The automotive industry has become a major user of magnesium components. However, use of magnesium sheet products is quite limited, due to difficulties in producing cost effective components. Any sheet currently produced is formed at elevated temperatures, making magnesium parts relatively expensive. Knowledge of the microstructural effects on magnesium formability will help reduce the cost of these products. In this thesis, the microstructural factors that affect the formability of rolled and extruded magnesium sheet were compared. It was found that the degree of dynamic recrystallization was the factor that controlled elongation. Dynamic recrystallization produced a finer grain size, which resulted in a transition in deformation mechanism from dislocation slip to grain boundary sliding. Digital image correlation was used to study local stresses during tensile deformation, and to determine if magnesium satisfies Considere's criterion before failure. The results indicated that local stresses developed during deformation satisfied Considere's criterion, although the global strains were lower than the theoretical predictions. / Thesis / Master of Applied Science (MASc)

Over the past decade, environmental concerns and safety regulation have led to increasing demand for vehicles with higher passenger safety and fuel economy. This has spurred intensive research on advanced high strength steels (AHSS). The quench and partitioning (Q&P) heat treatment is a novel approach that has led to development of one group of third generation AHSS alloys. In recent years most of the studies on the Q&P process were dedicated to the effect of the heat treatment parameters on microstructural evolution and mechanical properties. However, micromechanical deformation behavior of constituent phases and damage evolution in Q&P steels are not fully understood. In this study, damage micromechanisms in a commercial QP980 were investigated with the aid of in-situ tensile tests under a scanning electron microscope (SEM) followed by local strain mapping using microscopic digital image correlation (µ-DIC) analysis so as to quantify the microstructural deformation of constituent phases. Nano-hardness measurements were conducted to correlate the amount of plastic deformation of each phases to its strength. Ex-situ tensile tests coupled with electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) were conducted to study the influence of transformation induced plasticity (TRIP) of the retained austenite phase on microstructural damage and deformation. It was found that average local true strain in ferrite was approximately two times and three times greater than that of martensite and blocky retained austenite respectively, which was with the good agreement with nano-hardness measurements showing that retained austenite blocks was three times and two times harder than martensite and ferrite respectively. Damage in both ferrite and martensite starts at the same total strain; however, damage growth is faster in martensite leading to the formation of large cavities. The average local true strain ratio of ferrite to martensite decreases after total true strains higher than 0.1 and the reduction is more pronounced in regions with higher martensite volume fraction. EBSD results showed that at total true strain of 0.07 some of the retained austenite blocks located at the ferrite and martensite interfaces were almost fully transformed to martensite. According to XRD results at the point of necking 57% of retained austenite transformed to martensite. There is evidence of brittle cracking of large blocky retained austenite in regions with strain localization starting at relatively low strains but appear to have little impact on the final failure process. The good deformation ability of QP980 is attributed primarily to co-deformation of ferrite and martensite and secondarily to the TRIP effect. / Thesis / Master of Applied Science (MASc)

No / The performance of cement-based materials can be controlled and tailored by adjusting the characteristics of reinforced nano inclusions. Therefore, the lateral size effect of graphene on the nano/micro-structures of ultra-high performance concrete (UHPC) was explored, and then the mechanical properties were investigated to analyze the structure–property correlation of composites in this paper. The test results show that due to nucleation site effect and the formation of core–shell elements, incorporating graphene with lateral size of > 50 µm improves the polymerization degree and mean molecule chain length of C-S-H gel by 242.6% and 56.3%, respectively. Meanwhile, the porosity and average pore volume of composites is reduced by 41.4% and 43.4%. Furthermore, graphene can effectively inhibit the initiation and propagation of cracks by crack-bridging, crack-deflection, pinning and being pulled-out effect, and the wrinkling characteristic is conductive to the enhancement of pinning effect. These improvements on nano- and micro- structures result in that the compressive strength, compressive toughness and three-point bending modulus of UHPC are increased by 43.5%, 95.7% and 39.1%, respectively, when graphene with lateral size of > 50 µm and dosage of 0.5% is added. Compared to graphene with lateral size of > 50 µm, graphene with average lateral size of 10 µm has less folds and larger effective size, then reducing the distance between core–shell elements. Hence, the addition of graphene with average lateral size of 10 µm leads to 21.1% reduction for Ca(OH)2 crystal orientation index, as well as 30.0% increase for three-point bending strength. It can be, therefore, concluded that the lateral size of graphene obviously influences the nano/micro-structures of UHPC, thus leading to the significantly different reinforcing effects of graphene on mechanical behaviors of UHPC.

Requirements for increased operating efficiencies mean that future generations of aero-engines will need to operate at temperatures beyond the capabilities of current nickel-base superalloys. As a result, new alloy compositions for turbine disc applications are being developed. Optimising these alloy compositions requires balancing directly competing requirements. Increased Cr contents are needed to provide environmental resistance and increased concentrations of other refractory metals to improve solid solution strengthening. However, these elements compromise the alloyâs long-term microstructural stability by promoting the formation of topologically close-packed (TCP) phases, which are deleterious to alloy performance. High $\gamma^\prime$ volume fractions, which are needed to provide high-temperature strength, exacerbate the problem by increasing the concentration of these elements in the $\gamma$ phase. Therefore, an understanding of TCP formation and the compositional limits of stability is vital in the design of new alloys. This thesis presents a combination of fundamental studies of TCP phase formation in model alloys and microstructural assessment of the thermal stability of developmental alloy compositions. Knowledge of the effect of individual elements on thermal stability is important to enable the development of optimised alloy compositions. As a result, the first fundamental study investigated the effect of Co content on thermal stability. An unexpected transition in $\sigma$ precipitation behaviour after 500 hours at 800°C was observed between 12 and 16 at.\% Co. It is proposed that this behaviour may be due to the effect of Co on the $\gamma$/$\gamma^\prime$ partitioning behaviour of other elements. Preliminary results from further fundamental studies investigating the effect of the Mo/W ratio and B content on thermal stability are also presented. Decreasing the Mo/W ratio was found to reduce the quantity of $\sigma$ precipitation and promote the precipitation of a W-rich phase. B additions were found to promote the precipitation of the M$_3$B$_2$ phase. Thermodynamic predictions are frequently used to inform alloy design as an alternative to time-consuming and costly experiments. However, the accuracy of solvus temperature predictions for TCP phases has not been thoroughly considered. In this work, it was found that differential scanning calorimetry could be used as a means of measuring $\sigma$ solvus temperature in a series of alloys designed to be sufficiently unstable with respect to $\sigma$ precipitation. Comparison of experimental results with thermodynamic solvus temperature predictions revealed a significant underprediction of the $\sigma$ solvus temperatures for all of the studied alloys. This can inform our use of such predictions during alloy design. The ability to quantify the amount of TCP precipitation that occurs is extremely important when assessing the thermal stability of alloys. A new method was applied to the problem of TCP quantification, involving synchrotron X-ray diffraction of solid aged samples. This was an attempt to avoid some of the problems identified with the commonly used quantification method, which involves electrolytic extraction of minor phases, and assess the accuracy of the results produced by this method. Samples of a currently used commercial alloy, RR1000, were investigated following ageing for up to 5000 hours at 800°C, revealing the evolution of phases at this temperature. The presence of extremely low quantities of minor phases was successfully detected in the solid samples using this method. However, these quantities were too low for this to be a reliable method of quantification for commercial alloys. In parallel with these fundamental and technique-based studies, the thermal stability of a number of candidate alloys, which were developed during the design of a next-generation disc alloy by Rolls-Royce, was assessed. The alloys were characterised following a variety of thermal exposure temperatures and durations, which were determined by industrial needs at the time. Various minor phases were identified depending on the alloy compositions, including the TCP phases, $\sigma$ and $\mu$, as well as MC and M$_{23}$C$_6$ carbides and M$_3$B$_2$ borides.

Pasteurized skim milk was acidified using glucono-δ-lactone (GDL) at 10, 20, 30, and 40°C or with 1.2% freeze-dried yogurt starter culture at 40°C. Milk coagulation was followed by measuring turbidity, curd firmness, particle size, and casein micelle microstructural changes using transmission electron microscopy . The pH of milk was gradually lowered during acidification with GDL or starter culture. Acidification rate showed greater influence on turbidity change at 10°C than at 20, 30, or 40°C. Average casein micelle size increased with decreasing temperature. The patterns of average micelle size versus pH were not affected by temperature. No great variation of average micelle size was observed above pH 5.2. Below pH 5.0 the size increased exponentially as the milk gelled. Acidification rate did not influence average micelle size at 10°C. Acidification rate, types of acidifying agents, and temperature had no effect on the Formagraph gelation pH and the rate at which curd firmness developed. Casein micelles became less compact and less distinct with decreasing temperature before acidification. As pH was lowered, protein was dissociated from and then reassociated with casein micelles. Acidification rate had no effect on microstructure change of casein micelles at 10°C.

Photonic properties depend on both dielectric contrast in a microscopic composite and the arrangement of the microstructural components. No theory exists for direct prediction of photonic properties, and so progress relies on numerical methods combined with insight into manufacturable composite architectures. We present a discussion of effective photonic crystal production techniques and several numerical methods to predict dispersion relations of hypothetical but fabricable structures. / Singapore-MIT Alliance (SMA)

The study aimed at investigating the chemical composition and microstructure of raw iron ore from the deposits in Muko area (south-western Uganda). The quality of this iron ore was evaluated to establish its suitability to serve as a raw material for iron production. Samples were taken from the six hills of Muko ore deposits and tests carried out to establish their composition and properties. X-ray diffraction and scanning electron microscopy were employed in the investigation and chemical analysis performed to determine the compounds constituting the ore. The quality of this ore was compared to generalized world market standards and ores from other nations. It was found that Muko ore is a rich hematite grade with Fe content above 65%. It has little gangue (&lt;6% SiO2 and 3-4% Al2O3) and low contents of the deleterious elements (P ~ 0.02% and S &lt; 0.006%), which correspond to acceptable levels for commercial iron ores. / <p>QC 20130531</p> / Sustainable Technology Development in the Lake Victoria Region

The main objective of this study is to develop and validate a framework for microstructural modeling of asphalt composite materials using a coupled thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamage constitutive model. In addition, the dissertation presents methods that can be used to capture and represent the two-dimensional (2D) and three-dimensional (3D) microstructure of asphalt concrete. The 2D representative volume elements (RVEs) of asphalt concrete were generated based on planar X-ray Computed Tomography (CT) images. The 2D RVE consists of three phases: aggregate, matrix, and interfacial transmission zone (ITZ). The 3D microstructures of stone matrix asphalt (SMA) and dense-graded asphalt (DGA) concrete were reconstructed from slices of 2D X-ray CT images; each image consists of the matrix and aggregate phases. The matrix and ITZ were considered thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamaged materials, while the aggregate is considered to be a linear, isotropic elastic material. The 2D RVEs were used to study the effects of variation in aggregate shape, distribution, volume fraction, ITZ strength, strain rate, and temperature on the degradation and micro-damage patterns in asphalt concrete. Moreover, the effects of loading rate, temperature, and loading type on the thermo-mechanical response of the 2D and 3D microstructures of asphalt concrete were investigated. Finally, the model parameters for Fine Aggregate Mixture (FAM) and full asphalt mixture were determined based on the analysis of repeated creep recovery tests and constant strain rate tests. These material parameters in the model were used to simulate the response of FAM and full asphalt mixture, and the results were compared with the responses of the corresponding experimental tests. The microstructural modeling presented in this dissertation provides the ability to link the microstructure properties with the macroscopic response. This modeling combines nonlinear constitutive model, finite element analysis, and the unique capabilities of X-ray CT in capturing the material microstructure. The modeling results can be used to provide guidelines for designing microstructures of asphalt concrete that can achieve the desired macroscopic behavior. Additionally, it can be helpful to perform 'virtual testing' of asphalt concrete, saving numerous resources used in conducting real experimental tests.

Resumo: Os aços multifásicos são de grande interesse comercial e têm atraído a atenção, principalmente, da indústria automobilística devido a combinação de alta resistência e ductilidade. A fase austenita retida, presente nestes aços, tem sido objeto de especial interesse devido ao efeito TRIP, que contribui para a melhoria da tenacidade. Neste trabalho é utilizado o aço aeronáutico 300M e, com o objetivo de estudar a influência das fases sobre as propriedades mecânicas são realizadas as seguintes etapas: a otimização de rotas de tratamentos térmicos para a obtenção de uma microestrutura multifásica; caracterização microestrutural por microscopia óptica, microscopia eletrônica de varredura e microscopia de força atômica e avaliação do envelhecimento de deformação à temperatura ambiente. A principal contribuição é a determinação da fração volumétrica da austenita retida por meio de medidas de saturação magnética através de curvas de histerese magnética. As frações volumétricas de austenita retida obtidas por medidas de magnetização são comparadas com as obtidas por difratometria de raios X e microscopia óptica após ataques químicos de metabissulfito de sódio. Os resultados obtidos por microscopia óptica e pelo método magnético apresentam boa concordância com os valores encontrados na literatura. A alteração microestrutural devido aos tratamentos térmicos aplicados permite melhorias no limite de escoamento e resistência. Analisa-se, ainda, o efeito do envelhecimento por deformação à temperatura ambiente. Os resultados são surpreendentes, ocorrendo um aumento no limite de escoamento em até 87%, devido à associação do envelhecimento por deformação com o efeito TRIP. / Abstract: The multiphase steels are of great commercial interest and have attracted attention mainly of the automobile industry due to the combination of their high strength and ductility. The retained austenite phase present in these steels has been object of special interest due to TRIP effect contribution for the improvement of the toughness. In this work the influence of the phases of the aircraft 300M steel is carried out by the optimization of heat treatments routes to obtain multiphase microstructure. Microestructural characterization is evaluated by optical microscopy, scanning electronic microscopy, atomic force microscopy and analysis of the deformation ageing at room temperature. The main contribution is the determination of retained austenite volume fractions from magnetic hysteresis curves. These values are compared with x-ray diffraction data and optical microscopy after sodium metabisulfite etching, presenting good agreement with the literature. The microstructural alterations due the applied heat treatments also affect the yield strength and strength. The association of the deformation ageing and the TRIP effect resulted in a surprising increase of up to 87% in the yield strength. / Orientador: Antonio Jorge Abdalla / Coorientador: Tomaz Manabu Hashimoto / Banca: Marcelo dos Santos Pereira / Banca: Sergio João Crnkovic / Banca: Carlos de Moura Neto / Banca: Getulio de Vasconcelos / Doutor

Made available in DSpace on 2014-10-09T12:38:20Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:43Z (GMT). No. of bitstreams: 1 05682.pdf: 7598679 bytes, checksum: 0a8e36b4d2b6ba00958df86a2415829e (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP