<p> During heat treatment of polycrystalline materials, grain shape affects the rate of grain growth. In 2-D, the von Neumann-Mullins Law requires grains with fewer than six edges to shrink, greater than six edges to grow, and hexagonal grains to be stable regardless of edge lengths or curvatures. The 3-D analogue, described by the MacPherson-Srolovitz relation, does not explicitly depend on any topological feature (number of faces, edges, or vertices), yet there is bias in the observed grain topologies in 3-D metal polycrystals. In order to investigate this bias and determine its origins, numerical simulations of ideal polycrystalline materials, characterization of topologies, and comparisons to possible polyhedral shapes were performed.</p><p> Normal grain growth in polycrystalline materials is characterized by a self-similar distribution of topological properties: the average grain area increases with heat treatment time, but the average number of faces per grain remains constant. Therefore, distributions of the number of faces per grain are commonly reported characteristics of polycrystals. To investigate bias in grain topologies, the number of edges per face on each grain in the polycrystal was extracted, then the standard deviation of this quantity was computed for each grain. For grains resembling Platonic solids with equal numbers of edges on each face, such as the Platonic tetrahedron, hexahedron, and dodecahedron, this quantity is zero. In typical grains with more diverse faces, the standard deviation increases. The average, upper, and lower bound of standard deviations possible for all polyhedra with a given number of faces were determined by enumerating each using a graph theory-based code, plantri. Several polycrystalline datasets were then obtained and analyzed: two synthetic, two simulated grain growth, and one experimental reconstruction of titanium. The polycrystals all exhibited lower averages of the standard deviation of edges per face than the enumerated polyhedra, demonstrating bias. Specifically, the bias in grain growth favors more "regular" topologies, with a smaller spread in the number of edges per face than would occur at random.</p><p> One dataset, a synthetic microstructure with flat edges and faces, was biased more weakly than the rest. The remaining four datasets involved motion by mean curvature, the fundamental mechanism of grain growth, under which interfaces move toward their center of curvature with velocity proportional to that curvature: sharply curved faces move faster than more gently curved ones, and flat faces move not at all. To satisfy force balance at the vertices, three-edged faces in polycrystals become highly curved and quickly collapse during grain growth, but the laws of topology require that grains with between ten and sixteen faces have five edges per face, on average. This span covers the median and mean number of faces in polycrystalline grain populations. Therefore, as three-edged faces collapse, faces with more than 5 edges must also lose edges to maintain grain boundary network connectivity.</p><p> By changing the physics of grain growth to decrease edge and face curvature, the population of three-edged faces should increase, with the standard deviation in edges per face increasing proportionally. To test this hypothesis, a phase-field model of grain growth was implemented with lower mobility on triple junctions than on other features. This approach, known as a "vertex drag" model in 2-D, tends to straighten grain edges. From large-scale 2-D simulations, vertex mobility 100x lower than the edge mobility was found to increase the relative proportion of 3-edged grains by 25%. While the effect is small in magnitude, this result supports motion by mean curvature as the root cause of bias in polycrystalline grain topology.</p>

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Dynamic properties of soil, including modulus and damping, play essential roles in evaluating the response of the soil deposit and its supporting structures when subjected to dynamic loads induced by earthquakes, traffic, explosions, machine foundations, and so on. It is well recognized that the dynamic properties of soil are affected by many factors, such as strain amplitude, stress condition, void ratio, saturation and gradation. Despite tremendous works have been done, the macroscopic effects of several key factors on the dynamic properties of granular material are not yet fully understood, due primarily to its particulate and multiphase nature. Furthermore, the understanding of how the influencing factors affect the dynamic properties of granular material or the underlying fundamental mechanism is inadequate. This study thus is carried out to investigate the effects and the underlying mechanisms of these important factors, including strain amplitude, stress condition, void ratio, particle size, saturation, and initial fabric, by means of advanced laboratory tests and numerical simulations. To study the dynamic properties at the macro scale, a series of laboratory tests are carried out in a state-of-art resonant column (RC) apparatus incorporating bender element (BE) and torsional shear (TS). Test materials include artificial glass beads with different sizes, commercially available standard sands and natural completely decomposed granite (CDG). The specimens are prepared at various densities, confined at different pressures, tested both in dry and saturated conditions, and reconstituted by different preparation methods. In particular, the characteristics of wave signals (both S-wave and P-wave) at various conditions and the associated interpretation methods in BE tests are investigated in detail. The results obtained from BE, RC and TS are compared to clarify the potential effect of test method. Moreover, attempts are made to explain the test results from the viewpoint of micromechanics. Numerical simulations using discrete element method (DEM) are performed to study the dynamic properties of granular materials and explore the underlying fundamental mechanism at the micro scale. The simulations indicate that the elastic properties are closely related to the coordination number and the distribution of normal contact forces in the specimen. The effects of initial fabric and induced fabric, which are respectively achieved by different specimen generation methods and the application of anisotropic stress states, are investigated. The anisotropy of the specimen and its evolution during shearing are also studied. The results indicate that the anisotropy is resulted from the spatial distributions of contact force and contact number. The modulus reduction curve and damping curve obtained from the simulations are compared with those from laboratory tests. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Multiferroics which combine two or more order parameters of ferroelectricity, ferromagnetism and ferroelasticity, have drawn great interests in the past few years due to their promising potential of application in sensors, transducers, spintronics and multistate memories. Coupling between the ferroelectricity and ferromagnetism renders the induction of an electric polarization P upon applying a magnetic field, or the induction of a magnetization M upon applying an electric field which is called magnetoelectric coupling effect. There are single phase multiferroics which simultaneously possess ferroelectricity and magnetism in nature. However, these natural multiferroics only exhibit weak magnetoelectric coupling effect at very low temperature which hinders the practical applications. An alternative and more promising choice is to fabricate multiferroic composites. In the multiferroic composite systems, large magnetoelectric coupling effects can be produced indirectly from the strain-mediated interaction even at room temperature and great design flexibility can be obtained. In the present study, two types of multiferroic composite nanostructures are investigated: the vertical heteroepitaxial multiferroic thin films and film-on-substrate heterogeneous bilayers with incorporation of various influences, such as film thickness, misfit strains and flexoelectricity. Since the first fabrication of vertical epitaxial multiferroic nanostructures, great scientific interests have been attracted for the potential large magnetoelectric effects arising from the relaxed substrate constraint and large interfacial area between the ferroelectric and ferromagnetic phases. A three dimensional phase field model is devised to precisely describe the complex strain state of this nanostructure. The simulation results demonstrate that both film thickness and misfit strains are important in determining the magnitude of magnetoelectric effect. Due to the strong strain-mediated magnetoelectric coupling effect in film-on-substrate system with a ferromagnetic thin film directly growing on a thick ferroelectric substrate, precision electric control of local ferromagnetism, i.e. ferromagnetic domain pattern and domain wall properties, are achievable. The results show that the domain pattern of the ferroelectric substrate can be fully transferred onto the as-deposited ferromagnetic thin film. High stability of the magnetic domain is observed when the system is subjected to an external magnetic field. Under an applied electric field, the transferred domain pattern in magnetic film can be either maintained or erased depending on the direction of applied electric field. Moreover, when a pulse of in-plane electric field is applied, the magnetic domain wall motion can be observed in concurrence with the ferroelectric domain wall motion. With the decrease of material size, some effects that can be neglected in bulk materials may play an important role on the overall properties of material, such as flexoelectric effects which describe the induction of polarization from strain gradient. A two dimensional phase field model is adopted to study the influence of flexoelectric effects on the epitaxial ferroelectric films. A thermodynamic phenomenological model is then utilized to analyze the influence of flexoelectric effects on magnetic field induced electric polarization in the multiferroic nanocomposite bilayers. By decreasing the film thickness, the induced polarization from flexoelectric effects becomes more and more dominant and finally overcomes the electrostrictive induced polarization which is dominant when film thickness is large. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Porous structures are a particular type of solids, where a large number of pores exist in the geometric domain of interest. Research on porous structures have received increasingly keen interest in recent years and this is largely because of many unique and superior properties that porous structures possess. They can undertake special tasks which general solid materials are not competent to do. In recent twenty years numerous representations are put forward for porous structure modeling. But the challenges in practical porous structure design still exist and the structure heterogeneity brings many difficulties. This thesis is motivated to propose new porous structure modeling strategies which are more accurate, flexible and easy for porous structure description. An approach of porous structure modeling based on quadtree/octree and NURBS is proposed first. Quadtree and octree are tools for modeling domain partition. The pore size and pore distribution are controlled by the flexibility of quadtree and octree enumeration technique. Derived polygon and polyhedron are then introduced to assist the generation of NURBS curves and surfaces. These NURBS curves and surfaces form the boundaries of the porous structures. However there are limitations of the above method. The accurate control of porosity is not easily achieved in 3D porous structure modeling and seemingly adopting quadtree/octree for the modeling domain partition is also less than satisfactory. Hence a new representation for porous structures based on Centroidal Voronoi tessellation (CVT) and pore-network is put forward. CVT is utilized for modeling domain partition because the CVT cells are approximate hexagons which is widely existent in plants, animals and other cellular structures in nature. The density distribution function used in CVT generation also helps to build functionally graded porous structures. Pore-network, which is a mature and commonly used model in the research of multiphase flow in porous media, is subsequently introduced to build the porous structures. This modeling approach results in porous structures that could mimic the geometry and performance of structures in nature. To evaluate the object’s properties, finite element analysis (FEA) is conducted on the porous structure models represented by the two methods. The mechanics properties of the two types of models are analyzed. The stress-strain curve of each sample is plotted and the effective Young’s modulus is calculated. Comparison of these two types of models is also done. Besides, the contributions of the thesis and suggestions for future research are also discussed. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

<p> Grain boundary (GB) segregation and embrittlement of copper (Cu) by small amounts of bismuth (Bi) has been investigated on 6&deg;, 13&deg;, and 33&deg; Cu twist bicrystals. The results from micro-mechanical double edge notched testing showed no embrittlement effects in the 6&deg; GB. The 33&deg; GB has been shown to be significantly embrittled by the introduction of Bi. Single edge notch testing of the 13&deg; GB also showed a reduction in fracture toughness. These mechanical results have been interpreted through the use of analytical electron microscopy (AEM) studying the GB geometry, the atomic structure, the electronic structure, and the chemical compositions of the GBs. The 6&deg; and 33&deg; GBs were found to be close to pure twist boundaries but with more accurate twist angles of 4.3&deg; and 38.0&deg;, respectively. The electronic structure of the GBs was not found to be a good indication of the presence of Bi, which was confirmed on the 13&deg; and 33&deg; GBs. The Bi GB coverage was confirmed via quantitative XEDS on the 33&deg; GB to correspond to 0.12 &plusmn; 0.03 monolayers of Bi and through 3-dimensional scanning transmission electron microscope (STEM) through focus imaging to be 0.02 &ndash; 0.09 monolayers of Bi. The presence of edge dislocations along the 33&deg; GB was confirmed with Bi segregating to edge dislocation cores. The Bi atoms on the dislocation cores embrittle the GB by increasing the energy required to move a dislocation in response to an applied stress resulting in reduced plasticity at the crack tip which promotes GB cleavage.</p>

Note: / Stream current, electro-osmotic flow, conductance, and permeability measurements were carried out on a cylindrical pads composed of different fibrous materials over a range of solid concentrations. The Z-potentials calculated from stream current and electro-osmotic measurements on the same pad were shown to be identical. Some deficiencies of the bubble flowmeter used to meaure the electro-osmotic flow were investigated and explained theoretically; a new type of flowmeter, free from these defects, was proposed.

No description available.

No description available.

During operation of a CANDU reactor, corrosion occurs which results in free hydrogen that can diffuse into the tubes. Once the solid solubility of hydrogen in the zirconium matrix is exceeded, the hydrogen will precipitate as a flake-like brittle hydride phase. The natural orientation of the hydride flakes is in the circumferential direction but under a tensile hoop stress the hydrides are able to reorientate themselves to the radial direction. This makes the pressure tubes susceptible to delayed hydride cracking (DHC) which can cause failure of the tubes. A memory effect has been observed to cause hydrides that would otherwise form in the circumferential direction to form in the radial direction. Studies have been previously performed to examine the memory effect however they have not quantified the hydride orientation distribution. This study examined the memory effect in Zr-2.5Nb micro pressure tubes with three different microstructures and textures. A stepped micro pressure tube sample design was hydrided to 100 ppm(wt%) and pressurized to obtain a nominal hoop stress ranging from 65 MPa to 350 MPa. Each of the samples was heated to 350oC to dissolve all of the hydrogen followed by cooling under stress. Samples were then reheated to 350oC for 1 hour and 24 hours and cooled without stress. Almost complete reorientation was observed in typical pressure tube material which had a very fine microstructure and a large portion of basal plane normals in the transverse direction. After reheating, little memory effect was found in material similar to the commercially used pressure tube material. However a clear memory effect was observed in the other two samples. The memory effect was observed in the range of angles where hydrides are naturally present. A second flanged sample design was used to find the dilational strain associated with hydride reorientation from which the normal strain could be calculated. The strain normal to the hydride, εnormal, was calculated to be 0.11 ± 0.05. This study provides a valuable resource that can be used to improve DHC models which are used to determine the useful life of the pressure tubes. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-10-01 01:26:49.608