<p>Crystal-melt interface properties and their associated anisotropies playa crucial role during solidification in controlling the nucleation, crystallization rate and growth morphology. There are two solid-liquid interfacial (SLI) properties affecting the dendritic microstructures that form in the crystallization process and the SLI properties are interfacial free energy (γ) and kinetic coefficient (μ) . In this research work, atomic scale simulation techniques, such as Monte Carlo (MC) and Molecular Dynamics (MD), have been applied to compute the crystal-melt interface properties and their anisotropies of the AI-Mg system.</p> <p>An inter-atomic potential is utilized for describing the pair interactions of binary AI-Mg system during atomistic simulations. Actually the potential was developed particularly for the simulation of solid-liquid interface properties of AI-Mg alloys. Optimization of the potential is conducted by determining the equilibrium phase diagram employing Monte Carlo (MC) simulation techniques and comparing with the experimental results. A method is discussed for fitting the potential into phase diagram by varying the data for liquid solution energies. A good agreement of the AI-rich side of AI-Mg phase diagram, determined from this potential, is found with the experimental phase diagram. The inter-atomic potential is also optimized by comparing the liquid enthalpy of mixing of AI-Mg alloys with that of experiments.</p> <p>The crystal-melt interfacial energy (γ) and its anisotropies in AI-Mg binary alloys are computed utilizing a combination of MC and MD simulations in association with the analysis of capillary fluctuation method (CFM). The orientation averaged surface energy γ_o is observed to increase with increasing temperature which is consistent with other computational results of Lennard Jonnes (LJ) and Hard Sphere (HS) system. The anisotropy of y is found to follow the ordering of γ₁₀₀> γ₁₁₀> γ₁₁₁. Superimposition of the y anisotropy parameters on the orientation selection map, proposed by Haximali et al., predicts the primary dendrite growth in <100> direction for pure Al and the growth is examined to be stabilized in the same orientation with the addition of Mg atoms to Al i.e. in the concentrated alloys as well.</p> <p>Kinetic coefficient (μ) of pure Al is determined from the free solidification method utilizing Molecular Dynamics (MD) simulations employing multiple thermostats in the system which avoids the underestimation of μ due to slow dissipation of the generated latent heat at the solid-liquid interface. Kinetic coefficient is also extracted from equilibrium fluctuation analysis with a correction due to the contribution of thermally controlled interfacial kinetics. μ is computed for both (100) and (110) orientations of the crystal-melt interfaces and the values from both techniques are found to be equivalent. The magnitudes of interface mobility for pure Al is determined as μ₁₀₀ = 163 cm/s/K and μ₁₁₀= 129 cm/s/K.</p> / Master of Applied Science (MASc)

<p>Wrought AZ80 magnesium alloy from a spoke of forged automotive wheel was subjected to high cycle fatigue to study its fatigue properties and to understand the relationship between the material substructure and fatigue life. The results reveal that in axial tension-compression S-N testing the spoke material exhibits an endurance limit of 98MPa and a sharp bend in the S-N curve. Fracture surface observation by SEM revealed rapid crack growth after crack nucleation with micro voids and angled secondary cracks throughout. XRD analysis revealed a strong material texture beneficial to basal slip activation before and after fatiguing, with small amounts of refinement after cycling and no signs of twinning. TEM observations of samples cycled at high stress and stresses above and below the endurance limit revealed a large difference in the dislocation substructures developed, which may relate to the sharp bend in the S-N curve.</p> / Master of Applied Science (MASc)

<p>p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.5px Times; color: #202020} p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.5px Times; color: #383838} span.s1 {color: #383838} span.s2 {color: #202020} span.s3 {color: #525252}</p> <p>The production of aluminum sheet material can follow two distinct processing routes: the conventional semi-continuous process referred to as direct-chill (or DC) casting and, more recently, the continuous casting process (CC). The variation in processing routes can affect the alloy's microstructure, specifically in terms of particle size distribution and the concentration of alloying elements in solid solution, which can alter the materials mechanical properties. Therefore, the formability and fracture behaviour of AA5754 automotive sheet material in the O-temper, produced via two different processing routes, CC and DC casting, has been investigated by the use of the forming limit diagram (FLD).</p> <p>An in-plane forming test, developed by Marciniak (1973) was used to determine p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.5px Times; color: #202020} p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.5px Times; color: #383838} span.s1 {color: #383838} span.s2 {color: #525252} span.s3 {color: #6c6c6c} span.s4 {color: #202020}</p> <p>the intrinsic forming limits of the two materials, while full-field strain mapping based on</p> <p>digital image correlation analysis was used to follow the inhomogeneous plastic flow</p> <p>behaviour. As a result, PLC deformation bands were observed and their influence as</p> <p>geometric imperfections to initiate premature failure was shown to be dependent on the</p> <p>strain path. In addition, post-necking and fracture observations were used to understand</p> <p>the influence of microstructural variations and inhomogeneities on the total ductility of</p> <p>AA5754, in terms of the two processing routes.</p> / Master of Applied Science (MASc)

<p>Succinonitrile, commercially-pure Sn, Sn-0.3 wt% Cu, and Bi-Sn of several weight ratios were solidified under high-gravitational fields 287 times that of the earth's gravity simulated by centrifugal acceleration. The microstructure of the samples solidified in high-gravity was examined and compared with those solidified in normal gravity. Solidification was also done under varying cooling rates to determine its combined effect with high-gravity on microstructure development. The microstructure was quantified in terms of grain size, eutectic spacing, and primary phase distribution against the radial position of the samples. Vickers hardness of the samples was also measured by using both low load and a high load, in order to determine the solidified samples' relative strength.</p> <p>The microstructure of the Sn sample solidified in high-gravity possessed a higher percentage of small grains than that solidified in normal gravity. For Bi-Sn alloys solidified by slow-cooling, the eutectic phase formed in high-gravity had a complex but regular lamellar structure whereas that formed in normal gravity was irregular. In the hyper- and hypoeutectic Bi-Sn sample, the primary phase was segregated to the inner or outer radius of the samples formed in high-gravity, depending on the variation of density between the phases. The Sn-Cu alloy solidified in high-gravity had a cellular structure whereas that solidified in normal gravity had a dendritic structure.</p> <p>The effects of high-gravity on microstructure development are explained by the enhanced fluid flow and Rayleigh-Bénard convection during solidification of the melt. This convection is caused by thermally-induced density gradients within the melt and is confirmed by calculating the Rayleigh number. Other effects on microstructure are explained in terms of the Stokes equation and the Mullins-Sekerka criteria for solidification stability. Change in the solidification temperature as a result of increasing centrifugal acceleration was calculated from the Clausius-Clapeyron equation, and its magnitude is discussed.</p> / Master of Applied Science (MASc)

<p>Superplastic forming (SPF) represents one feasible method to improve the formability of wrought magnesium alloy sheets at high temperatures. A fine grain structure not only improves the ductility but also increases the optimum strain rate thus, reducing the cost of SPF. Microstructural stability of AZ31 sheets have been characterized following different heat treatments. Second phase particles help to suppress grain growth due to the pinning effect. Thus once the temperature exceeds 450°C pmticles dissolve leading to abnormal grain growth. Superplastic behavior based on the fine grain structure of AZ31 was therefore evaluated at temperatures ranging from 200°C to 400°C with constant strain rates of 2.7*10^-4 s^-1 to 8.8*10^-3 s^-1. The dominant defonnation mechanism is grain boundary sliding, accommodated by dislocation creep. Under the current test conditions the initial (12μm grain size) structure is unstable and leads to dynamic recrystallization (DRX). Competition between DRX and grain growth leads to a variation of mechanical response with test conditions. Further grain refinement of AZ31 sheets was investigated through warm rolling tests for both symmetric and asymmetric types at 200°C and 300°C, followed by 200°C annealing. Asymmetric rolling was more effective for grain refinement and achieved better mechanical properties than symmetric rolling.</p> / Master of Applied Science (MASc)

<p>p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Times}</p> <p>Due to the environmental burden of fossil fuel combustion, a major strategic shift towards renewable green sources and environmentally benign power generation technology makes biofuel cells a possible alternative, though their extremely low current density remains a bottleneck for the further practical applications. Recent progress shows some promise to increase the current by platinum (Pt) electrodes, and alkaline solution to replace the enzymes/microbes. However, the approach involves high cost of noble metals as well as their poisoning effect. We report here a glucose biofuel cell based on nickel (Ni) electrodes and alkaline medium without the catalyst poisoning found in Pt systems. Surprisingly, a six-fold current increase over time, and a final current density equivalent to 1.5 times that of Pt have been achieved. They are found to be caused by the transformation of glucose to an enediol form, the expansion of triple phase boundaries where cathode reactions take place, and the enhancement of reaction kinetics by alkaline solution. The results not only provide a dramatic increase in current and overall biofuel cell performance, but also demonstrate a low cost approach to renewable source utilization, if corresponding designs can be implemented.</p> / Master of Applied Science (MASc)

<p>Electrochemical supercapacitors (ES) have become an attractive technology in electrical energy storage devices and found many applications in a number of areas. The development of ES requires fabrication of advanced electrodes with novel materials and new techniques. Conducting polymer polypyrrole (PPy) has been found to be a promising electrode for ES due to its high pseudo capacitance and good electrical conductivity. The polypyrrole based composite electrode with multiwall carbon nanotubes (MWCNTs) has been proved to enhance the electrochemical performance of ES.</p> <p>Anodic electrochemical deposition of polypyrrole film and composite with MWCNTs has been successfully achieved on a stainless steel substrates in aqueous solutions. Novel additives, such as tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt) and sodium salicylate have been employed for the electro-synthesis of PPy. The role of additives in the electrodeposition process have been discussed. The electrochemical properties of PPy and PPylMWCNT composite have been investigated and compared by using different characterization techniques.</p> <p>The results showed that good quality PPy film and PPy/MWCNT composite can be obtained on stainless steel without anodic dissolution of anode using novel additives in aqueous solutions. The PPy films provided corrosion protection for stainless steel in aqueous solutions. High electrochemical performance of PPy film was achieved, the electrochemical behavior of PPy electrodes was significantly enhanced when MWCNTs were incorporated into the film matrix. Hence, investigations in this work indicated that PPy and PPy/MWCNT composite deposited on the stainless steel substrates are promising electrode materials for ES.</p> / Master of Applied Science (MASc)

<p>The electrochemical supercapacitors (ESs) are an emerging technology that promises to play an important role in meeting the demands of electronic devices and systems both now and in the future. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. Nanostructured manganese oxides in various forms have been found to be promising electrode materials for ES.</p> <p>Cathodic electrodeposition method has been developed for the fabrication of nanostructured manganese dioxide films. Manganese oxide films were obtained by galvanostatic, pulse and reverse pulse electrodeposition from KMnO₄ solutions. The diffusion-controlled deposition mechanism is based on the reduction of anionic MnO₄^- at the cathode surface. It was shown that film porosity is beneficial for the charge transfer during deposition, crack prevention in thick films and electrolyte diffusion in fabricated ES electrodes. The microstructure, chemical properties and charge storage properties of films prepared by different deposition methods are investigated and compared.</p> <p>Novel chemical precipitation methods have been developed to produce manganese dioxide and Ag-doped manganese dioxide nanoparticles. Composite electrodes for ES were fabricated by impregnation of slurries of the manganese dioxide nanoparticles and carbon black into porous nickel foam current collectors. The microstructure and chemical properties of the powders were characterized. The capacitive behaviour of the composite electrodes was studied.</p> / Master of Applied Science (MASc)

<p>New electrochemical deposition methods have been developed for the fabrication of advanced composite coatings for biosensors applications. The methods are based on electrodeposition of biopolymers, such as cathodic electrodeposition of chitosan, anodic electrodeposition of alginic acid and hyaluronic acid. Another approach is based on electrolytic deposition and electrophoretic deposition of ceramic materials and chitosan. Electrochemical strategies have been discovered for the electrochemical co-deposition of polymers with enzymes, such as glucose oxidase and hemoglobin. Glucose oxidase was used as a model enzyme for the development of new electrochemical strategies for the fabrication of composite coatings for applications in biosensors. New strategies have been further utilized for the fabrication of novel composites containing hemoglobin. It was found that co-deposition of biopolymers and enzymes from the solutions resulted in the fabrication of composite materials which can keep the activity of the enzymes.</p> <p>Electrochemical methods have been developed for the deposition of composite coatings containing ceramic materials (ZnO) in the matrix of chitosan. The composite coatings can be utilized for the immobilization of enzymes by the electrostatic attraction. The composition and microstructure of the composite coatings were investigated. The composition of these nanocomposite coatings can be varied by variation of bath composition for electrodeposition. The deposition yield was studied at various deposition conditions. Electrochemical deposition mechanisms have been investigated and discussed. Obtained results pave the way for the fabrication of novel coatings for immobilization of enzymes and for application in advanced biosensors.</p> / Master of Applied Science (MASc)

<p>The development of new materials with a combination of high strength and ductility is required for the automotive industry, due to the demand for increased fuel efficiency while maintaining vehicle safety and performance. High-Mn steels combine exceptional strength and ductility to achieve the sustained rates of high work hardening required to achieve these objectives. Fe-22Mn-C alloys containing strain-induced deformation products (twins and ε-martensite) contribute to the high work hardening rates by acting as boundaries for dislocation motion. Three Fe-22Mn-C alloys were investigated with varying carbon contents of 0.6, 0.4 and 0.2 wt% and stacking fault energies (SFEs) of 37.2, 33.4 and 29.6 mJ/m² , respectively. Their microstructural evolution and mechanical properties were evaluated.<br /> The as-annealed Fe-22Mn-0.6C alloy comprised an austenitic microstructure, produced twins during deformation and had the highest sustained work hardening rate of the three alloys. The kinematic hardening contribution was due to the production of twins during deformation, adding to the overall flow stress. The flow stress was successfully modeled with contributions from the yield strength, isotropic hardening and kinematic hardening. The main damage mechanism was the separation of grain boundaries and the production of twins during defonnation classified the 0.6C alloy as a TWIP steel.<br /> The Fe-22Mn-OAC alloy displayed an austenitic matrix ill the as-annealed microstructure with twins and ε-martensite produced during deformation. The work hardening rate was sustained from the continuous production of deformation products. The kinematic hardening contributed to the overall flow stress as a result of twins and ε-martensite acting as dislocation barriers. The mechanical behaviour of the alloy was modeled successfully by combining the yield strength, isotropic and kinematic hardening contributions to obtain the overall flow stress. Decohesion at γ – ε interfaces was observed to be the primary fracture mechanism. With the production of both twins and ε-martensite, the OAC alloy was labelled as a TWIP / TRlP steel.<br /> The Fe-22Mn-0.2C alloy had the lowest carbon content of the three alloys and contained an initial dual phase microstructure of austenite and ε-martensite plates. Straininduced ε-martensite was created during tensile deformation, with the kinematic hardening contribution resulting from the production of ε-martensite. An iso-work model was applied with contributions from the isotropic hardening of austenite and kinematic hardening of ε-martensite to the overall flow stress. Fracture was caused by separation along austenite - ε-martensite interfaces. The strain-induced ε-martensite created a TRlP effect within the O.2C alloy. <br /> Overall, the effect of carbon content on the microstructural evolution and mechanical properties within the Fe-22Mn-C system was determined. As the carbon content decreased, the SFE was lowered and a shift from the TWIP to TRlP effect was observed. The SFE phase map predictions were correct in predicting the as-annealed microstructure and deformation mechanism as determined by Allain et al. (Allain 2004b) and Nakano (Nakano 2010). The transformation kinetics and the role of carbon were not included in the SFE phase map predictions and were also factors to consider on the effect of carbon content on the Fe-22Mn-C alloys.</p> / Master of Applied Science (MASc)