By sensitising the magnetic resonance signal to the diffusion of water molecules in tissue, diffusion-weighted magnetic resonance imaging provides a means of assessing tumour microstructure non-invasively. Such measurements have the potential to provide important information about tumour development and the response of tumours to treatment, but the way in which different tissue properties affect the diffusion-weighted signal remains unclear. Through simulations, in vivo studies and phantom experiments, this thesis investigates the relationship between the diffusion-weighted signal, the pulse sequence parameters used for acquisition, and microstructural properties of tumours. The use of oscillating gradient pulse sequences on a clinical scanner was investigated initially, with theoretical and practical considerations leading subsequent work to focus on pulsed gradient sequences. The forward problem of predicting the diffusion-weighted signal for given combinations of tissue properties and sequence parameters was addressed numerically through Monte Carlo simulations, focussing on how tumour cell size, intracellular volume fraction and membrane permeability affect the signal. These simulations allowed the sensitivity of the signal to changes in these tissue properties to be investigated, revealing how sensitivity depends on sequence parameters as well as the specific microstructural configuration. By repeating the simulations using the specific sequence parameters used in a clinical and preclinical study, the sensitivity of the implemented protocols was assessed, and linked to the experimental findings. The preclinical study illustrated the importance of the diffusion time in determining the sensitivity to treatment-induced changes in tumours, with larger post-treatment signal changes observed at longer diffusion times. These trends were qualitatively reflected in the sensitivity analysis derived from the simulations. Finally, the inverse problem of estimating microstructural properties from the diffusion-weighted signal was addressed using a physical phantom designed as a simple mimic of tumour tissue. By fitting a biophysical model to the diffusion data, the size and volume fraction of the approximately spherical 'cells' were estimated. The radius was slightly underestimated compared with that determined from independent measurements, the fitted volume fraction was plausible, and parameters were found to be estimated with reasonably good precision.

The research in this thesis aims to develop new mechanistic knowledge for coating processes at 2024-Al alloy surfaces, ultimately to aid the design of new protective coatings. Coatings formed by phosphating, chromating, and permanganating were characterized especially by scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy, and scanning electron microscopy . The objective was to learn about growth (nm level) as a function of time for different coating baths, as well as a function of lateral position across the different surface microstructural regions, specifically on the μm-sized Al-Cu-Mg and Al-Cu-Fe-Mn particles which are embedded in the alloy matrix . The research characterizes coating thickness, composition, and morphology. The thesis emphasizes learning about the effect of different additives in zinc phosphating baths . It was found that the Ni²⁺ additive has two main roles : first, the rate of increase in local solution pH is limited by the slower kinetics of reactions involving Ni²⁺ compared to Zn²⁺, leading to thinner zinc phosphate (ZPO) coatings when Ni²⁺ is present. Second, most Ni²⁺ deposition occurs during the later stages of the coating process in the form of nickel phosphate and a Ni-Al oxide in the coating pores on the alloy surface, increasing the corrosion resistance. Aluminum fluoride precipitates first during the initial stages of the coating process, followed by aluminum phosphate, zinc oxide, and finally ZPO. When Ni²⁺ is present in the coating solution at 2000 ppm, ZnO predominates in the coating above the A-Cu-Fe-Mn particle while ZPO dominates on the rest of the surface. The Mn²⁺ additive gives a more even coating distribution (compared with Ni²⁺) across the whole surface. The Mn²⁺ -containing ZPO coating is similar to the chromate coating in terms of evenness, while there is more coating deposition at the second-phase particles for permanganate coatings. The oxides on the Al-Cu-Fe-Mn and matrix regions are similar before coating, thereby confirming that a variety of observed differences in ZPO coating characteristics at these regions arise from the different electrochemical characteristics of the underlying metals. Upon exposure to a corrosive solution, the ZPO coating provides more protection to the second-phase particles compared to the matrix. / Science, Faculty of / Chemistry, Department of / Graduate

This dissertation focuses on the investigation of the microstructures of two nuclear grade matrix graphites. These graphites were intended for use in the core components of a high temperature test reactor (HTTR) of the pebble bed modular reactor (PBMR) design. The graphites were provided in the form of fuel spheres and a reflector block. The techniques used in the analysis of the materials include fracturing, etching, scanning electron microscopy (SEM), nano-indentation, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructures of the materials were characterized successfully. The fuel sphere material consisted of a high concentration of curved graphite flakes and grains in contact with turbostratic matrix graphite. The well graphitized flakes and grains were polycrystalline in nature. Delamination cracks were prevalent in the graphite crystallites. There was no significant difference in the microstructures of the center, interior and surface regions of the fuel sphere material. No evidence of amorphous carbon or resin residues was found. The reflector material consisted of a low concentration of graphite crystallites embedded within turbostratic matrix graphite. Delamination cracks were observed within the graphite crystallites, and many cavities were present in the material. TEM observation also revealed the presence of diamond crystallites. It was concluded that the fuel sphere graphite was most probably suitable for use as is, provided that the material also possessed other required properties for use in a HTTR. The reflector material however was considered to be unsuitable for use in a HTTR. It was thus suggested that the reflector material could be made more suitable by sufficient graphitization of the turbostratic graphite which formed the bulk of the material.

The microstructure of a steel is often developed by solid-state transformation from austenite. The major transformation products are allotriomorphic ferrite, pearlite, Widmanstatten ferrite, bainite and martensite, differentiated by morphological features, and their nucleation and growth mechanisms. A steel often consists of several phases as a result of dynamic evolution during continuous cooling. The complexity of the calculation of all the transformations simultaneously poses a challenge. There have been a few attempts at integrating all these transformations into an unified scheme. They involve varying degrees of empiricism. For the first time, a model that can predict simultaneously the volume proportions of all the major transformation products has been developed. The algorithm has taken full account of the thermodynamics and kinetics of individual transformations, instead of empirical equations, so the model should in principle generalise well. The predictions of the model are based on a number of input parameters: the chemical composition, austenite grain size and cooling conditions. The model can simulate cooling at constant rates, or isothermal transformations. Therefore it can also generate continuous cooling transformation (CCT), or time-temperature transformation (TTT) diagrams. The model has demonstrated a consistency in its predictions. The validations of the model against published experiment data and experiments conducted in this work have shown the predictions in most cases are reasonable with errors less than a few volume percent. Further research opportunities presented by the work are reviewed.

Slurry coating technique, as one of the most popular deposition methods, is widely used to produce various material coatings. This method includes two processes: spraying, brushing or dipping of slurry, and sintering heat treatment of the coated specimen. Superalloys and stainless steels are the most common materials used as either coating materials or substrate materials because of their excellent corrosion, wear, high-temperature and mechanical properties. This research is aimed at investigating the influence of the sintering heat treatment in the slurry coating process developed at Kennametal Stellite Inc. on the microstructure, hardness and wear behavior of superalloys and stainless steels. Low-carbon Stellite 22, cobalt-based Tribaloy T-400C, martensitic AISI 420 and AISI 440C stainless steels are studied in this research. The microstructure, hardness and wear resistance of these alloys before and after the heat treatment are investigated, stressing the influence of the heat treatment on these material characteristics. The hardness and wear tested are conducted on these alloys at both room temperature and at elevated temperatures. The worn surfaces of each specimen are analyzed using a Scanning Electron Microscope (SEM) with backscatter electron imaging (BEI) and energy dispersive X ray (EDX) spectrum. It is demonstrated that the heat treatment alters the microstructures of these alloys differently; it increases the hardness but affects the wear resistance more complexly than hardness. At room temperature, the wear resistance of these alloys is governed by their microstructures. However, at high temperatures, oxidation, resulting in formation of oxide films on the specimen surface, influences the wear resistance significantly.

Nous avons caractérisé de manière détaillée les  fortes hétérogénéités de texture et de microstructure (macrozones) par EBSD dans des zones déformées différemment d’un disque en IMI834 de structure bimodale αP/αS, utilisé en motorisation aéronautique. Des échantillons spécifiques ont permis de montrer que l’amplitude des signaux ultrasonores rétrodiffusés dans ces échantillons dépendait de la présence et de l’organisation des macrozones. Partant de ces observations, une analyse ultrasonore des signaux rétrodiffusés dans les parties du disque soumis à de fortes déformations lors du forgeage a permis de sélectionner un jeu d’éprouvettes ayant des caractéristiques microstructurales différentes et destinées à des essais en fatigue-dwell. Une forte dispersion des durées de vie liée aux microstructures présentes a été observée. L’analyse fractographique des surfaces de rupture et leurs études par EBSD ont permis de localiser les sites d’amorçage et les chemins de propagation des fissures ainsi que leurs caractéristiques cristallographiques. Les résultats des essais en fatigue-dwell montrent que la réduction de la tenue en service est fortement influencée par la présence de macrozones formées d’agglomérats de grains αP allongés associés à des lamelles αS plus épaisses et organisées en colonies et d’autre part présentant majoritairement des axes c à moins de 30° de l’axe de sollicitation / We have characterized in detail the strong heterogeneity of texture and microstructure (macrozones) by EBSD in areas, differently deformed, of a disk in IMI834 with a bimodal structure αP/αS, used in aircraft engines. Specific samples have shown that the amplitude of back-scattered ultrasonic signals depended on the presence and organization of macrozones. Based on these observations, an analysis of back-scattered ultrasonic signals in parts of the disc subject to strong deformation during forging led to select a set of specimens with different microstructural features for fatigue-dwell tests. A high dispersion of lifetimes associated with these microstructures was observed. The fractographic analysis of fracture surfaces and their EBSD studies were used to locate crack initiation sites and propagation paths and their crystallographic characteristics. The results of fatigue-dwell tests showed that the lifetime reduction is strongly influenced by the presence of macrozones formed by agglomerates of grains elongated αP associated with thicker αS lamellae, organized in colonies and with c-axis predominantly at less than 30 ° of the solicitation axis

This thesis describes the formation of and applications of self-assembled structures on metals. Primarily the focus of this PhD project is on the formation of surfaces structures on stainless steel (AISI 304) but other metals have been studied. Laser generated surface structures have been applied to the modification of wettability and reflectivity with a view towards developing these processes for industrial applications. Compared to conventional techniques for the modification of wettabililty, lasers offer the advantage of being a relatively simple technique for the modification of surface structure, reducing the need for complex processes. It is hoped that investigations into the reduction of surface reflectivity will have applications in the conversion of solar energy into useable power in the form of solar thermal energy. The production of self assembled structures is demonstrated using diode pumped solid state (DPSS) Nd:YVO4 lasers operating at wavelengths of 532 and 1064 nm. It is shown that the production of surface microstructures is highly dependant on the correct laser fluence and requires multiple pulses and processing passes. At 1064 nm wavelengths, it has been found highly reproducible surface structures can be formed by carefully controlling laser fluence and scanning speed while keeping the optical arrangement relatively simple. In addition to microstructure formation, the use of ultrafast femtosecond lasers, operating at 400 and 800 nm wavelengths has verified the production of laser induced periodic surface structures. Additionally, the stationary method used to produce these surfaces has been adapted to cover large surface areas with sub wavelength ripple structures with periods of ~295nm and 600nm. Applications of laser surface microstructures on metals have been studied in an effort to produce hydrophobic and superhydrophobic surfaces on metals. It has been found that the roughness change produced by laser processing induces composite wetting when water droplets are introduced to the surface. Contact angle measurements and small angle XRD analysis of laser processed stainless steel (AISI 304) have shown that surface wettability decreased over a period of approximately one month, leading to steady contact angles of over 140°. This is attributed to the formation of a magnetite (Fe3O4) oxide layer in the period after laser processing. The effect of surface microstructure on surface reflectivity has also been studied. It was found that laser induced surface microstructures on copper can decrease surface reflectivity by almost 90%. A comparative study of the effects of surface roughness and chemistry on the optical absorption of copper is given, finding that these surfaces are competitive with contemporary coatings.

Flow forming is a fairly new technique used for the production of dimensionally accurate near net shaped hollow components. The process has many advantages such as cost effectiveness and eliminating further operations like welding, machining, etc. This study focuses on the characterization of flow formed components to understand the process. Flow formed components are composed of different reductions and characterization techniques are applied to reveal the resulting microstructural differences. Effect of number of passes on the material is also investigated. Metallographic analysis was conducted by optical microscope, electron micro probe analyser (EPMA) and the electron back scatter diffraction technique (EBSD) in a scanning electron microscope (SEM). Texture evolution of the samples was examined either by laboratory X-ray diffraction or EBSD technique. Furthermore, residual stresses were measured by neutron diffraction (at StrainAnalyzer for Large and Small Scale Engineering Applications (SALSA) and PulseOverlap Diffractometer (POLDI) instruments), laboratory X-ray diffraction and hole drilling. Stress relief heat treatments were carried out at 500°C for either 4 or 16 hours to mitigate residual stresses without losing much of the strength. The experiments conducted show that flow forming is a process resulting in heterogeneous microstructure with grains elongated along the deformation direction. Texture evolution is different from the typical rolling of steels with body centred cubic crystal structure. Any significant effect of the number of passes was not observed. Due to the nature of the process, residual stresses in the axial and hoop directions are critical. Therefore, stress distributions through thickness of the samples are plotted. It is observed that in the thick section, the stresses are higher. Heat treatments applied at 500°C for 4 or 16 hours are effective in diminishing the stresses.

Les aciers bainitiques sans carbure font partie d'une nouvelle génération d’aciers à très haute résistance, présentant des limites d’élasticité élevées et un excellent compromis entre résistance mécanique et ductilité. Leurs propriétés sont liées à la spécificité de leur microstructure qui fait intervenir plusieurs constituants (bainite, austénite et martensite) imbriqués selon une topologie particulière (colonies lamellaires et ilots résiduels). De nombreuses questions restent cependant en suspens quant aux liens existant entre paramètres microstructuraux et propriétés mécaniques. Ce travail de thèse vise ainsi à explorer ces relations dans différents aciers bainitiques sans carbure élaborés par traitement de trempe étagée. Diverses techniques de caractérisation in situ et post mortem ont été mises en oeuvre pour analyser qualitativement et quantitativement les microstructures résultant de la décomposition de l'austénite en conditions isothermes. L'influence de la température de maintien isotherme et de la concentration en carbone sur la microstructure et la microtexture a ainsi été mise en évidence. Un traitement de trempe étagée réalisé sous Ms a également permis d'élaborer une microstructure composite constituée de martensite revenue, de ferrite bainitique et d'austénite résiduelle. Des essais de traction et de cisaillement ont permis d'évaluer les propriétés mécaniques de ces aciers et notamment d'estimer les contributions isotropes et cinématiques de leur écrouissage. Les résultats ont ensuite été analysés à la lumière des informations microstructurales et l'influence de certains constituants a été mise en évidence. Le comportement de la bainite sans carbure élaborée sous Ms a été appréhendé par une approche micromécanique basée sur une loi des mélanges entre la martensite revenue et le composé bainite – austénite résiduelle / Carbide-free bainitic steels are part of the 3rd generation of advanced high strength steels, which exhibit high yield strength and an excellent compromise between tensile strength and ductility. These ground – breaking properties are achieved thanks to the characteristics of their microstructure which is constituted of different phases (bainite, austenite and martensite) organized in a specific way (typical bainitic colonies and residual islands). However, relationships between microstructural features and mechanical properties are yet to be thoroughly established. In the frame of this PhD, we investigated these relationships in carbide-free bainitic steels elaborated by an austempering process. Various means of characterization were used in situ and post mortem to analyze qualitatively and quantitatively microstructures elaborated by the decomposition of the austenite in isothermal conditions. The influence of the austempering temperature and the carbon content on the microstructures has been highlighted. Moreover, austempering under Ms allowed elaborating a microstructure constituted of tempered martensite, bainitic ferrite and residual austenite. Tensile and shear tests were performed in order to evaluate their mechanical properties and to estimate the kinematical and isotropic contributions of the workhardening. Results were analyzed in the light of the microstructural characterizations and the effects of some microstructural features have been highlighted. The mechanical behavior of the bainite elaborated under Ms was estimated by a micromechanical approach based on a law of mixtures between the tempered martensite and the compound made of bainitic ferrite and residual austenite

Dans ce travail, les caractéristiques cristallographiques des martensites d’alliages Ni-Mn-Ga ont été étudiées en détail. En utilisant l’information de la superstructure de martensite 5M de Ni50Mn28Ga22 et de martensite 7M de Ni50Mn30Ga20 pour des mesures en EBSD, les structures cristallines ont été confirmées. Le nombre de variantes, les relations d’orientation entre les variantes adjacentes et les plans d’interface des variantes ont été déterminées sans ambiguïté. Sur la base de données d’orientations précises des variantes de martensite, les relations d’orientation de transformation de l’austénite en martensite 5M et de l’austénite en martensite 7M ont été déterminées, sans présence de l’austénite résiduelle. Pour la martensite NM de Ni54Mn24Ga22, les lamelles de macles à l’échelle nanométrique dans les platelets martensitiques ont été révélées. Les interfaces entre les platelets et entre les lamelles ont été analysées. Dans un alliage Ni53Mn22Ga25 avec coexistence de l’austénite et de la martensite à température ambiante, la formation de la microstructure martensitique en forme de losange avec quatre variantes lors de la transformation de l’austénite en martensite 7M a été mise en évidence. La nature de la martensite 7M a été clairement précisée dans ce travail. Elle est thermodynamiquement métastable et intermédiaire entre l’austénite parent et la martensite NM finale. La martensite 7M possède une structure cristalline indépendante, plutôt que la combinaison de macles nanométrique de martensite non modulée. Le rôle de la martensite 7M dans la transformation est d’atténuer le décalage important entre la maille de l’austénite cubique et celle de la martensite tétragonale et d’éviter la formation d’interfaces incohérentes entre les platelets de martensite NM, qui constituent une barrière énergétique infranchissable / In this work, the crystallographic features of martensites in Ni-Mn-Ga alloys were detailed studied. By using superstructure information for EBSD mapping on 5M martensite in Ni50Mn28Ga22 alloy and 7M martensite in Ni50Mn30Ga20 alloy, the crystal structures were confirmed and the variant number, twin orientation relationships of adjacent variants and twin interface planes were unambiguously determined. Based on the accurate orientation data of martensite variants, the transformation ORs for austenite-5M and austenite-7M were indirectly determined with no presence of initial austenite. For the NM martensite of Ni54Mn24Ga22, the nano-scale twin lamellae in martensitic plates were revealed, and the inter-plate interfaces and inter-lamellar interfaces were analyzed. In a Ni53Mn22Ga25 alloy with co-existence of austenite and martensite at room temperature, the formation of characteristic diamond-like martensite microstructure with four variants during the austenite-7M martensite transformation was evidenced. The 7M martensite occurs on cooling as a thermodynamically metastable phase that is intermediate between the parent austenite and the final NM martensite. 7M martensite possesses an independent crystal structure, rather than the nanotwin combination of normal non-modulated martensite. The role of 7M martensite in the transformation from the cubic austenite to the tetragonal NM martensite has been clarified, which is at the request of mitigating the large lattice mismatch between the cubic austenite and the tetragonal NM martensite and avoiding the formation of the incoherent NM plate interfaces that represent insurmountable energy barrier