<p>Two model systems, Co-Rh and Ni-Co, were used to study surface segregation and oxidation phenomena on binary alloys.</p> <p>Cobalt was found to sputter preferentially from five Co-Rh alloys ranging in composition from 19.3 to 84.7 at % Rh when irradiated by 4 keY Ar⁺ ions. Surface segregation experiments on this system in the temperature range 623 to 1123 K showed that cobalt was enriched on the surface of a 84.7Rh alloy while rhodium enrichment was determined for a 19.3Rh alloy. No conclusive segregation could be measured on the surface of a 47.8Rh alloy. The variation of surface concentration with temperature indicated enthalpy values of 1 ± 1 kJ mol‾¹ and -3 ± 7 kJ mol‾¹ for the 19.3Rh and 84.7Rh alloys, respectively. Reasonable agreement was found between these measured enthalpy values and those calculated from the combined bond breaking and lattice strain models of surface segregation.</p> <p>Oxidation experiments were performed on (110) Ni-3.86Co and Ni-4.0Co polycrystalline alloys at temperatures in the range 673 to 1073 K and a pressure of 5x10‾³ Torr. The effects of surface pretreatment and orientation of the alloy surfaces were correlated to differences in oxide morphology and growth rates. Single crystal specimens which were chemically polished, annealed in vacuum and initially exposed to oxygen at room temperature showed a tendency to form (100) (NiCo)O ⎮⎮ (110) Ni-3.86Co. These layers also exhibited the slowest growth rates and greater enrichment of CoO from the inner to outer oxide surfaces. Single crystal surfaces prepared by chemical polishing, annealing in vacuum and initial exposure to oxygen at the reaction temperatures developed additional oxide orientations parallel to the underlying alloys. Finally, the polycrystalline Ni-4.0Co specimens which were prepared by mechanical polishing followed by exposure to room temperature oxygen developed polycrystalline oxide films. These types of structures were associated with higher rates of oxide growth and lower enrichments of CoO from the inner to the outer oxide surfaces.</p> <p>The experimentally measured composition profiles were analysed according to the Wagner(⁶) model of oxidation, modified to include the effects of short circuit grain boundary diffusion. Effective alloy interdiffusion coefficients were calculated which showed that the rate of transport in the alloy zone beneath the growing oxide layer was enhanced relative to that expected solely on the basis of volume diffusion in this phase. The results were rationalized according to a dislocation model for the accomodation of plastic strain and vacancy annihilation in cation conducting scales(¹⁴⁹).</p> / Doctor of Philosophy (PhD)

<p>Atomic mixing resulting from heavy-ion bombardment of thin-film Ni/Pd bilayers and thin Pd markets sandwiched between two Ni layers has been investigated. Experiments were carried out using 120 keV Ar⁺ and 145 keV Kr⁺ ions for doses 0-4x10¹⁶cm⁻² over a wide temperature range (40-673K). The resulting interdiffusion was measured using 2 MeV ⁴He⁺ ion backscattering in-situ. In some cases, additional depth analysis was undertaken using Auger Electron Spectroscopy (AES) with sputter depth profiling. The influence of film microstructure and bombardment-induced microstructural changes on the mixing have been assessed with transmission electron microscopy (TEM). The electrocatalytic behavior of the ion-mixed bilayer samples was determined for the hydrogen evolution reaction in strong KOH solution, using potentiostatic polarization techniques. The amount of ion beam mixing is found to vary with the square root of the ion dose for temperatures ≤ 573K. Also, at 40K, where it is assumed that no significant thermally activated process contributes to the mixing, it is found that the mixing is dependent on the damage energy, FD(subscript, see original file), deposited at the interface region. Between 40 and ~400K, the mixing parameter shows a weak temperature dependence with an apparent activation energy Ea ~ 0.1eV. This suggests that some interstitial diffusion contributes to the inter-mixing in this temperature regime. Above ~ 473K, the mixing (at each temperature) increases rapidly with ion dose for ir radiation doses ≤ 5x10¹⁵cm⁻² with high initial mixing efficiencies of 6-35 atoms/ion between 573-673K. This initial rapid mixing level eventually slows to a less rapid (~1.2 atoms/ion at 673K) rate for doses ≥ 5x10¹⁵cm⁻². TEM observations of the irradiated and unirradiated films suggests that the change to a less rapid mixing rate is correlated with an increase in the average grain diameter (10-80nm) in the film following ion bombardment, and therefore with a reduction in the contribution of grain boundary diffusion to the intermixing. For samples annealed, without irradiation, below ~ 473K, no measurable interdiffusion of the Ni/Pd layers was observed. For samples annealed above ~473K, Whipple analyses of the interdiffusion show two distinct regions - lattice diffusion-dominated region near the original interface and grain boundary diffusion dominated region further away from the interface. The lattice diffusivities, Dℓ(subscript), under thermal annealing conditions were evaluated and ranged from ~ 0.22 to 52x16⁻¹⁶cm⁻²-sec⁻¹ for Pd diffusing in Ni, and 0.33 to 56.6x10⁻¹⁶cm²-sec⁻¹ for Ni diffusing in Pd between 523-673K. These lattice diffusivities are high because of enhancement of diffusion by defects present in the as-prepared films. The grain boundary diffusion coefficients (for Pd diffusion in Ni boundaries) DB(subscript) varied from ~ 5.4x10⁻¹⁴cm²-sec⁻¹ at 523K to ~ 3x10⁻¹¹cm²-sec⁻¹ at 623K, with an apparent activation energy EB(subscript) ~ 1.35 eV and a prefactor D⁰B(subscript) ~ 0.75 cm²-sec⁻¹. Under irradiation conditions, ion irradiation results in an enhancement of the mixing especially below ~ 400K where almost an order of magnitude enhancement over thermal anneal only, was observed. At ~ 673K, the contribution of equilibrium defects to interdiffusion over-whelms the irradiation effects. The diffusion coefficients under irradiation, Drad(subscript), were also evaluated and ranged from ~4 to 70x10⁻¹⁶cm²-sec⁻¹ at 523 and 673K with apparent activation energy Ea ~ 0.57 eV. For Pd films deposited on large-grain Ni substrates, thermal annealing and/or bombardment result in mixing levels that are almost two orders of magnitude lower than observed from small-grained Ni/Pd couples, though under irradiation conditions, radiation enhancement of the mixing was also observed. As potential electrocatalysts for the H₂ evolution reaction in strong KOH solution, the ion-beam mixed surface alloys (in the over-potential range ≤ 0.4 V) show superior performance over smooth Ni, and Ni coated with evaporated Pd.</p> / Doctor of Philosophy (PhD)

<p>This thesis is a report on a series of measurements of ion-bombardment effects on α-quartz crystals. Damage was produced in α-quartz (single-crystal SiO₂) by bombarding with ions of ⁴He⁺ to ²⁰⁹Bi⁺⁺ in the energy range of 15 - 200 keV, at both 300 K and ≤ 50 K. The samples were analysed in - situ with Rutherford backscattering/channeling, using 1 - 2 MeV ⁴He⁺ ions; data was obtained from both the oxygen and silicon peaks, so that damage stoichiometry could be calculated. At low ion fluences (~ 10¹⁰ - 10¹¹ ions/mm²) the apparent damage level increases linearly with fluence, but at rates of about 3 - 19 times greater than predicted by the modified Kinchin-Pease equation. At higher fluences (~ 10¹¹ - 10¹² ions/mm²) the rate of damage increases with fluence for elastic energy deposition rates of less than :::= 0.08 eV/atom, but remains constant for greater elastic energy deposition rates. At even higher ion fiuences saturation of the damage occurs. It has been observed that the analysis beam creates damage at a rate dependent on the level of damage already present in the crystal; furthermore, the damage created by the analysis ions appears to be predominantly due to the inelastically deposited energy. Strain effects seem to greatly exaggerate the level of damage indicated by simple channeling calculations. A simple model is proposed to explain both the high rates of damage production and the non-stoichiometry of the damage. The model indicates that the effects of elastic energy deposition, inelastic energy deposition, strain produced by damage, and lattice relaxation into a '"quasi-amorphous" state. all contribute to the apparent damage levels synergistically.</p> / Doctor of Philosophy (PhD)

<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)