Accumulative roll bonding was adapted to fabricate a carbon nanotube reinforced aluminum matrix composite. The microstructure was investigated by transmission electron microscopy, and it was confirmed that the nanotubes were embedded into the metal matrix while maintaining their multiwalled structure. Measurements revealed that the as-received carbon nanotubes had a bimodal diameter size distribution, while only nanotubes with diameters >30 nm and more than 30 walls were retained during four consecutive rolling operations at 50% reduction. The elastic deflection and vibration damping properties of the laminated composite were investigated by cantilever bending test and by impulse excitation method in samples with different concentrations of carbon nanotubes. Measurements by both methods revealed that a 0.23wt% concentration of nanotubes increased the elastic modulus according to the rule of mixtures and the damping behavior of the composites increased by the addition of nanotubes up to 0.1wt%. / Materials Engineering

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The main focus of this thesis is the characterization of defects and microstructure in high-energy ball milled magnesium hydride powder and magnesium-based multilayered composites. Enhancement in kinetics of hydrogen cycling in magnesium can be achieved by applying severe plastic deformation. A literature survey reveals that, due to extreme instability of -MgH2 in transmission electron microscope (TEM), the physical parameters that researchers have studied are limited to particle size and grain size. By utilizing a cryogenic TEM sample holder, we extended the stability time of the hydride phase during TEM characterization. Milling for only 30 minutes resulted in a significant enhancement in desorption kinetics. A subsequent annealing cycle under pressurized hydrogen reverted the kinetics to its initial sluggish state. Cryo-TEM analysis of the milled hydride revealed that mechanical milling induces deformation twinning in the hydride microstructure. Milling did not alter the thermodynamics of desorption. Twins can enhance the kinetics by acting as preferential locations for the heterogeneous nucleation of metallic magnesium. We also looked at the phase transformation characteristics of desorption in MgH2. By using energy-filtered TEM, we investigated the morphology of the phases in a partially desorbed state. Our observations prove that desorption phase transformation in MgH2 is of nucleation and growth type, with a substantial energy barrier for nucleation. This is contrary to the generally assumed core-shell structure in most of the simulation models for this system. We also tested the hydrogen storage cycling behavior of bulk centimeter-scale Mg-Ti and Mg-SS multilayer composites synthesized by accumulative roll-bonding. Addition of either phase (Ti or SS) allows the reversible hydrogen sorption at 350C, whereas identically roll-bonded pure magnesium cannot be absorbed. In the composites the first cycle of absorption (also called activation) kinetics improve with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Up to a point, increasing the loading amount of the second phase also accelerates the kinetics. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH2 formed exclusively on various heterogeneous nucleation sites. During activation, MgH2 nucleation occurred at the Mg-hard phase interfaces. On the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of internal free surfaces such as cracks. / Materials Engineering

In recent years with the exponential growth in web-based applications the amount of data generated has increased tremendously. Quick and accurate analysis of this 'big data' is indispensable to make better business decisions and reduce operational cost. The challenges faced by modern day data centers to process big data are multi fold: to keep up the pace of processing with increased data volume and increased data velocity, deal with system scalability and reduce energy costs. Today's data centers employ a variety of distributed computing frameworks running on a cluster of commodity hardware which include general purpose processors to process big data. Though better performance in terms of big data processing speed has been achieved with existing distributed computing frameworks, there is still an opportunity to increase processing speed further. FPGAs, which are designed for computationally intensive tasks, are promising processing elements that can increase processing speed. In this thesis, we discuss how FPGAs can be integrated into a cluster of general purpose processors running iterative algorithms and obtain high performance. In this thesis, we designed a heterogeneous cluster comprised of FPGAs and CPUs and ran various benchmarks such as PageRank, Katz and Connected Components to measure the performance of the cluster. Performance improvement in terms of execution time was evaluated against a homogeneous cluster of general purpose processors and a homogeneous cluster of FPGAs. We built multiple four-node heterogeneous clusters with different configurations by varying the number of CPUs and FPGAs. We studied the effects of load balancing between CPUs and FPGAs. We obtained a speedup of 20X, 11.5X and 2X for PageRank, Katz and Connected Components benchmarks on a cluster cluster configuration of 2 CPU + 2 FPGA for an unbalancing ratio against a 4-node homogeneous CPU cluster. We studied the effect of input graph partitioning, and showed that when the input is a Multilevel-KL partitioned graph we obtain an improvement of 11%, 26% and 9% over randomly partitioned graph for Katz, PageRank and Connected Components benchmarks on a 2 CPU + 2 FPGA cluster.

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This study focused on the material characterization of granular materials containing different percentages of “RAP”. A series of laboratory tests results were carried out to determine the physical and mechanical properties of natural aggregates and various aggregate-RAP blends. The results were used to evaluate methods to develop high-performance granular layer for pavement construction through proper compaction and control of RAP usage. The resilient modulus and accumulative deformation characteristics were determined in relation to RAP content, relative density, compaction method, stress level, stress state and the number of load applications. The effects of RAP content and density on the CBR values of aggregate-RAP blends under various conditions were also investigated. In addition, the effect of small strain cyclic loading on shear strength of aggregate-RAP blends was observed in laboratory tests. Results from this investigation demonstrated that: 1) adding RAP to natural aggregates may increase the resilient modulus of natural aggregates, and optimum content can be found to achieve the highest resilient modulus; 2) resilient modulus generally increases with density; higher density of aggregate-RAP blends can be achieved by using methods combining vibration and static loading. 3) deviatoric stress has more pronounced influence on accumulative deformation than confining pressure. 4) proper compaction method can reduce accumulative deformation of samples. 5) addition of RAP into aggregates results in little change in accumulative deformation when the RAP content is less than a threshold. 6) CBR value decreases with increasing RAP content and decreasing compaction effort or compacted dry density. 7) shear strength of an aggregate-RAP blend tends to increase after small strain cyclic loading. / Thesis / Master of Applied Science (MASc)

Magnesium alloys are widely used in engineering applications, including aerospace and automobile industries, due to their desirable properties, such as lower density, high damping capacity, relatively high thermal conductivity, good machinability, and recyclability. Researchers have, therefore, been developing new magnesium materials. However, mechanical and corrosion properties are still limiting many commercial applications of magnesium alloys. In this Ph.D. thesis research, I developed Mg-Ti composite materials to offer some solutions to further improve the mechanical behavior of magnesium, such as titanium-magnesium (Ti-Mg) claddings, Mg-Ti multilayers, and Ti particle enforced Mg alloys. Low cost manufacturing processes, such as hot roll-bonding (RB) and accumulative roll-bonding (ARB) techniques, were used to produce Mg-Ti composites and sheets. The microstructural evolution and mechanical properties of composites were investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), nanoindentation, and tensile tests. In the first part of this study, I investigated the bonding strength of the AZ31/Ti to understand the mechanical properties of Mg/Ti composites. Using a single pass RB process, I fabricated AZ31/Ti multilayers with the thickness reduction in a range of 25% to 55%. The hot-rolled AZ31/Ti multilayers were heat-treated at 400 °C for 6, 12, and 24 hours, respectively, in an argon atmosphere. Tensile-shear tests were designed to measure the bonding strength between AZ31/Ti multilayers. Furthermore, the experimental results revealed good bonding of the AZ31/Ti multilayers without forming any intermetallic compounds in the as-rolled and heat-treated AZ31/Ti multilayers. The good bonding between Ti and AZ31 is the result of diffusion bonding whose thickness increases with increasing heat-treatment time and thickness reduction. The shear strength of the Ti/AZ31 multilayer increases with increasing bonding layer thickness. In the second part of this study, I characterized the microstructure and texture of three-layered Ti/AZ31/Ti clad sheets which were produced by single-pass hot rolling with a reduction of thickness 38% (sheet I) and 50% (sheet II). The AZ31 layer in sheets I and II exhibited shear bands and tensile twins {1012}⟨1001⟩ . The shear bands acted as local strain concentration areas which led to failure of the clad sheets with limited elongation. Heat treatment caused changes in the microstructure and mechanical properties of clad sheets due to static recrystallization (SRX) on twins and shear bands in the AZ31 layer. Recrystallized grains usually randomize the texture which causes weaken the strong deformed (0001) basal texture. Twins served as nucleation sites for grain growth during SRX. Tensile tests at room temperature showed significantly improved ductility of the clad sheets after heat treatment at 400°C for 12h. The results showed that the mechanical properties of clad sheets II are better than clad sheet I: The clad sheet II shows elongation 13% and 35% along the rolling direction (RD) for as-rolled and annealed clad sheet, respectively whereas the clad sheet I shows elongation 10% and 22% along RD for as-rolled and annealed clad sheet, respectively. In the final part of this study, I examined the effects of dispersed pure titanium particles (150 mesh) with 0, 2.3, 3.5, 4.9, and 8.6 wt. % on the microstructure and mechanical properties of AZ31-Mg alloy matrix. Mg-Ti composites were processed through three accumulative roll bonding (ARB) steps using thickness reductions of 50% in each pass followed by heat treatment at 400 °C for 12 h in an argon atmosphere. ARB is an efficient process to fabricate Mg-Ti composites. Mechanical properties of Mg- 0Ti and Mg-2.3Ti composite were enhanced by ~ 8% and 13 % in yield strength and ~ 30% and 32 % in ultimate tensile strength, respectively. Meanwhile, the elongation of the composites were decreased by 63% and 70%, respectively. After heat treatment, the results showed a decrease in yield strength and increase in elongation to fracture. The mechanical properties of the Mg-0 and Mg-2.3Ti composite were enhanced: ultimate tensile strength by 9% and 7%, and elongation by 40% and 67%, while the yield strength was decreased by 28% and 36% compared with the initial AZ31. Enhancements of strength and ductility were the results of two mechanisms: a random matrix texture by ARB and ductile titanium particle dispersion.

M. Tech. (Mechanical Engineering, Faculty of Engineering and Technology): Vaal University of Technology / Presently there exist a lot of controversies about the mechanical properties of nanomaterials. Several convincing reasons and justifications have been put forward for the controversies. Some of the reasons are varying processing routes, varying ways of defining equations, varying grain sizes, varying internal constituent structures, varying techniques of imposing strain on the specimen etc. It is therefore necessary for scientists, engineers and technologists to come up with a clearer way of defining and dealing with nanomaterials’ mechanical properties. The parameters of the internal constituent structures of nanomaterials are random in nature with random spatial patterns. So they can best be studied using random processes, specifically as stochastic processes. In this dissertation the tools of stochastic processes have been used as they offer a better approach to understand and analyse random processes. This research adopts the approach of ascertaining the correct mathematical models to be used for experimentation and modelling. After a thorough literature survey it was observed that size and temperature are two important parameters that must be considered in selecting the relevant mathematical definitions for nanomaterials’ mechanical properties. Temperature has a vital role to play during grain refinement since all severe plastic deformation involves thermomechanical processes. The second task performed in this research is to develop the mathematical formulations based on the experimental observation of 2-D grains and 3-D grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing. The experimental observations revealed that grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are elongated when observed from the rolling direction, and transverse direction, and equiaxed when observed from the normal direction. In this dissertation, the different experimental observations for the grain size variants during grain refinement were established for 2-D and 3-D grains. This led to the development of a stochastic model of grain-elongation for 2-D and 3-D grains. The third task was experimentations and validation of proposed models. Accumulative Roll-Bonding, Equal Channel Angular Pressing and mechanical testing (tensile test) experiments were performed. The effect of size on elongation and material properties were studied to validate the developed models since size has a major effect on material’s properties. The fourth task was obtaining results and discussion of theoretical developed models and experimental results. The following facts were experimentally observed and also revealed by the models. Different approaches of measuring grain size reveal different strains that cannot be directly obtained from plots of the corresponding grain sizes. Grain elongation evolved as small values for larger grains, but became larger for smaller grains. Material properties increased with elongation reaching a maximum and started decreasing as is evident in the Hall-Petch to the Reverse Hall-Petch Relationship. This was alluded to the fact that extreme plastic straining led to distorted structures where grain boundaries and curvatures were in “non-equilibrium” states. Overall, this dissertation contributed new knowledge to the body of knowledge of nanomaterials’ mechanical properties in a number of ways. The major contributions to the body of knowledge by his study can be summarized as follows: (1) The study has contributed in developing a model of elongation for 2-D grain and 3-D grains. It has been generally reported by researchers that materials deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are generally elongated but none of these researchers have developed a model of elongation. Elongation revealed more information about “size” during grain refinement. (2) The Transmission Electron Microscopy revealed the grain shape in three directions. The rolling direction or sliding direction, the normal direction and the transverse direction. Most developed models ignored the different approaches of measuring nanomaterials’ mechanical properties. Most existing models dealt only with the equivalent radius measurement during grain refinement. In this dissertation, the different approaches of measuring nanomaterials’ mechanical properties have been considered in the developed models. From this dissertation an accurate correlation can be made from microscopy results and theoretical results. (3) This research has shown that most of the published results on nanomaterials’ mechanical properties may be correct although controversies exist when comparing the different results. This research has also shown that researchers might have considered different approaches to measure nanomaterials’ mechanical properties. The reason for different results is due to different approaches of measuring nanomaterials’ mechanical properties as revealed in this research. Since different approaches of measuring nanomaterials’ mechanical properties led to different obtained results, this justify that most published results of nanomaterials’ mechanical properties may be correct. This dissertation revealed more properties of nanomaterials that are ignored by the models that considered only the equivalent length. (4) This research has contributed to the understanding of nanomaterials controversies when comparing results from different researchers.

O presente trabalho teve como objetivo a caracterização mecânica, microestrutural e inspeção fratográfica do compósito de matriz de alumínio Al-1100 reforçado com partículas de carbeto de silício-SiC (40 μm) fabricados por meio de laminação acumulativa (ARB- do inglês Accumulative Roll Bonding), assim como, para efeito comparativo, foram estudados o Al-1100 processado por ARB sem adição de partículas e Al-1100 como recebido. Ensaio de desgaste microadesivo com esfera fixa e ensaio de tração unidirecional quase estático que foram realizados em amostras sem entalhes e em amostras contendo diferentes geometrias de entalhe. Microscopia óptica, microscopia eletrônica de varredura nos modos: elétrons secundários, elétrons retroespalhados, espectroscopia de energia dispersiva por raios-X e difração de elétrons retroespalhados, difração de raios-X e microtomografia computadorizada foram utilizados para caracterizar as amostras. Os resultados obtidos mostraram êxito da incorporação de partículas de SiC na matriz de Alumínio por meio do processo ARB. Houve ganhos relevantes na resistência máxima à tração, na rigidez e na deformação máxima no momento da ruptura, devido à incorporação de SiCp. Essas propriedades foram bastante influenciadas na presença de concentradores de tensão (entalhes). A resistência ao desgaste do compósito foi excepcionalmente incrementada comparativamente aos demais materiais. Todos os resultados foram corroborados pelas análises microetrutural e fratográficas. / The present study aims to characterize mechanical, microstructural and through fractographic inspection laminates Al-1100 aluminum matrix composite reinforced with silicon carbide particles, SiCp (40 μm), manufactured by accumulative roll bonding (ARB), as well as, for comparative effect, were studied Al-1100 processed by ARB without the addition of particles and Al-1100 received. Micro-adhesive wear test with fixed ball and test almost static unidirectional traction were performed on samples without scoring, and in samples containing different geometries notches. Optical microscopy, scanning electron microscopy modes: secondary electrons, backscattered electrons, energy dispersive X-ray and electron backscatter diffraction, X-ray diffraction and computed microtomography, these were used to characterize the samples. The results indicated successful incorporation of SiC particles in the aluminum matrix by ARB process. There have been significant gains in maximum tensile strength, stiffness and maximum deformation at the time of rupture, due to incorporation of SiCp. These properties were strongly influenced in the presence of stress concentrators (notches). The resistance of the composite wear was exceptionally increased compared to Al-1100 ARB. All results were corroborated by microstructural and fractographics analysis.