In engineering, materials are often assumed to be homogeneous and isotropic; in actuality, material properties do change with sample direction and location. This variation is due to the anisotropy of the individual grains and their spatial distribution in the material. Currently there is a lack of communication between the design engineer, material scientist, and processor for solving multi-objective/constrained designs. If communication existed between these groups then materials could be designed for applications, instead of the reverse. Microstructure sensitive design introduces a common language, a spectral representation, where both design properties and microstructures are expressed. Using Hill bounds, effective elastic properties are expressed within the spectral representation. For the elastic properties, two FCC materials, copper and nickel, were chosen for computation and to demonstrate how symmetry enters into the methodology. This spectral representation renders properties as hyper-surfaces that translate through a multi-dimensional Fourier space depending on the property value of the hyper-surface. Property closures are generated by condensing the information contained within the multi-dimensional Fourier space into a 2-D representation. This compaction of information is beneficial for a quick determination of property limits for a particular alloy system. The design engineer can now dictate the critical design properties and receive sets of microstructures that satisfy the design objectives.

Polymer-derived ceramics (PDCs) are a new class of high-temperature materials synthesized by thermal decomposition of polymeric precursors. These materials possess many unique features as compared with conventional ceramics synthesized by powder metallurgy based processing. For example, PDCs are neither amorphous nor crystalline. Instead, they possess nano-domain structures. Due to the direct chemical-to-ceramic processing, PDCs can be used for making components and devices with complex shapes. Thus, understanding the properties and structures of these materials are of both fundamental and practical interest. In this work, the structures and electronic behavior of polymer-derived amorphous silicon carbonitrides (SiCNs) were investigated. The materials were synthesized by pyrolysis of a commercially available liquid precursor. Ceramic materials with varied structures/properties were successfully synthesized by modifying the precursor and using different pyrolysis temperatures. The structures of the obtained materials were studied using XRD, solid state NMR, EPR, FTIR and Raman Spectroscope. The electronic behavior of the materials was investigated by measuring I-V curves, Hall effects, temperature dependent conductivity. The experiments were also performed to measure UV-Visible absorption and dielectric properties of the materials. This work leads to the following significant progresses: (i) developed quantitative technique for measuring free carbon concentration; (ii) achieved better understanding of the electronic conduction mechanisms and measured electronic structures of the materials for the first time; and (iii) demonstrated that these materials possess unusual dielectric behavior and provide qualitative explanations.

Yes / In this study, the enhancement of stainless steel wires (SSWs) to the flexural fatigue performance of reactive powder concrete (RPC) including fatigue life and fatigue stress-strain hysteresis relationship as well as fatigue damage were investigated, and the underlying mechanisms were explored through microstructure observation and characteristic analyses of hydration products. The average flexural fatigue life of RPC is increased by 636.6%, 558.3% and 1010.7% at the maximum stress levels of 0.7, 0.8 and 0.9 when 1.5 vol.% SSWs are incorporated. The method of moments and method of maximum likelihood are employed to calculate the scale and shape parameters for fatigue life subscribed to Weibull distribution. The calculated ratio of flexural fatigue endurance limit to static flexural strength for SSWs reinforced RPC reaches up to 0.64. The incorporation of SSWs decreases the flexural failure damage of RPC by 41.5% and converts the long and link-up micro cracks into emission cracks centered on SSWs. Benefited from the large specific surface area of SSWs, abound of silica fume with pozzolanic activity absorbs on the surface of SSWs and continues to hydrate, reducing the surrounding water-binder ratio to form a microstructure enhancement zone with SSWs as the core and improve the homogeneity of RPC. This can be confirmed by the decrease of porosity, Ca(OH)2 crystal orientation index and molar ratio of CaO to SiO2 for calcium silicate hydrate gels. SSWs can also enhance the fatigue performance of RPC by transmitting hydration heat, inhibiting the initiation and propagation of micro cracks especially at the initial stage of fatigue load, bridging cracks and being pulled-off. The excellent flexural fatigue properties and homogeneous microstructures of SSWs reinforced RPC make it particularly suitable for large-span and ultra-thin elements in extreme service environments.

No / This paper presents the results of a recent comprehensive investigation of selective laser melting (slm) of prealloyed gas and water atomised M2 and H13 tool steel powders. The objective of the study was to establish the parameters that control the densification of single and multiple layers with the aim of producing high density parts without the need for infiltration. Powders were processed using continuous wave (CW) CO2 and Nd:YAG lasers. Relationships between alloy composition, powder particle size and shape, flowability, microstructure (phases present, their size, morphology and distribution), track morphology, post scanned density, surface finish and scan conditions (Laser power, spot size and scan speed) are discussed for single track, single layer and multi-layer (up to 25 layers) constructions. Processing with a Nd:YAG laser with powders placed on substrates rather than on a loose powder bed gave more stable builds than with the CO2 laser. Using the Nd:YAG laser densities up to ~90% relative were possible with H13 powder compared with a maximum of ~70% for M2 in multi-layer builds. Maximum density achieved with CW CO2 processing was only ~60%, irrespective of powder composition. The paper compares the processibility of these materials with stainless steel powders processed to higher densities (up to 99% relative) under similar conditions. The results of the work show that a crucial factor for high density processing is melt pool wettability and this is controlled largely by carbon content; low carbon contents producing better wettability, flatter tracks and higher densities. The significance of this observation for the processing high alloy steels by slm will be discussed.

Laser-based additive manufacturing is inherently associated with extreme, unprecedented, and rapid thermokinetics which impact the microstructural evolution in a built component. Such a unique, near to non-equilibrium microstructure/phase evolution in laser additively manufactured metallic components impact their properties in engineering application. In light of this, the present work investigates the unique microstructural traits as a result of process induced spatial and temporal variation in thermokinetic parameters in laser directed energy deposited CoCrMo biomedical alloy. The influence of such a unique microstructural evolution in laser directed energy deposited CoCrMo on electrochemical response in physiological media was elucidated and compared with a conventionally manufactured, commercially available CoCrMo component. Furthermore, while investigation of the electrochemical response, such a microstructural evolution in laser directed energy deposited CoCrMo led to in-situ surface modification of the built components in physiological media via selective, non-uniform electrochemical etching. Such in-situ surface modification resulted in enhanced biocompatibility in terms of mammalian cell growth, cell-substrate adhesion, blood compatibility, and antibacterial properties indicating improved osteointegration, compared to a conventionally manufactured, commercially available CoCrMo component.

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<p>Pages that are darker in colour were scanned one at a time to produce a high quality image.</p> / <p>Following a review of the gas nitriding process, of the Fe-N system and of the formation of the compound layer, this study investigates the interaction between the diffusing nitrogen and the material to be nitrided.</p> <p>The formation of the compound layer and diffusion case is discussed for pure iron. AISI 1070 low alloy steel is investigated using different microstructures and the resulting effect on the nitrided microstructure is presented. The interaction of a nitride forming element is studied with a Fe/Fe-5Cr diffusion couple specimen. The strengthening mechanisms resulting from the nitriding process for all these materials are discussed using standard models from the literature. The scale of the precipitates produced in pure iron as well as in the 1070 steel pearlitic and spheroidized microstructure is found to provide marginal hardening. The interaction between N and carbides obtained during the nitriding of the 1070 martensitic microstructure gives rise to a significant hardness increase up to 130HV. The most significant hardening effect (up to 900HV) is obtained with the formation of a high density of fine CrN precipitates in the Fe-Cr specimens.</p> / Master of Applied Science (MASc)

Le développement d’une troisième génération d’aciers Fe-Mn-Al-C à structure duplex, pour des teneurs en Mn et Al inférieures à 8 %mass, pourrait être une réponse prometteuse aux objectifs d’allègement de 20% des véhicules automobiles, tout en garantissant des propriétés de haute résistance mécanique et haute formabilité.Le choix des nuances et l’optimisation des conditions d’élaboration nécessitent de prévoir en particulier les compositions et proportions des phases existantes en fonction de la route métallurgique. Une base de données thermodynamiques fiable et précise est donc requise. Cependant les données de la littérature sur le système quaternaire Fe-Mn-Al-C, dans les domaines de composition envisagés, sont limitées.Ce mémoire est consacré à l’établissement des équilibres de phases ferrite-α, austénite-γ et carbure-κ (Fe,Mn)3AlC entre 700 et 1000°C par une approche couplée d’expériences ciblées et de modélisation thermodynamique. Pour appuyer l’évolution expérimentale des fractions de phases et des compositions, une modélisation cinétique (DICTRA) est proposée. La cinétique de formation de l’austénite en fonction de la composition de l’alliage et de la température de maintien dans le domaine intercritique a été caractérisée. Les phases en équilibre, caractérisées par DRX, MEB, microsonde, sont représentées sous forme de conodes α/γ, γ/κ, α/γ/κ, ce qui permet de définir les domaines de stabilité de l’austénite et du carbure κ. Ces données expérimentales sont utilisées pour affiner la description thermodynamique du système quaternaire mais il est nécessaire de réviser la modélisation du carbure κ. / A third generation of Fe-Mn-Al-C steels with a duplex structure, for Mn and Al contents less than 8%mass, could be a promising response to the 20% weight lightening of automotive vehicles, by keeping a high strength and a high formability.The knowledge of the corresponding quaternary phase diagram serves as a roadmap for the choice of compositions and the optimization of elaboration conditions. A reliable and precise thermodynamic database is therefore required. However, the literature data on the Fe-Mn-Al-C quaternary system in the targeted domains are limited.This study is devoted to the establishment of phase equilibria involving ferrite-α, austenite-γ and carbide-κ (Fe,Mn)3AlC between 700 and 1000°C by a coupled approach of experiments and thermodynamic modeling. A kinetic model (DICTRA) is proposed to support the experimental evolution of phase fraction and composition. The kinetics of austenite formation as a function of the alloy composition and of the maintaining temperature in the intercritical domain have been calculated. The phases in equilibrium, characterized by XRD, SEM, EPMA, are represented as α/γ, γ/κ, α/γ/κ tie-lines in order to specify the stability fields of γ and κ. These data are used to refine the thermodynamic description of the quaternary system but it is necessary to revise the modeling of κ carbide.

Les structures optiques diffractives sous forme d’hologramme de sécurité sont largement employées contre la falsification et la contrefaçon.Elles sont présentent sur les billets de banque, les documents de voyage et d’identité, etc. Leurs techniques de fabrication sont de plus en plus accessibles, augmentant les risques de fraudes et la concurrence sur le marché des hologrammes de sécurité. Pour endiguer les fraudes et gagner des parts de marché, il faut innover. Ces travaux de thèse de doctorat s’articulent autour du développement de structures optiques diffractives multiéchelles innovantes. Ces structures diffractives multiéchelles sont la combinaison de structures diffractives microscopiques permettant la mise en forme de la lumière incidente avecdes structures nanoscopiques qui permettent la création d’effets colorés. Ces travaux accordent une grande place au développement de la technique de photolithographie multiniveaux par écriture directe massivement parallèle. Ils abordent également le développement d’un modèle hybride permettant de simuler physiquement le comportement des structures diffractives (notamment de nos structures multiéchelles) en temps réel. Ce rendu en temps réel est possible grâce à l’utilisation du processeur graphique (GPU) au travers d’OpenGL et des programmes Shader, ainsi qu’avec l’utilisation de données précalculées. Le développement de ces structures multiéchelles permet la création et la commercialisation de nombreux nouveaux effets visuels, ce qui participe aux doubles objectifs de contrer les fraudes et de gagner en part de marché. / Diffractive optical structures on the fon of security holograms are widely used against forgery and counterfeiting. They are present on banknotes, travel and identity documents, etc. Their manufacturing techniques are becoming more and more accessible, increasing the risk of fraud and competition in the security hologram market. To stem fraud and gain marketshare, hologram procedures must innovate continously. This Ph.d focuses on the developmentof innovative multi-scale diffractive optical structures. These multi-scale diffractive structures result from combination of microscopic diffractive structures that shape the incident light and nanoscopic structures that generate colored effects. This work places emphasis on the development of the massively parallel-direct-write greyscale photolithography fabrication process. We also discuss the development of an hybrid model for physically simulating the behaviour of diffractive structures (especially our multiscale structures) in real time. This real time rendering is possible thanks to the use of the graphical processor unit (GPU) through OpenGL and Shader programs, as well as the use of precomputed data. The development of these multiscale structures has led to the creation and commercialisation of many new visual effects and contributed to the dual objectives of counter fraud and gain market share.