<p>A study has been conducted in mildly aggressive saline solutions to indentify subtle yet important difference in the anodic and cathodic process of three Mg-Al alloys, AZ31B, AM30 and AM60B in a partially passive state and a localized corrosion state. The influence of metallurgical factors and environment variables on the corrosion resistance and surface film breakdown process has been investigated using potentiodynamic and potentiostatic tests combined with optical microscopy.</p> <p>All three Mg-Al alloys corroded in a partially protected state under open circuit conditions in the test solution and the surface film formed on each exhibited a similar breakdown potential. This indicates that metallurgical factors such as alloying additions and the presence of the β-phase (Mg₁₇Al₁₂) did not significantly influence the surface film breakdown process. AM60B exhibited improved corrosion resistance at potentials below the breakdown potential due to the formation of a more protective surface film. The β- phase, however, did not strongly influence either the anodic process at potentials above the breakdown potential or the cathodic process. It was determined that increasing the alloy Al content increases the corrosion potential of Mg-Al alloys, but also increases the risk of localized corrosion. The similar anodic and cathodic polarization behaviour exhibited by AZ31B and AM30 indicates that a 1 wt % Zn alloying addition does not strongly influence the corrosion resistance of these alloys. The die-cast skin of AM60B exhibited better corrosion resistance than the interior at potentials below the breakdown potential due to the higher fraction of Al-rich β-phase, which improved the protective ability of the surface film. The semi-continuous β-phase network did not provide a strong micro-galvanic activity to drive anodic dissolution of the α-phase, therefore, did not significantly affect the corrosion resistance above the breakdown potential. The crystallographic texture exhibited by the extruded AM30 did not strongly affect the short-term and long-term corrosion resistance. AM30 showed a relatively steady passive state during long time exposures in a room temperature 0.01 M NaCl solution.</p> / Master of Science (MSc)

Metallic glasses (MGs) usually have high strength, high hardness and high elastic strain limit. However, the deformation mode and mechanism in metallic glasses are radically different from those in conventional crystalline materials with a long-range ordered structure. For crystalline materials, the intrinsic relationship between their mechanical properties and crystal structures has been well described by dislocation theory. In contrast, for amorphous materials, theories on the structures and controlling factors of localized shear-band formation are far from being complete. In this thesis, shear-banding behavior of MGs under nanoindentation was first reviewed. The hardness of MGs was found to be independent on the shape of indenter tip. The hardness drop during each pop-in was a constant for a given indenter tip. A nanoindentation-based method for measuring the shear resistance of MGs was further developed. The hardness of MGs was largely affected by residual stresses, especially the tensile residual stress. Significant softening could be caused by tensile residual stress and the softening was attributed to the creation of extra free volume. The hardness of MGs was demonstrated to be extremely sensitive to the initial free volume in the material. Spherical indentation was also conducted on stressed MG sample to study the effect of residual stress on the first shear-band formation. It was found the critical shear stress for the shear-band formation was essentially a constant. The constant critical shear stress was correlated with a critical free volume in the material. Spherical indentation was further carried out at elevated temperature but well below glass transition temperature to explore the temperature effect on shear-band nucleation. Localized shear-banding was observed to be the dominant deformation mode at all temperatures. The shear stress at first pop-in or the onset of yielding decreased with temperature, and the activation energy and the size of shear transformation zone (STZ) were measured. Shear-band nucleus was estimated to be 10~20 nm and independent on temperature. Micro-compression tests were further performed on micro-sized pillar samples at different temperatures. The strength-temperature relationship could be explained by the constant viscosity concept, suggesting shear-banding was a stress-induced glass transformation.

This Thesis examines the effect of soil characteristics, cement content and fibers on the strength and durability of compressed earth blocks (CEBs). This work expands on the available information regarding the constituent properties which affect the compressive and tensile strengths and durability of CEBs. Additionally, little research on the subject of synthetic fibers and their effect on strength and durability of CEBs is available and this work provides an initial study in this area. To study the effects of fibers, as well as confirm the trends of previous research regarding the effects of clay and cement, 27 unique batches of CEBs were pressed and tested using a Vermeer BP 714 block press. Three different soil types and two fiber types were utilized. The compressive strength, modulus of rupture (MOR), absorption, and durability were measured on over 185 specimens. The strength and absorption tests were adapted from common ASTM International test methods for similar materials, while the durability test was a uniquely developed method to quantify durability by measuring mass loss during drying and wetting cycles. After the testing regimen was completed, a trend between clay content and strength could not be determined. Durability testing suggested that as clay content decreases, durability increases. A linear relationship was found between cement content and strength, which was confirmed during durability testing. The addition of different fibers did not have an effect on the peak strength of CEBs, although it appears they may decrease the durability. The gross versus net unit strengths of CEBs were examined due to the unique shape of the CEBs utilized. Additionally, a mathematical expression relating the MOR to the compressive strength was developed.

This thesis focuses on the development, parameterization and optimization of a novel meso-scale pattern used to induce auxetic behavior, i.e., negative Poisson's ratio, at the bulk scale. Currently, the majority of auxetic structures are too porous to be utilized in conventional load-bearing applications. For others, manufacturing methods have yet to realize the meso-scale pattern. Consequently, new auxetic structures must be developed in order to confer superior thermo-mechanical responses to structures at high temperature. Additionally, patterns that take into account manufacturing limitations, while maintaining the properties characteristically attached to negative Poisson's Ratio materials, are ideal in order to utilize the potential of auxetic structures. A novel auxetic pattern is developed, numerically analyzed, and optimized via design of experiments. The parameters of the meso-structure are varied, and the bulk response is studied using finite element analysis (FEA). Various attributes of the elasto-plastic responses of the bulk structure are used as objectives to guide the optimization process

<p>This thesis presents an investigation into the structure, composition and performance of naturally formed surface films on AZ alloys as a function of pH and alloyed Al concentration. STEM verified the film structure was bi-layer, consisting of an inner barrier layer, which was visibly deteriorated, and an outer porous layer. EDS SmartMaps™ coupled with the Inca™ software package determined the inner barrier layer was predominantly composed of MgO, whereas the outer layer was primarily Mg(OH)₂. However, both layers appeared to posses mixed oxide/hydroxide components according to ToF-SIMS analysis.</p> <p>Environmental pH had the largest effect on the structure and composition of the surface film. The near-neutral sample showed significant breakdown within the inner layer, which was attributed to natural hydration of MgO to Mg(OH)₂. This favourable hydration reaction is slower in alkaline environments and as such, the stability of the inner barrier layers of the pH 14 samples were noticeably improved. The effect of alloyed Al concentration was less significant however; increased enrichment of Al into the surface film structure appeared to cause a reduction in the thickness of the corrosion film itself.</p> <p>Drastic differences in corrosion performance were observed between the near-neutral and alkaline environments. Significantly better corrosion resistance to anodic dissolution was present in the alkaline environment coupled with a noticeably lower corrosion rate. The absence of breakdown potentials along with the presence of mass transport controlled anodic kinetics signified that the improved stability of the inner barrier layer was responsible for improved corrosion performance. In contrast, severe pitting and a narrow range of anodic stability were present for the near-neutral samples where the inner barrier layer was significantly compromised. This deterioration was deemed responsible for accelerated cathodic kinetics as well as minimal impedance to aggressive Cl^- ions from initiating wide scale electrochemical breakdown of the surface film.</p> / Master of Applied Science (MASc)

<p>Sheet specimens of alloy AZ31 were cross-rolled to equivalent strains of 0.05, 0.10, 0.30, 0.40, 0.56, and 0.77. The microstructure evolution was examined using a combination of optical metallography (OM), Electron Backscattered Diffraction (EBSD), Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). The results revealed significant activity of basal and non-basal slip as well as twinning. The twins were mainly of the contraction and double-twin (contraction-extension) types. In addition to the micron scale (1-5μm) twins observed on EBSD patterns, nano-scale twins were observed. The nano twins had a width of less than 0.20μm and existed either as individual/isolated twins or as twin-bundles that are several microns thick. The number of nano twin-bundles increased with increasing strain. Shear bands were also observed to form at high strains and eventually led to the failure of the sheet. As for the texture evolution, analysis of the pole figures shows an evident strengthening of the basal texture during the cross-rolling.</p> <p>Specimens of Mg alloy AZ31 cold-rolled to equivalent strains of 0.10 and 0.30 were selected and annealed at 250^oC. The progress of recrystallization was followed using OM, EBSD and TEM with special emphasis on the nucleation of recrystallization. The distribution of recrystallization nuclei was very heterogeneous due to the heterogeneity of the as-deformed microstructure. Twin/grain-boundary and twin/twin intersections as well as twin interiors were the dominant recrystallization nucleation sites. Significant recovery was observed in the non-recrystallized regions and this limited the growth of the recrystallized grains.</p> / Master of Applied Science (MASc)

<p>Bainite is a low temperature transformation product of austenite decomposition in steels. Its unique range of microstructures offers promising combinations of strength with ductility. At low transformation temperatures the crystallography of a phase transformation often plays an important role in the overall microstructure and how it develops. Therefore in this study the structures of ferrite/cementite and ferrite/austenite interfaces in bainite were investigated from a crystallographic viewpoint. After describing these interfaces, the idea of interphase boundary nucleation of cementite on a ferrite/austenite interface was investigated.</p> <p>An O-line model (a special case of the O-Lattice) was used to explain the observed experimental results on orientation relationship, habit plane and good matching direction between ferrite and cementite. The calculated orientation relationship was used in an NCS (near coincident site) model to describe several possible edge facets of cementite precipitates. The major observed edge facet in cementite is deviated from the more favored interfaces based on the NCS model. This deviation could imply that the edge facets are non-equilibrium interfaces whose orientations and morphologies are kinetically determined.</p> <p>Focused Ion Beam sectioning, conventional transmission electron and optical microscopy were used to shed more light on the three dimensional nature of a complex cementite-free bainitic microstructure. The faceted interfaces of bainitic ferrite were characterized and it was shown that the habit plane contains edge misfit dislocations. The orientation of the bainitic ferrite lath did not match an O-line model. Transformation time was considered to play an important role on the orientation and morphology of the bainitic laths and interfacial dislocation character.</p> <p>Finally, with the aid of known crystallographic relations and interfaces between the ferrite/cementite, ferrite/austenite and austenite/cementite phases, a model for cementite nucleation was proposed. This interphase boundary nucleus is assumed to form on a coherent ferrite/austenite interface and to possess ferrite/cementite and austenite/cementite calculated habit planes as two main facets surrounding the nucleus. It was shown that cementite nucleation would be viable if interfacial energies of all surrounding facets of a nucleus are in a semi-coherent energy range.</p> / Doctor of Philosophy (PhD)

<p>A framework is described using the phase-field crystal model for the study of premelting in binary alloys through short-range interfacial interactions that arise from the structure of grain boundaries. A nonconserved model A formulation of PFC was used to model grain boundaries in two dimensions for several different angles of misorientation: 27.8, 21.8, 17.8, 13.2, and 5 degrees. The character of the premelting transition, whereby a liquid-like film develops at a defect at temperatures below the melting point, changed with misorientation angle. An excess mass over the grain boundary can be defined as an analog to the liquid layer thickness due to premelting. It is found that low-angle grain boundaries remain at a relatively constant value of excess mass, and indeed can remain solid above the melting point. High-angle grain boundaries have a logarithmically increasing width that diverges at the melting point. A width-dependent energy can be defined called the disjoining potential that takes into account structure, interfacial and bulk energies to describe the liquid-layer width. The form of this disjoinging potential was found to be exponential and monotonically decreased as width increased for high angles and produced an attractive minimum for low angles. The results of this work were compared to a pure material from a previous study.</p> / Master of Applied Science (MASc)

<p>Tension and compression experiments have been carried out on a series of Mg-Y alloys with Y content up to 1.3 at.%, in a range of temperatures between 4.2K and 298K, to study the effect of Yttrium on mechanical properties and strain hardening. The alloys show strong difference in the hardening behavior under tension and compression attributed to the effect of texture. The yield strength scales with concentration of the solute as cⁿ, where c is the concentration of the solute in atomic percent and n~2/3. The results suggest that in addition to the atomic size and modulus misfit effects, the valence may be responsible for the enhanced strengthening of Y in Mg. Strain rate sensitivity measurements carried out under tension and compression reveal that Mg-Y alloys show decreasing SRS with increasing Y content at 298K and exhibit a negative SRS in highly concentrated alloys. At low temperatures the alloys show positive SRS increased with Y content. Texture measurements suggest that increasing Y content in alloys decreases the amount of basal component and enhances non-basal orientations. The reduced yield asymmetry between tension and compression observed in higher Y content alloys is being attributed to the weakening of the basal texture.</p> / Master of Applied Science (MASc)

Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material. In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges. To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor. To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.