<p>This thesis consists of two parts. The first describes work on swelling - the second, on creep damage accumulation.</p> <p>The first part develops a model for pressure-driven diffusional dedensification (or swelling) in a polycrystal containing isolated pores. The model is sufficiently general to be directly applicable also to pressure sintering. It is tested against experimental data for hot-pressed alumina (HPA) which swells during high-temperature, pressureless anneals in air.</p> <p>The model includes two independent, coupled, pore growth rate components: (1) diffusional (de)densification, driven by gas pressures in closed pores, and (2) coalescence of pores attached to grain boundaries, driven by grain growth against pore drag. Internal pore pressures may be due to inert trapped gas or to gas generated by chemical reactions.</p> <p>In a poly crystal with inert trapped gas, the trapped gas drives early swelling, but is self-dissipating. Pore coalescence, by progressively releasing capillarity constraint, becomes the dominant cause of swelling at longer times.</p> <p>The model predicts what magnitudes of chemically-generated pore pressure will increase swelling rates beyond those arising from inert trapped gas alone. And, for research on creep damage accumulation, it predicts what precreep anneals are required to dissipate an initial trapped-gas pore pressure, and the range of distinct, predamaged porous microstructures available.</p> <p>The second part of the thesis presents experimental data from flexural and uniaxial tensile creep tests. The data demonstrated that HPA's cavitation damage tolerance and failure strains are high enough that creep fracture tests should be done in uniaxial tension rather than in bending. A high temperature tensile test system, with optical extensometry, was designed, built and tested. The uniaxial tensile data suggest design improvements for testing ductile ceramics.</p> <p>The combined mechanical and microstructural data indicate that, at 1250 C, failures at high stress are controlled by slow crack growth from microstructural heterogeneities. An abrupt transistion at about 200 MPa seperates rapid failures from failures at strains on the order of 0.1. At about 50 MPa, a more gradual transition occurs, to fracture at higher strains, controlled by strain-driven damage. The internal microstructural data show internal creep damage at levels at which cavity coalescence generates macrocracks in HPA.</p> / Doctor of Philosophy (PhD)

<p>An industrial B.O.F., located at the #1 Melt Shop of Dominion Foundries and Steel Ltd., was used to develop and evaluate a method of measuring the rate of fume emissions during steelmaking.</p> <p>The fume rate was observed to decrease with increasing time into the blow. Approximately 60% of the total fume iron losses are emitted during the first one-third of the blowing time. The fume rate is influenced by such process variables as: metal carbon content, slag volume, metal temperature and lance practice.</p> <p>The mechanism of fume formation was assessed using the size, shape and chemical analyses of the fume material. The major mechanism is that of the explosive oxidation of metal droplets in the oxygen impact zone. Evidence of vaporization was also observed, but this represented less than 10% (by weight) of the total fume iron losses during the blowing period.</p> / Master of Engineering (ME)

<p>The mechanical deformation behaviour of the age-hardenable aluminum alloy 2036 was used to study the effects of differences in the microstructure due to (a) variations in commercial processing for one ageing condition (T4), and (b) variations in the ageing condition for one processing route. The effects of loading along different proportional straining paths have been considered with the aim of establishing correlations between the mechanical properties and formability, and for the case of (a) above, some rationalisation of selected press performance in terms of particular microstructural constituents has also been attempted. Changes in the fracture behaviour due to both (a) and (b) above are discussed. These changes emphasise the need to consider the role of fracture events in forming operations and their dependence on microstructure.</p>

<p>The role of damage on the mechanical response of a heterogeneous material was investigated through both mechanical testing and x-ray tomography. X-ray tomography was used to obtain quantitative information on the evolution of damage through processes occurring at both the local and global scale. The results indicate that heterogeneity in the spatial distribution of particles does influence the damage process. However, the influence of having one particle or more interacting with another is limited to reducing the tensile deformation required to initiate damage. The rate at which damage evolves is similar for both isolated and non-isolated particles and increasing the number of neighbors around a non-isolated particle was determined to have no additional influence on the evolution of damage. These results, coupled with mechanical testing measurements of both global and local properties, were used to develop models describing the flow response of composite materials as damage accumulates. Models were developed to predict the effects of both particle multiple cracking and micro-crack linkage on the composite flow response. The models predict that both damage processes reduce the load bearing capability of the material over that of an undamaged composite, however the loss in load bearing capability is much more severe when micro-crack linkage occurs. Micro-crack linkage rapidly leads to a loss in global stability so that the strain at which the composite fails is significantly less than previous models suggest. The experimental behavior of the composite materials investigated in the modelling work favors that predicted by the micro-crack linkage model. Ductility predictions resulting from the micro-crack linkage model were sensitive to both the volume fraction and the matrix work hardening exponent. By varying the matrix work hardening exponent the micro-crack linkage model captured the experimentally observed range of ductility values present in literature.</p> / Doctor of Philosophy (PhD)

<p>This work involves the study of crack-particle interaction in model brittle particulate composites where,</p> <p>i) the interfacial bonding strength between second phase and matrix was continuously varied (glass - partly oxidized Ni system) and</p> <p>ii) the presence and absence of elastic mismatch between second phase and matrix which gives rise to stress concentration around the particle was monitored. An ultrasonic fracture surface modulation technique was used which imprints the details of local crack-particle interaction on fracture surface.</p> <p>The important results of this study are:</p> <p>i) Both the interfacial strength and elastic mismatch important roles in improving the toughness of a particulate composite; optimum bond strength alone; without consideration of elastic mismatch may not improve toughness of a composite be a significant amount.</p> <p>ii) For optimum toughening, second phase particles with a high intrinsic toughness which are well-bonded to the matrix and whose elastic moduli are equal to or less than that of matrix should be used.</p> <p>iii) Due to the presence of elastic stress concentration, for second phase particles whose elastic moduli exceeds that of the matrix, a rather weak interfacial bonding is preferred to a strong interfacial bonding for effective crack particle interaction which will improve the toughness of a composite.</p> <p>In summary, this work provides some guidelines for choice of proper second phase particles and matrix in the design of brittle particulate composites for optimum toughness.</p> / Master of Engineering (ME)

<p>The occurrence of brittle intergranular fracture during sheet metal forming operations or during in-service use in low carbon steels is often termed cold work embrittlement (CWE). Interstitial-free steels are especially susceptible to brittle intergranular fracture, as there is little free carbon in solid solution left to segregate to the grain boundaries, where its presence is thought to intrinsically strengthen the grain boundaries and indirectly strengthen the grain boundaries by impeding the segregation of deleterious elements such as phosphorus and tin. These embrittled grain boundaries, coupled with an increased flow stress from cold working, result in intergranular fracture, especially after deep drawing, with cracks propagating in the drawing direction. The purpose of the present work was to identify the key microstructural parameters controlling cold work embrittlement. Tensile tests performed on well-characterized undeformed and predeformed IF steel sheet indicated that the grain shape was a key microstructural parameter. A strain path dependent fracture criterion was developed and used to predict the occurrence of cold work embrittlement for forming operations from the evolution of the resulting intergranular fracture surface and the yield surface with deformation. A series of experiments on deep drawn cups were used to test the key predictions of the model. The role of grain shape, amount of deep drawing, segregation levels, macroscopic residual stresses, strain rate, and surface condition were further clarified. Although a fracture path investigation indicated that low angle boundaries were resistant to fracture, the density of these boundaries were insufficient to affect the bulk macroscopic fracture properties. The potency and amount of segregant at the grain boundary controls the cohesive strength of the grain boundary. Quantitative analytical electron microscopy detected higher levels of phosphorus segregation for batch annealed steel as opposed to continuously annealed steel. The higher level of segregation resulted in a greater susceptibility to CWE especially for severe deep drawing. The process of phosphorus segregation during recrystallization annealing was modeled by accounting for the migrating grain boundary.</p> / Doctor of Philosophy (PhD)

<p>A technique has been developed for the assessment of the magnitude and direction of the principal finite strains from measurements made on a deformed pair of lines. The analytical procedure leads to the establishment of a symmetric second order tensor which is not one of the classical large deformation tensor defined in many Continuum Mechanics texts, but is much simpler in form and readily applicable to the determination of natural strains in sheet metal forming operations.</p> <p>Another aspect of this work has been an attempt to provide a new method (Geometric Modelling) for blank development and the investigation of possible strain distributions in forming sheet metal components. It is a computer technique simulating the traditional manual calculations performed by experienced tool designers.</p> <p>The present work describes the formulation of the fundamental theory of the method and the basic geometric assumptions which are employed. Two particular examples have been considered; one is forming a sheet on a smoothly analytically defined surface and the other is forming the corner section of an automotive stamping.</p> <p>The detailed analytical procedure is implemented in FORTRAN code. The analysis has been performed without access to advanced computer graphics. However; the results suggest that the basic approach is feasible and that future modelling using interactive computer graphics may well be attainable.</p> / Master of Engineering (ME)

<p>Methods for controlling slopping (slag overflow) were studied in the Steel Company of Canada's Hilton Works basic oxygen furnace installation. A link was established between slopping and various operating conditions by drawing on the experience of the furnace operators and by examining ingot yield data. An on-line control system was developed which used changes in the temperature of the furnace waste gases to signal when corrective action should be taken to prevent slopping. During trials, this system reduced slopping and increased ingot yield by approximately one percent.</p> <p>The chemical composition of slag from normal and slopping heats was studied in an attempt to determine why slopping occurs. The results of this study support a mechanism proposed by F. Bardenheuer (Ref. 21) which relates increases in slag foaming in the B.O.F. to the precipitation of dicalcium silicate and overoxidation.</p> / Master of Engineering (ME)

<p>In this thesis, the growth and structure of nickel oxide on three single crystal faces of nickel, viz the (100) (110), and (111), in the temperature range from 500 to 800°C have been investigated.</p> <p>A study has been made on the morphological development of the oxide and its structures by electron microscopy, X-ray diffraction and scanning electron microscopy in order to correlate these structures with the kinetics. Measurements of the kinetics for oxide growth on the different nickel crystal faces have shown non-parabolic behaviour and marked anisotropy in reaction rates due to the differences in crystallographic orientations. The deviation from parabolic behaviour and anisotropy in reaction rates are shown to be associated with the structural defects arising in the oxide layer during the growth process, epitaxial relationships between the metal and oxide, and orientation relationships between the oxide grains in the scale. The development of epitaxy can be justified in terms of the coincidence of a close packed direction between the metal and the oxide. Preferred growth leads to the formation of a duplex scale.</p> <p>A model for growth of the nickel oxide films and scales based on the simultaneous diffusion of nickel across the films via lattice defects and short-circuit paths is advanced to correlate the oxidation kinetics with the oxide structure. It is assumed in this model that the decrease in the density of the short-circuit diffusion sites at nickel oxide grain boundaries is determined by oxide grain growth processes. This model is shown to account satisfactorily for the observed oxidation kinetics of the (100) and (111) nickel faces. Also, the values for nickel grain boundary diffusion coefficients have been calculated for the first time for oxide on these two faces.</p> / Doctor of Philosophy (PhD)

<p>Instrumental Neutron Activation Analysis has been used to measure the materials loss from the cutting tools during machining. Using this technique, the contributions from mechanical and dissolution wear have been individually quantified, and the dominance of dissolution wear during the high speed machining of a medium carbon steel using uncoated tungsten carbide-cobalt tools has been established.</p> <p>SIMS analysis of the chip material next to the tool-chip interface showed concentration profiles of tungsten and cobalt to a depth of about 0.5 micron. The maximum concentration of tungsten is seen to increase with cutting speed, in agreement with the principle of dissolution wear. There is consistency between the results of the two independent analytical techniques (INAA and SIMS), used to study the dissolution wear process.</p> <p>The microstructure of the secondary shear zone of the chips was studied by Scanning Electron Microscopy and Transmission Electron Microscopy. In the immediate vicinity of the tool-chip interface of water quenched chips, a changed ultrafine equiaxed grain structure (0.2μ) was observed. Enhancement of the diffusivity of tungsten resulting from this ultrafine structure was estimated and incorporated into a thermokinetic model for dissolution wear. However, the dissolution wear predicted by the model was lower than the experimentally measured values at all the speeds; e.g. the predictions based on complete austenitization and a 0.2 μm grain size in the immediate vicinity of the tool-chip interface accounted for 34.7% of the experimentally measured value at 240 m/min. It is proposed that entire dissolution wear could possibly be accounted for by the thermokinetic model based on further enhancement in the diffusivity due to grain boundary migration or the presence of approximately 10nm size grains during the tool-chip contact. The joint role of solubility of the tool material into the chip and the enhanced diffusivity has been shown to be important in determining dissolution wear, and the performance of HfN coated tools is discussed in light of the above model. The 'hard' alumina inclusions lead to an increase in the mechanical wear rate of the cutting tool, and this effect is demonstrated quantitatively by the INAA technique.</p> / Doctor of Philosophy (PhD)