<p>The structures in the PbS-rich part of the Ag₂S-PbS-Bi₂S₃ system are composed of PbS-like slabs obtained by twinning the parent PbS (NaCl structure) along the (311)PbS planes. A number of phases, some new and some already encountered in mineral samples, have been found by HREM. In the observed structures the slabs 4 and 7 octahedra wide were the most common. Slabs with width of 8 and 5 octahedra were also encountered, but not those with 6 octahedra. The material was sensitive to the electron beam and the structures often decomposed to galena during the observation. Disappearance of the twin boundary was accompanied by formation of an edge dislocation.</p> <p>The structures in the PbS-rich part of the PbS-Sb₂S₃ system are composed of ribbons made of back to back square pyramids and arranged in three kinds of topology thus forming three homologous series. The structures of 5 phases were determined and the cation occupancies in the ribbons show that lead atoms occupy the 8- or 7-coordinated sites where the ribbons join, edge to face, while antimony atoms prefer the 7-coordinated sites where the parallel parts of the ribbons overlap.</p> <p>The observations of the topology and strength of bonding in the PbS-Sb₂S₃ structures served as basis for developing models in which the structures. built of ribbons, are bonded together in variety of ways. The model correctly predicts: 1) stability of three kinds of ribbon arrangements 2) stability limits of the structures in each homologous series 3) lead and antimony distribution in the structures.</p> / Doctor of Philosophy (PhD)

<p>Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used to investigate the morphologies, distributions, compositions, particle size distributions and growth kinetics of niobium carbide and titanium-niobium carbonitride precipitates in steels containing low levels of Ti, Nb, C and N. The microalloyed steels were solution treated at 1390°C for 2 hr and quenched, and then aged at 1000°C and 1100ºC for various times.</p> <p>The precipitates, mostly with spherical shapes for short aging times and polyhedral and cubic shapes for long aging times, were found to be not quite uniform due to preferential sites for heterogeneous nucleation. After rapid growth within the first few minutes, insignificant ripening takes place for precipitation at 1000ºC while fast coarsening occurs at 1100ºC. During the aging, only complex carbonitride precipitates of the form (Tiᵪ Nb₁₋ᵪ)(CyN₁₋y) were found among the newly nucleated and growing particles. There is a strong size dependence of particle compositions for short aging times. The youngest of these particles. which approach the size of critical nuclei. tend to be Ti-rich. Titanium nitride tends to form at very high temperatures and titanium-niobium carbides go to completion at low temperatures.</p> <p>A thermodynamic model was applied to the alloy systems to predict the compositions and mole fractions of the complex carbonitrides. The calculated precipitation time-temperature (PTT) diagrams for carbonitrides exhibit a C shape for all of the steels. A model predicting the growth kinetics of carbonitrides and composition variation within the precipitates for the initial growth stage (before coarsening) has been developed based on equilibrium thermodynamics with the inclusion of capillarity and multicomponent diffusional kinetics. In this theory the precipitates in Fe-Mi-Xj austenite containing two substitutional transition metals M (Ti, Nb or V) and two interstitial elements X (C and N) have been considered. Satisfactory agreement with the experimental results has been demonstrated.</p> / Doctor of Philosophy (PhD)

<p>The interaction between hydrogen isotopes and irradiation defects in nickel is studied using Gas-phase Permeation and Thermal Desorption Analysis techniques. The work was motivated by the need for the understanding and control of tritium diffusion and trapping in structural materials of the first generation of D-T fusion reactors, as well as by the need for minimizing helium "ash" in the plasma. Major attention was given to the study of relationships between helium defect configuration and deuterium diffusion and trapping behavior.</p> <p>Permeation results show that crystalline defects such as dislocation loops produce no measurable effect on deuterium permeation in the temperature range of 373 K - 573 K, whereas helium defects decrease the apparent diffusivity and, sometimes, the permeability of deuterium by several orders of magnitude, indicating a strong trapping effect for hydrogen isotopes. Thermal treatments at elevated temperatures of samples pre-implanted with helium ions resulted in a newly observed abnormal aging history dependence on deuterium diffusion and permeation behavior. The basic aspects of this abnormal behavior were examined and analyzed.</p> <p>Thermal desorption technique was used to obtain additional information on helium trapping mechanisms: for examples, the effective binding energies of deuterium to helium defects consist of a distribution of values and are in the range of 0.4 - 0.6 eV with higher values observed when helium bubbles formed; the trapping efficiency decreases from> 1 to < 0.3 when the helium fluence increases from 1x10¹⁵ to 4x10¹⁷/cm²; thermal treatments of these samples resulted in a decrease in the trapping efficiency that was also dependent on the initial helium fluences. An interesting phenomenon, "H" enhancement of helium release, was observed and analyzed using the Hydrogen-Assisted Cracking model.</p> / Doctor of Philosophy (PhD)

<p>In this thesis, the results of oxidation tests carried out on iron- nickel alloys in carbon dioxide - carbon monoxide atmospheres at 1000°C are presented. Linear oxidation kinetics were observed for the formation of wustite on alloys containing up to 50 weight % nickel. Spinel oxides appear on alloys containing greater than 50 % nickel. A subscale developed in all alloys investigated. An oxidation model is presented for the constant uptake of oxygen based on detailed balancing of the individual reaction steps, assuming that dissociation of carbon dioxide is the rate controlling process. The subscale formation mechanism is based on the principles of diffusion in multicomponent metallic systems. Theoretical relationships are presented which adequately account for the experimental observations.</p> / Doctor of Philosophy (PhD)

<p>The strength of ceramics is related to their fracture toughness and the fracture-initiating defects. The present thesis reports investigations of the strength/defect relationships that increase understanding of strength-improvement pedagogies and strength prediction methodologies for ceramics. The strength improvement of a 4.5wt% yttriapartially-stabilized zirconia (Y-PSZ) via defect elimination is demonstrated. Defect types were identified and eliminated via several novel techniques. These procedures were combined with a conventional sintering route. A crack model is developed and used to examine the discovered flaw/strength relationships. Four-point-bend flexural strengths were systematically improved from 880 to 1380 MPa, a value comparable to hot-isostatically-pressed or slip-cast materials. The flaw/strength relationships for various fracture origin types were examined, i.e. fiber inclusions, agglomerates, iron inclusions, pores and alumina inclusions. An order of flaw-type severity was discovered and defect types of the same "size" were found to act in a dissimilar fashion in fracture initiation. The "severity" is related to local residual stresses around defects due to thermal mismatch or differential sintering. The relative defect-severity was quantitatively ranked via the apparent fracture toughness and a "relative defect severity parameter" developed. The temperature dependence of this parameter for the various defect types was investigated and the behavior explained via residual stress relief. Combined high-temperature four-point-bend and chevron-notched-bar fracture toughness (Kio) tests were conducted. It was found that the residual stress associated with alumina defects in tetragonal zirconia was temperature-dependent, that associated with agglomerates, temperature-independent and could be annealed. No residual stress was associated with pores. The residual-stress relaxation temperature (Tg) was determined for tetragonal zirconia. The size equivalence of inclusion defects and the resulting flaws was investigated. The fracture behavior of alumina inclusions was investigated via an Al₂O₃-ZrO₂ composite layer sintered onto the tensile surface of PSZ four-point-bend bars. It was found that inclusion fracture exclusively occurred and the fracture was associated with weak interfaces between the grains of the alumina inclusion or between the inclusions and matrix. Crack initiation in the fracture-toughness chevron-notched-bend bar was studied. Ideal testing conditions for valid Kio values strongly depend on specimen preparation and material. An in-test precracking techniques was developed for Kio testing at room and elevated temperatures. A Y-PSZ ceramic was tested and ideal testing conditions proven. Defect-initiated fracture models for ceramics developed. Fracture initiation is a typical inclusion with a thermal expansion coefficient and fracture toughness lower than the matrix and a Young's modulus higher than the matrix was analyzed and modeled for a spherical inclusion using a weight function method to compute the residual stress intensity of factor and a part-through elliptical crack model to follow the crack extension. The model was used to predict the strength of PSZ containing α-Al₂O₃ inclusions. It was also used to compute the Tg for tetragonal zirconia. The predictions were in good agreement with the experimental data. For agglomerate-initiated fracture, an agglomerate sintering/ residual-stress-retention model is proposed and used to estimate the fracture stress for agglomerate-matrix decohesion. The residual stress levels after elastic and thermal recovery were estimated and, agreed with experimental data.</p> / Doctor of Philosophy (PhD)

<p>The crystallography and the interfacial structure of interphase boundaries in a Zr-2.5 wt% Nb alloy have been studied by transmission electron microscopy (TEM), and the results compared with the predictions of a model developed in this work. The α phase (HCP), when precipitated from β (BCC), usually forms as intragranular plates. The habit plane of an α plate is characterized by the regularly spaced dislocations, parallel to the long axis of the α plate. These dislocations, about 10nm apart, have a Burgers vector [0 1 0] with respect to β phase. The orientation relationship between α and β phases was found to deviate slightly from the ideal Burgers orientation relationship. Based on the O-lattice and invariant line analysis a geometrical model has been further developed. The cell structure of O-lattice, which is the key information in predicting the configuration of the interfacial dislocations, has been constructed by a simple calculation made in reciprocal space of the O-lattice. This is expected to improve the general application of the existing O-lattice model. The plane of the least lattice mismatch, determined by the O-lattice plane of the smallest spacing, is consistent with the observed habit plane. The optimum orientation relationship suggested based on the analysis of mismatch in the habit plane is shown to deviate slightly from the Burgers orientation, which is supported by experimental evidence. The analysis also indicated that the misfit in the habit plane could be accommodated completely by a set of to [0 1 0]ᵦ dislocations of ~10nm in spacing, lying along the invariant line, in agreement with the experimental observations.</p> / Doctor of Philosophy (PhD)

<p>A high temperature electrochemical formation cell</p> <p>[equation removed]</p> <p>has been used to determine the standard free energy of formation of alumina at steelmaking temperatures. The temperature variation of the free energy was determined as</p> <p>[equation removed]</p> <p>in the temperature range 1700º-1850ºK. These results are in good agreement with those reported in the literature at ~1300ºK using a similar technique but are 3.3% more positive than the generally accepted thermochemical value at 1800ºK. In order to confirm the absence of any systematic errors in the above cell, the following cell was designed and operated in the temperature range 1641º-1785°K</p> <p>[equation removed]</p> <p>The results from this cell are in good agreement with those of the first cell within the experimental error.</p> <p>The Third Law analysis of the present data gives an average value of the standard heat of formation of alumina at 298ºK, Hº₂₉₈, of -392.6 (± 0.92) kcal/mole which is in good agreement with the Third Law analysis of lower temperature cell (~1300ºK) data and the values of ΔHº₂₉₈ calculated from phase equilibria and solution calorimetric data in mineralogical systems. All these values are at variance with the presently accepted "best" value of ΔHº₂₉₈ = -400.40 (± 0.3) kcal/mole from oxygen bomb calorimetry, although the average of all bomb calorimetric data is in agreement. Using the presently determined ΔGºf (Al₂O₃), the temperature dependence of K = h²Al h³₀, the aluminum deoxidation constant, can be calculated as</p> <p>[equation removed]</p> <p>The value of K = 4.7x10⁻¹³ at 1600ºC is in good agreement with the most recent work of Jacquemont et al. (1973) and is an order of magnitude higher than previous calculations. An independent calorimetric determination of ΔHº₂₉₈ using a method other than oxygen bomb calorimetry is therefore suggested.</p> / Doctor of Philosophy (PhD)

<p>The photochemical etching behaviour of n-InP was examined by producing holes or vias through the samples. The rate of material removal and quality of the resulting vias were studied as a function of: electrolyte pH, temperature, and type of metal ion impurity, illumination level, frequency, and duty cycle, sample surface quality, area, and roughness, and a second light source. The project was undertaken to prepare thin samples for the transmission electron microscope (TEM). The quality of sample produced for the TEM was acceptable, as determined by comparing the thin sections to thin sections produced by other traditional sample preparation techniques. This new method of sample preparation has the important advantages of being able to thin at a precisely predetermined region and that the resulting thin regions are surrounded by a thick built-in stabilizing structure which improves handleability.</p> / Doctor of Philosophy (PhD)

<p>It has been appreciated for some time that low oxygen activity is required for effective steel desulphurization, and recently this has been recognized for hot metal desulphurization. In the present experiments, calcium carbide was injected into 70 kg melts of iron during which desulphurization and deoxidation were studied by continuously measuring oxygen activity and frequently sampling for sulphur content. In some experiments, carbon dioxide was used either as a carrier gas or as generated from calcium carbide/limestone mixtures. The experiments confirmed that the oxygen activity in hot metal was controlled by the silicon-silica equilibrium, except when aluminum was added. It was found that the rate of desulphurization was associated with the oxygen activity. The desulphurization rate increased as oxygen activities decreased in order from carbon dioxide-containing injections into iron-carbon-silicon melts, to calcium carbide/nitrogen injections into iron-carbon-silicon melts, to finally calcium carbide/nitrogen injected into iron-carbon-aluminum melts. The dependence of the incubation period with the melt oxygen activity was recognized. The incubation times coincide with periods of deoxidation before desulphurization proceeds. In order to interpret the experimental results, a kinetic model was developed that takes simultaneous desulphurization and deoxidation into account. This model demonstrates clearly that the rate of desulphurization is controlled by the melt oxygen activity and the powder feed rate. Thus, low oxygen is required for effective hot metal desulphurization. In a log h$\sb{\rm o}$-log h$\sb{\rm s}$ diagram, the refining trajectories determined from the kinetic model were found to be consistent with those obtained from the experiments. From the model calculations, it was found that the commercial calcium carbide which normally contains 30 per cent of lime can be employed for the melts pre-deoxidized with silicon or aluminum, without causing significant delay of desulphurization. Another aspect of this work is the particle-liquid contact. The contact was measured by monitoring the melt temperature during injection, and applying calorimetric principles to analyze the results. It was found that a significant fraction of particles does not come in contact with the melt during their rise through it. The particle-liquid contact fraction is approximately 30-50 per cent. The same conclusion was obtained by analyzing the experimental rate constants of desulphurization on the basis of mass transfer theory.</p> / Doctor of Philosophy (PhD)

<p>The primary purpose of this investigation was to examine the strengthening that arises when reinforcing an aluminum alloy with approximately equiaxed shaped SiC particles. The results indicate that the majority of strengthening that is derived from the particles develops at low strains. For a variety of test conditions explored, about 90% of the total strengthening derived is developed by a plastic strain of about.5%. This is primarily a result of the high strain hardening rate which is exhibit by the composite materials during the onset of plastic flow. An examination of the Bauschinger effect in the composites indicates that an internal stress, resulting from inhomogeneous plastic flow, develops rapidly in the low strain regime. Microstructural observations suggest that the nature of this inhomogeneous plastic flow is influenced by the distribution of the SiC particles. Initial plasticity appears to be associated with regions of the microstructure which are relatively particle free. A self-consistent model, based on Eshelby's equivalent inclusion method, has been developed which is capable of calculating the stress partitioning which occurs between the Al matrix and SiC particles during the elastic/plastic transition and beyond. Excellent quantitative agreement between the experimentally measured elastic modulus and that predicted by the model was achieved. When considering a uniform distribution of second phase particles, the model predicts an increase in the initial strain hardening rate. However, it underestimates the actual strain hardening rate which was measured experimentally. The influence of particle distribution was examined by representing the composite microstructure by a bimodal mixture consisting of a particle rich and particle free phase. The model predicts an increase in the initial strain hardening rate as a result of clustering the microstructure in this way. The magnitude of this increased hardening is comparable to that measured experimentally.</p> / Doctor of Philosophy (PhD)