orientation for activation. The effects of shear strain outside the necking band, rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations were individually explored.

Ultra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.

Magnesium, with a Hexagonal Close-Packed (HCP) structure, is the eighth most abundant element in the earth’s crust and the third most plentiful element dissolved in the seawater. Magnesium alloys exhibit the attractive characteristics of low densities and high strength-to-weight ratios along with good castability, recyclability, and machinability. Replacing the steel and/or aluminum sheet parts with magnesium sheet parts in vehicles is a great way of reducing the vehicles weight, which results in great savings on fuel consumption. The lack of magnesium sheet components in vehicle assemblies is due to magnesium’s poor room-temperature formability. In order to successfully form the sheets of magnesium at room temperature, it is necessary to understand the formability of magnesium at room temperature controlled by various plastic deformation mechanisms. The plastic deformation mechanisms in pure magnesium and some of its alloys at room temperature are crystallographic slip and deformation twinning. The slip systems in magnesium at room temperature are classified into primary (first generation), secondary (second generation), and tertiary (third generation) slip systems. The twinning systems in magnesium at room temperature are classified into primary (first generation) and secondary (second generation, or double) twinning systems. A new comprehensive rate-dependent elastic-viscoplastic Crystal Plasticity Constitutive Model (CPCM) that accounts for all these plastic deformation mechanisms in magnesium was proposed. The proposed model individually simulates slip-induced shear in the parent as well as in the primary and secondary twinned regions, and twinning-induced shear in the primary and secondary twinned regions. The model also tracks the texture evolution in the parent, primary and secondary twinned regions. Separate resistance evolution functions for the primary, secondary, and tertiary slip systems, as well as primary and secondary twinning systems were considered in the formulation. In the resistance evolution functions, the interactions between various slip and twinning systems were accounted for. The CPCM was calibrated using the experimental data reported in the literature for pure magnesium single crystals at room temperature, but needs further experimental data for full calibration. The partially calibrated model was used to assess the contributions of various plastic deformation mechanisms in the material stress-strain response. The results showed that neglecting secondary slip and secondary twinning while simulating plastic deformation of magnesium alloys by crystal plasticity approach can lead to erroneous results. This indicates that all the plastic deformation mechanisms have to be accounted for when modelling the plastic deformation in magnesium alloys. Also, the CPCM in conjunction with the Marciniak–Kuczynski (M–K) framework were used to assess the formability of a magnesium single crystal sheet at room temperature by predicting the Forming Limit Diagrams (FLDs). Sheet necking was initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field was assumed inside and outside the band, and conditions of compatibility and equilibrium were enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs were simulated under two conditions: a) the plastic deformation mechanisms are primary slip systems alone, and b) the plastic deformation mechanisms are primary slip and primary twinning systems. The FLDs were computed for two grain orientations. In the first orientation, primary extension twinning systems had favourable orientation for activation. In the second orientation, primary contraction twinning systems had favourable orientation for activation. The effects of shear strain outside the necking band, rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations were individually explored.

Hexagonal close packed (hcp) crystal structures, such as magnesium and titanium, provide formidable strength in relation to density. Current interests in reducing CO2 emissions, hold magnesium as a contender to lightweight passenger vehicles. Although significant decreases in mass could be achieved through magnesium, poor formability and energy absorption capacity limit the possibility for cost-effective production. This Master’s thesis aims to observe the microstructure and micro texture evolution induced by twinning using interrupted electron backscattered diffraction (EBSD) characterization in order to determine potential mechanisms causing early failure of magnesium alloys. This study revealed {10-11} contraction twins at stress levels contrary to the basic hypothesis of the Schmid effect revealing the importance of non-Schmid effects in damage. Furthermore, it was observed that crack nucleation occurs in magnesium alloys, due mainly to interaction between twins and microstructure defects and form inside contraction twins, causing cleavage-like terraces in the fracture surface.

Abstract  Aggregatibacter aphrophilus, a Gram negative bacterium, found in the oral cavity, causing cerebral abscesses and infective endocarditis, has been shown to be serum resistant in previous studies. Bacterial secretion systems are important for bacteria as they transfer virulence factors into other bacteria or host cells as an attack. A. aphrophilus encodes a type VI secretion system, which is a spike-like membrane protein, mainly consisting of a hemolysin co-regulated protein (Hcp). In this work, it was tested if Hcp would contribute to serum resistance of A. aphrophilus. Firstly, to assess Hcp contribution to serum resistance, a bacterial serum killing assay-method was used and data was collected from three independent experiments. Two strains of A. aphrophilus were used in the experiments: the laboratory strain HK83 and a HK83 hcp mutant strain. The results showed that Hcp provided no significant effect on serum resistance of A. aphrophilus. Secondly, optical density measurements were made for growth curve analysis, to determine if the HK83 hcp mutant strain had an impact in growth compared to HK83. The results indicated that the HK83 hcp mutant strain had a somewhat reduced growth compared to its parental strain.

La mousse métallique a des propriétés mécaniques et thermodynamiques uniques qui pourraient s'avérer utiles dans de nombreux domaines, tels que la construction légère et ingénierie automobile. Cependant, la mousse métallique n'est pas encore établie en génie.Une des raisons sont les difficultés et les prix élevés dans le processus de fabrication. Causée par drainage gravitaire en état liquide, peuvent se produire des distributions de matériel non homogènes. En outre, dépassant le drainage peut provoquer rupture de bulle et la génération de soufflures. Ces effets négatifs potentiellement peuvent être évités en ajoutant magnétique ou champs électromagnétiques au cours du processus de génération.Dans cette thèse, l'influence de ces champs est donc étudié en réalisant la phase de résolution des simulations. Ces simulations sont effectuées avec le code interne premier. Une modification de la modélisation de la collision était nécessaire pour enquêter sur l'agglomération de bulles dans le métal liquide.Calcul d'une configuration statique-drainage, les mécanismes de l'agglomération sont étudiés sans la présence de champs électriques ou magnétiques. Aux vitesses élevées de drainage, les bulles flottent. À des vitesses inférieures de drainage, les bulles s'agglomèrent dans la partie supérieure du domaine, formant des structures cristallines compacte.La préférence expérimentalement bien connue de commande cubes axés sur le visage, plus hexagonale compacte vous passez votre commande de volume égal bulles est reproduit numériquement. Appliquant davantage des simulations et expériences, un mécanisme de l'instabilité de la commande de façon hexagonale compacte est identifié, ayant pour résultat la préférence de commande de cubes axés sur le visage.Afin de déterminer les propriétés mécaniques de la mousse métallique solide avec la fraction de gaz faibles et aux fonctionnalités avantageuses et désavantageuses d'état d'arrangements de bulle, simulations par éléments finis de la mousse métallique solide avec cavités sphériques sont réalisées et comparées. Une influence significative de la quantité de cristaux de bulle sur la mécanique de la mousse se trouve. Le type de l'ordre cristallin est moins important.On étudie l'influence d'un champ magnétique horizontal sur l'agglomération de bulle. La résistance de drainage peut être augmentée sensiblement en ajoutant un champ magnétique. La structure résultante des bulles est moins sensible à un champ magnétique.Combinant un courant électrique horizontal et une perpendiculaire, champ magnétique horizontal se traduit par une force verticale de Lorentz. Cette force peut équilibrer la gravitation et donc, provoquer la rotation des bulles. Simuler cet État révèle une distribution homogène de bulle. Dans le même temps, friser les champs de force dans le voisinage de chaque bulle induire un mouvement continu en remuant. Petit champ électromagnétique forces n'empêchent pas les bulles d'agglomération, mais peuvent varier le montant de la commande cristallisé et par conséquent, les propriétés mécaniques de la mousse solide qui en résulte.En conclusion, un champ magnétique horizontal augmente la résistance de drainage, tandis que sa combinaison avec un courant électrique provoque des distributions de bulle homogène et peut modifier la structure de la mousse et la fraction de gaz. Les résultats de cette thèse pourraient aider à améliorer le processus de fabrication industrielle de mousse métallique ou même permettre la production de métal poreux avec la fraction de gaz définie par l'utilisateur. / Metal foam has unique mechanical and thermodynamic properties which could prove useful in many fields, such as light-weight construction and automotive engineering. However, metal foam is not yet established in engineering. One reason are the difficulties and high prices in the fabrication process. Caused by gravity-driven drainage in liquid state, inhomogeneous material distributions can occur. Also, exceeding drainage might cause bubble rupture and the generation of blow-holes. These negative effects potentially can be avoided by adding magnetic or electromagnetic fields during the generation process. In this thesis, the influence of these fields is therefore investigated by conducting phase resolving simulations. These simulations are carried out with the in-house code PRIME. A modification of the collision modelling was necessary in order to investigate the agglomeration of bubbles within liquid metal. Computing a static-drainage setup, the agglomeration mechanisms are investigated without the presence of electric or magnetic fields. At high drainage velocities the bubbles float. At lower drainage velocities the bubbles agglomerate in the upper part of the domain, forming close-packed crystalline structures. The experimentally well known preference of face-centred cubic ordering, over hexagonally close-packed ordering of equal-volume bubbles is reproduced numerically. Applying further simulations and experiments, an instability mechanism of hexagonally close-packed ordering is identified, resulting in the preference of face-centred cubic ordering. In order to determine the mechanical properties of solid metal foam with low gas fraction and to state advantageous and disadvantageous features of bubble arrangements, Finite-Element simulations of solid metal foam with spherical voids are carried out and compared. A significant influence of the amount of bubble crystals on the foam mechanics is found. The type of the crystalline ordering is less important. The influence of a horizontal magnetic field on the bubble agglomeration is investigated. The drainage resistance can be increased significantly by adding a magnetic field. The resulting structure of the bubbles is less sensitive to a magnetic field. Combining a horizontal electric current and a perpendicular, horizontal magnetic field results in a vertical Lorentz force. This force can balance gravitation and thus, cause rotation of the bubbles. Simulating this state reveals a homogeneous bubble distribution. At the same time, curling force fields in the vicinity of each bubble induce a continuous stirring motion. Small electromagnetic field strengths do not prevent the bubbles from agglomerating, but can vary the amount of crystallized ordering and therefore, the mechanical properties of the resulting solid foam. In conclusion, a horizontal magnetic field increases the drainage resistance, while its combination with an electric current causes homogeneous bubble distributions and can alter the foam structure and the gas fraction. The results of this thesis could help improve the industrial fabrication process of metal foam or even allow production of porous metal with user-defined gas fraction.

Mg alloys have spurred a renewed academic and industrial interest because of their ultra-light-weight and high specific strength properties. Hexagonal close packed Mg has low deformability and a high plastic anisotropy between basal and non-basal slip systems at room temperature. Alloying with Li and other elements is believed to counter this deficiency by activating non-basal slip by reducing their nucleation stress. In this work I study how Li addition affects deformation mechanisms in Mg using atomistic simulations. In the first part, I create a reliable and transferable concentration dependent embedded atom method (CD-EAM) potential for my molecular dynamics study of deformation. This potential describes the Mg-Li phase diagram, which accurately describes the phase stability as a function of Li concentration and temperature. Also, it reproduces the heat of mixing, lattice parameters, and bulk moduli of the alloy as a function of Li concentration. Most importantly, our CD-EAM potential reproduces the variation of stacking fault energy for basal, prismatic, and pyramidal slip systems that influences the deformation mechanisms as a function of Li concentration. This success of CD-EAM Mg-Li potential in reproducing different properties, as compared to literature data, shows its reliability and transferability. Next, I use this newly created potential to study the effect of Li addition on deformation mechanisms in Mg-Li nanocrystalline (NC) alloys. Mg-Li NC alloys show basal slip, pyramidal type-I slip, tension twinning, and two-compression twinning deformation modes. Li addition reduces the plastic anisotropy between basal and non-basal slip systems by modifying the energetics of Mg-Li alloys. This causes the solid solution softening. The inverse relationship between strength and ductility therefore suggests a concomitant increase in alloy ductility. A comparison of the NC results with single crystal deformation results helps to understand the qualitative and quantitative effect of Li addition in Mg on nucleation stress and fault energies of each deformation mode. The nucleation stress and fault energies of basal dislocations and compression twins in single crystal Mg-Li alloy increase while those for pyramidal dislocations and tension twinning decrease. This variation in respective values explains the reduction in plastic anisotropy and increase in ductility for Mg-Li alloys.

This thesis reveals the tribological mechanisms behind the intrinsic low friction potential of the Co-based family of alloys called Stellites. Although being an established and important group of materials, a satisfactory explanation to why they exhibit low-friction properties under severe sliding conditions has not previously been found in the literature. The main part of this thesis is dedicated to the clarification of the tribological performance of Stellites in highly loaded sliding contact. The results should assist the development of Co-free alternatives, suitable for replacing Stellites in nuclear applications. Owing to their beneficial properties they are today the most commonly used material in the sealing surfaces on gate valves in the primary circuits of boiling water reactors (BWR). The underlying reason for the replacement in the nuclear applications is an undesired contribution to the background radiation level, originating from the Co in the Stellite surfaces. The Stellites mainly consist of Cr-rich carbides in a solid solution dominated by Co. The commonly used Stellite 6 and Stellite 21 were chosen as primary test materials and applied by laser cladding, providing a metallically bonded clad layer with a fine dendritic microstructure. By combining information from a series of dedicated tribological tests and modern high-resolution analysis instruments (e.g. SEM, XRD and TEM) available at the Ångström Laboratory at Uppsala University, the following conclusions can be made regarding the tribological performance of Stellites under high load sliding. Mechanisms. The (tested) Stellites form a thick deformation hardened layer, topped with a superficial easily sheared layer of hcp basal planes aligned parallel to the worn surface. The easy-shear layer is continually regenerated, replacing worn off material. Technical benefits. The Stellites offer low-friction properties thanks to their easily sheared surface layers. The risk of severe galling is also avoided by restricting shear and adhesive transfer to very thin superficial layers. In closed sliding contacts, self-generated protective layers formed by re-deposition of wear fragments are also offered.

L'application d'un temps de maintien, même de faible durée, lors d'un chargement cyclique, modifie de façon très sensible à la fois le comportement contrainte-déformation et le nombre de cycles à amorçage dans l'alliage base titane TI-6242. Ceci est lié à un régime de fluage cyclique, conduisant à de la déformation progressive d'une part, et à une forte interaction fatigue-temps de maintien pour ce qui concerne le nombre de cycles à amorçage. Les différents phénomènes sont pour le moment assez mal analysés, si bien qu'il n'est pas possible d'effectuer une conception optimale des pièces, de larges marges de sécurité étant nécessaires. Le but du travail est de mieux comprendre les mécanismes locaux qui régissent le comportement et l'amorçage des fissures, dans le but de suggérer des microstructures optimales, et de calibrer des modèles macroscopiques utilisables en bureau d'études. En s'appuyant sur une base expérimentale fournie par Snecma et l'ENSMA, une approche multiéchelles a été mise en place pour représenter les hétérogénéités locales qui ont un rôle significatif sur les comportements observés. Dans les calculs des microstructures, faisant intervenir une étape d'évaluation statistique, on se focalise sur la représentation explicite des "plumes", grains de taille exceptionnelle, qui sont à l'origine des premières microfissures en raison du contraste cristallin qu'ils introduisent avec l'environnement. Une revue des différentes configurations de plumes, afin de retenir celles qui sont le plus critique, a été établie. Cette analyse a permis de mettre en évidence la présence de plumes simples, doubles ou triples, les domaines se présentant sous formes de bandes. Les configurations à étudier comportent comme paramètres critiques l'orientation géométrique de la bande par rapport à la direction du chargement macroscopique, mais surtout l'orientation cristallographique au sein de cette (ces) bande(s). Des calculs systématiques ont été effectués afin de mener une étude statistique et de déterminer les configurations les plus sensibles.