Phase field models have played an important role in shaping our understanding of a variety of micro structural phenomena in materials. Their attractive features include (a) their ability to capture instabilities in microstructures, and (b) their ability to handle topological transitions { such as splitting or coalescence { gracefully. Therefore, we have chosen to use a phase field model in our study of instabilities in cylindrical pores in nanoporous membranes which eventually lead to their failure. Our study is motivated by recent studies on thermal stability of nanoporous membranes of alumina, titania and zirconia. The key feature in our model is its ability to incorporate surface discussion as the mechanism for mass transport. We first benchmark the model through a critical comparison of our results on early stages of surface evolution during Rayleigh instability and grain boundary grooving with those from linear theories of these phenomena. We have then used longer simulations (which go beyond early stages, and therefore, can incorporate non-lineare effects) to study instabilities in a hollow cylinder in three different systems: single crystal or amorphous solid (which fails through Rayleigh instability), a model sys-tem with parallel grain boundaries (which fails through grain boundary grooving), and a polycrystal (whose failure depends on a combination of grain growth and grooving). In all the cases, the surface energy is assumed to be isotropic, and the operative mechanism for mass transport is assumed to be surface discussion.

La synthèse par métallurgie des poudres de nickel à grains ultrafins (UFG) a été effectuée, et l’effet de l’affinement de la microstructure sur le comportement mécanique et les propriétés physiques a été étudié. La possibilité de coupler le broyage et le frittage flash est étudiée avec des résultats prometteurs. Des échantillons de haute densité avec des tailles de grains d = 0.65 – 4 µm, caractérisés par une fraction élevée des joints de grains Σ3 et un faible niveau de contrainte ont été synthétisés. Les propriétés mécaniques des échantillons UFG montrent une bonne combinaison ductilité-résistance mécanique, avec un impact mineur des porosités présentes. L’étude de l’influence de la taille de grain dans le régime UFG sur les propriétés mécaniques montre une limite d’élasticité supérieure à celle attendue et une capacité d’écrouissage plus faible. Ces observations sont cohérentes avec la microstructure déformée à rupture, étudiée par diffraction d’électrons rétrodiffusés et microscopie électronique en transmission. Une haute diffusivité, mesurée par des expériences de traceurs radioactifs, montrent des profils de pénétration très différents liés aux structures de porosités diverses présents dans les échantillons. Ces différentes structures sont aussi responsables de la densification rétrograde observée, uniquement pour les échantillons frittés à partir de poudres broyées. / The present manuscript concerns the synthesis of ultrafine grained (UFG) Ni by powder metallurgy, and the study of the influence of UFG microstructures on the mechanical behavior and physical properties. The possibilities of coupling ball milling and Spark Plasma Sintering are presented showing promising results. Highly dense homogeneous specimens are obtained, with average grain sizes d = 0.65 - 4 µm, and microstructures highlighted by a high fraction of Σ3 grain boundaries dependent on grain size. The mechanical properties in tensile testing for UFG samples are evaluated showing a good combination of strength and ductility, with little impact from porosities, the major drawback of powder metallurgy. The influence of grain size in the UFG regime on the mechanical properties is investigated, showing strength values that deviate from the expected behavior for grain refinement. Likewise, a reduced strain hardening capacity is depicted which correlates to the microstructural observations performed on the deformed state. High diffusivity measured by means of radiotracer experiments is observed in the sintered samples, displaying different penetration profiles that relate to diverse porosity structures. Such structures are also responsible for retrograde sintering observed exclusively in samples processed from BM powders.

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<p dir="ltr">Nanocrystalline materials have unique properties such as high ultimate strength and superior hardness. However, they also exhibit some disadvantages, such as low thermal stability. An effective strategy to address this issue is alloying with other materials. Grain boundaries play a pivotal role in property prediction due to their orientation between grains and the complexity of their structure. The prediction of structure-property relationships for GBs with microstructural complexity represents a difficult challenge.</p><p dir="ltr">To understand the effects of dopants on the material properties of grain boundaries, we constructed some bicrystal models for Al and Mg-doped Al (Al-Mg) alloys. Findings from shearing simulations of these GBs indicate that the GB structure and dopant distribution can influence GB migration. Dopants inhibit GB migration at certain GBs, effectively reinforcing these GBs. Shear-coupled GB migration in pure Al, as well as dopant inhibition of GB Al-Mg alloys, both contribute to the mechanisms of GB migration.</p>

Aluminium alloys have the possibility to be infinitely recycled. By only generating 5% of the emissions compared to primary aluminium, great CO2 savings can be made. One of the issues in manufacturing components entirely from post-consumer scrap is the presence of trace elements and impurities. Such elements can be Fe, Cu, Cr, P and Pb. In sheet metal manufacturing, these elements can also react with process agents such as Ti, B, Na and Sr and affect the recrystallization behavior and in turn mechanical properties.In this work, a derivative of the 3003 alloy made entirely from post-consumer scrap has been analysed. The alloy achieved insufficient formability due to lack of recrystallization and grain growth. With the use of scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) together with focused ion beam (FIB) lamella preparation, the microstructure was characterized.The characterization shows both larger particles of α-Al15Si2M4 (M=Mn,Fe,Cr) from solidification and dispersoids from heat treatment, pinning the grain boundary movement together with Q-AlCuMgSi. With the use of high throughput computational thermodynamics, Thermo-Calc was used to effectively screen compositions lowering the amount of α-Al15Si2M4 and removing the Q-AlCuMgSi phase. The new alloy was cast using directional solidification at different cooling rates to study the particle morphology, which in turn plays a role in the particle break up and distribution during cold working as the interparticle spacing affects the grain growth.Varying cooling rates was seen to affect morphology and distribution. Hot compression was utilized to examine the particle redistribution before cold work. It was however shown that hot compression was not sufficient in redistributing the particles as would be the case in rolling.

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Hf₁₋ₓZrₓO₂ (x ∼ 0.5–0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca2₁) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

<p>In this dissertation, the Phase Field Crystal (PFC) Method is used to study a number of problems in which interfaces and elastic effects play important roles in alloys. In particular, the three topics covered in this work are grain boundary thermodynamics in alloys, dislocation-mediated formation of clusters in binary and ternary alloys, and solutal effects in explosive crystallization. Physical phenomena associated with grain boundaries, such as Read-Shockley-like behaviour and Gibbs adsorp- tion theorem, were shown to be accurately captured in both PFC- and XPFC-type models. In fact, a connection between the solute segregation behaviour and physical properties of the system—such as energy of mixing, mismatch, and undercooling—were shown. Also, grain boundary premelting was investigated. It was shown how solute can change the disjoining potential of a grain boundary and a mechanism for hysteresis in grain boundary premelting was discussed. Regarding the phenomenon of cluster formation, a general coexistence approach and a nucleation-like approach were used to describe the mechanism consistently with observations; the process is facilitated by lowering the energy increase associated with it. The final phenomenon studied was explosive crystallization. It was shown that the temperature oscillations due to unsteady motion of an interface could be captured with PFC-type models and that this behaviour leaves patterns, such as solute traces, in the material. The versatility of this PFC formalism was demonstrated by capturing the underlying physics and elucidating the role of misfit strain in altering interface oscillations during explosive crystallization. Finally, it was demonstrated in all projects how PFC model parameters relate to coarse-grained material properties, thereby connecting these phenomena on larger scales to atomistic-scale properties.</p> / Doctor of Philosophy (PhD)

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD. / QC 20100819