<p>This research concerns a study of the deformation behavior and dislocation substructure characteristics resulting from plastic flow in single phase Al-Mg (0-4.11 at%Mg) and Al-Cr (0-0.36at%Cr) alloys. Tensile tests are performed at 298K, 78K, and 4.2K, and strain rate sensitivity tests are performed at 78K for the Ai-Mg alloys, and at 298K and 78K for the Al-Cr alloys. Resistivity measurements are carried out during tensile tests for all alloys deformed at 4.2K. The resulting structures are then studied using SEM and TEM microscopy. Solute strengthening is seen to occur in both systems, along with significant increases in strength and work hardening capacity in all alloys accompanying decreases in temperature. The limit for benefits from solute strengthening appears to lie close to the solubility limit for the Al-Mg system, but no clear limit is observed in the Al-Cr system. Resistivity data seems to indicate that a critical dislocation density is reached before fracture in all Al-Mg alloys studied, but that this critical density decreases with Cr content. Portevin Le-Chatelier (PLC) type instabilities are observed at room temperature in the Al-Mg alloys only, though both systems exhibit adiabatic shearing processes at 4.2K. A dislocation substructure resembling those observed in other Al-Mg alloys is observed, but the Al-Cr alloy dislocation substructure more closely resembles that observed in pure Al. Both substructures are seen to show greater dislocation density, distributed more homogenously over the structure as temperature decreases.</p> / Master of Applied Science (MASc)

<p>p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Times; color: #242424} p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Times; color: #3c3c3c} span.s1 {color: #3c3c3c} span.s2 {color: #242424}</p> <p>This thesis report the results of a simulation study of the melting kinetics of multiple, randomly distributed, steel scrap pieces. The model used is that previously developed by Li [Li, 2006]. The aim of this study was to better understand the universal features governing the kinetics of multi-piece scrap melting in a liquid pool. We observed the formation of a solidified shell and interfacial gap both in a single scrap piece as well as in randomly distributed multiple scrap melting cases. It is shown that the multiple scrap pieces agglomerate throughout the sample due to solidified shell formation.</p> <p>The key factors affecting melting kinetics of a heat examined were: heat transfer coefficient, initial solid fraction, initial liquid (preheating) and solid temperatures, scrap size and thermal conductivity.</p> <p>A scaling analysis of simulation data of melting kinetics was conducted, identifying suitable characteristic length and time scales through which the melting kinetics across different parameters and processing conditions could be scaled and thus understood in the context of a unified mathematical description.</p> / Master of Applied Science (MASc)

<p>Despite all efforts during the last 30-40 years, the formation of metastable Fe-rich and Si-rich intermetallics in dilute aluminum alloys is still an unsolved mystery. Based on the equilibrium Al-Fe-Si phase diagram, in dilute aluminum alloys only one equilibrium intermetallic, namely Al₁₃Fe₄, is expected. It is known, however, that a rapid solidification, i.e. solidification at a high cooling rate, results in dozens of metastable phases seen in the as-cast alloys. It is firmly established that the greater the cooling rate (i.e. the rate of heat extraction), the greater the supercooling (supersaturation) achieved in the course of solidification.</p> <p>Understanding the nature and a sequence of formation of these intermetallic phases precipitating from the supersaturated melt is at the centre of this research. In fact, this endeavor was launched to answer the following fundamental question: "What governs the formation of intermetallic c phases from a rapidly solidifying alloys, in general, and from aluminum alloys, in particular?" Prior to starting this investigation, it was believed that the concept of the driving forces for the beginning of precipitation originated by Miroshnichenko, Cahn and Hillert, could be used to explain experimental findings. Was that belief justified? Although a definite answer to this question has not been found, there are strong indications that the concept is likely operative, although it has to be refined by taking into account the surface energies.</p> <p>To evaluate applicability of this concept to the formation of Fe-rich and Si-rich intermetallics in aluminum alloys, in this research an array of experimental information related to microstructures of as-cast alloys having different compositions are obtained. Then, the collected experimental results are interpreted using the concept of the driving forces for the beginning of precipitation.</p> / Master of Applied Science (MASc)

<p>TRIP-assisted steels combine high strength and good ductility, which makes them attractive to the automotive industry. Simultaneously, galvanizing is essential for the corrosion resistance of these steels. In industrial practice, the processing of TRIP-assisted steels and galvanizing process must be combined.</p> <p>Two Al-alloyed TRIP-assisted steels (<strong>1.0Al: </strong>O.2C-1.SMn-O.SSi-1.0Al (wt.%) and <strong>1.5Al: </strong>O.2C-1.SMn-1.SAl (wt.%)) were investigated in the present research work, where two points are noteworthy. First, the experimental processing routes in the present work are compatible with the continuous galvanizing process; second, it has been shown that the two steels exhibit good galvanizability. The initial microstructure and its evolution during plastic deformation of the two steels were examined. The kinetics of phase transformations taking place during thermal processing and plastic deformation were discussed. These results were linked to the work hardening behaviour with kinematic hardening taken into account.</p> <p>It was confirmed that the retained austenite in the steels obtained through the present galvanizing heat treatments contributed to the work hardening behaviour by transforming to martensite during plastic deformation. The 1.SAl steel exhibited a better work hardening behaviour due to the more stable retained austenite.</p> <p>Retained austenite of higher stability transformed to martensite more gradually during plastic deformation, efficiently attenuating decreases in the instantaneous work hardening rate, dσ/dε, and giving rise to a smoother evolution of the incremental work hardening coefficient, d(Lnσ)/d(Lnε). One of the attenuating mechanisms was the development of back stresses, which contributed kinematic hardening to the overall work hardening. Gradual transformation of retained austenite to martensite continuously supplied new obstacles to dislocations and delayed the saturation of back stresses.</p> <p>Based on the present work and the previous work, it seems possible to process galvanizable Al-alloyed TRIP assisted steels using continuous galvanizing thermal cycles, which implies the possibility to combine continuous galvanizing and thermal processing of TRIP-assisted steels.</p> / Master of Applied Science (MASc)

<p>New electrochemical deposition methods have been developed for the fabrication of advanced composite coatings for biomedical 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 anodic electropolymerization of polypyrrole. Electrochemical strategies have been discovered for the electrochemical co-deposition of polymers with other functional biomaterials, such as proteins, drugs, bioactive ceramics and bioglass. Bovine serum albumin was used as a model protein for the development of new electrochemical strategies for the fabrication of composite coatings containing proteins. New strategies have been further utilized for the fabrication of novel composites containing hemoglobin. It was found that biopolymers can be used for efficient electrosteric dispersion of bioceramics and bioglass in suspensions. Co-deposition of biopolymers with bioceramics and bioglass from the suspensions resulted in the fabrication of composite organic-inorganic bone substitute materials.<br /> Electrochemical methods have been developed for the deposition of composite coatings containing functional biomaterials in the matrix of conductive polypyrrole. New additives have been developed for the deposition of polypyrrole on low cost stainless steel substrates. The additives enabled the passivation of the stainless steel substrates and charge transfer during anodic electropolymerization. The composite coatings were obtained as monolayers, multilayers or materials of graded composition. <br /> The composition and microstructure of the composite coatings were investigated. The composition of these nanocomposite coatings can be varied by variation in 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 the surface modification of biomedical implants and for application in advanced biosensors.</p> / Master of Applied Science (MASc)

<p>Nanostructured manganese oxides in amorphous or various crystalline forms have been found to be promising electrode materials for electrochemical supercapacitors (ES). Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 3-10 nm were prepared by a chemical precipitation method. Electrophoretic deposition (EPD) method and mpregnation techniques have been developed to fabricate thin film and composite electrodes for ES. As-prepared nanofibers and electrodes were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The capacitive behavior of electrodes was investigated by cyclic voltammetry (CV) and chronopotentiometry method using a three-electrode cell in the mild Na₂SO₄.</p> <p>The composite electrodes fabricated by impregnation of manganese dioxide nanofibers and multi-walled carbon nanotubes (MWCNTs) into porous nickel foam and nickel plaque current collectors showed excellent capacitive performance with large material loading of 7-40 mg cm^-2 in 0.1-0.5 M Na₂SO₄. MnO₂ nanofibers and MWCNTs can form a porous fibrous network, which is beneficial for the electrolyte access to the active materials. In addition, MWCNTs formed a secondary conductivity network within the porous nickel structures. The highest specific capacitance (SC) of 185 F g^-1 was obtained at a scan rate of 2 mV S^-1 in the 0.5 M Na₂SO₄ solutions. The effect of the electrolyte concentration, scan rate and active material composition on the capacitive behavior was discussed.</p> <p>Obtained thin film and composite electrodes by EPD showed a capacitive behavior in the 0.1 M Na₂SO₄ aqueous solutions with a potential range of 0-1.0 V. The highest SC of 412 F g^-1 was obtained for the thin film electrodes at a scan rate 2 mV S^-1 in the 0.1 M Na₂SO₄. The SC decreased with increasing deposit mass and scan rate. It was found that the addition of MWCNTs can improve the capacitive performance of manganese dioxide electrodes with smaller equivalent series resistance (ESR). The mechanisms and kinetics of all the deposition methods were discussed.</p> / Master of Applied Science (MASc)

<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)