<p>Single phase pure metals and alloys deformed at high temperatures show large variations in the form of their strain-time curves and in the stress dependence of the strain rate. At present, it is not possible to predict theoretically the mechanical responses of a given alloy.</p> <p>This thesis is concerned with the deformation of pure copper, copper-5 wt.% tin and iron-3¼ wt.% silicon at temperatures above half the melting point. Mechanical experiments involving both constant load and constant strain rate teats have established that these three materials exhibit a spectrum of behaviour as wide as wide as any previously reported. Detailed microstructural examination has revealed corresponding variations in dislocation arrangements and in the extent of grain boundary distortion in crept specimens.</p> <p>It is argued that the form of the primary creep curve and strain transients following a change in stress, obtained in a given metal, is closely related to the mobility of mobile dislocations, rather than to an internal resistance arising from the substructure. To explain the observed mechanical responses, a model has been proposed which provides some semi-quantitative predictions of material behaviour. Both the mechanical and microstructural results suggest that the recovery theories of creep, in which hardening and recovery are considered as separately definable parameters, do not deal realistically with the creep process.</p> / Doctor of Philosophy (PhD)

<p>Ion bombardment has been used as a technique for the sputtering of metals for several years; very few studies have, however, been made on alloys and compounds, especially studies where the target itself was analyzed after ion bombardment. In this thesis results are presented concerning the changes in targets of Nb₂O₅ and related systems due to ion bombardment. The targets used were in the form of anodic thin films and sintered pellets. Bombardments were carried out using Kr⁺ and O₂⁺ ions with energies of 2 to 35 keV and at doses varying from 10¹³ to 10¹⁷ ions/cm². The structural changes induced by the bombardment were studied with an electron microscope and the changes in electrical conductivity were studied with a four point probe.</p> <p>The results indicate that an altered layer is formed which has the stoichiometry of NbO when Nb₂O₅ targets are bombarded with 35 keV ions at high doses (≥ 1 x 10¹⁷ ions/cm²). A technique based on the changes in the chemical reactivity of the altered layer as compared to the substrate is used to estimate the thickness of the altered layer. That on Nb₂O₅varies.from 18 nm at very low doses (5x10¹³ ions/cm²) to 23 nm at high doses (1x10¹⁷ ions/cm²). The conductivity of the altered layer formed at high doses is about 4x10³ s/m, an increase by a factor of 2x10⁸. An energy threshold for conductivity changes was found using an oxygen ion beam, whereas Kr ion bombardment reveals conductivity changes at all.</p> <p>Oxygen loss from bombarded Nb₂O₅ can be considered to have three alternative origins based on a vaporization model, an internal displacement model and a preferential sputtering model. The results favour the preferential sputtering of oxygen as the likely mechanism to explain the altered layer in the target. A modified sputtering model explains the energy threshold effect observed when an oxygen beam is used.</p> <p>Several applications of the techniques and results presented here have been considered. They include, among others, dating of minerals, limitations of surface analytical techniques, and preliminary results for Ta₂O₅.</p> / Doctor of Philosophy (PhD)

<p>This investigation is concerned with the study of the localized corrosion of nickel and the effect of the addition of molybdenum on its corrosion properties. To this end, alloys containing up to 30 wt % Mo have been studied in 1N H₂SO₄ and chloride containing environments by potentiostatic and galvanostatic techniques and 'in-situ' microscopic observations.</p> <p>The anodic oxidation of nickel has been studied by the galvanostatic technique. The experimental results have been used to develop the kinetic equation describing the film growth on nickel. It has been shown that an inverse, logarithmic behaviour is obtained in that the film growth current density is inversely proportional to the charge stored in the film. The resistivity of the film has been calculated and its value suggests that the film is pure and almost stoichiometric NiO.</p> <p>The significance of the Flade potential of nickel in 1N H₂SO₄ solution is discussed in view of the results obtained and those existing in literature. The implications of the critical potential for determining the susceptibility to pitting corrosion of metals and alloys has been investigated for Ni and Ni-Mo alloys. It is concluded that the critical potential does not adequately describe the electrochemical conditions required for the pitting corrosion of Ni and Ni- Mo alloys in sulfuric acid solutions containing chloride ions.</p> <p>The passive electrode has been treated as a metalsemiconductor system and justification has been provided for the assumption of a 100% efficiency for the current density during cathodic reduction.</p> <p>The charge required to reduce the film formed on Ni and Ni-Mo alloys in 1N H₂SO₄ and in the presence of chloride ions is reported. Some information is provided on the nature of the film on the Ni-Mo alloys in chloride containing solutions. Studies are reported on the polarization behaviour of the Ni-Mo alloys in synthetic sea water and on the corrosion morphology obtained after 30 day immersion in synthetic sea water and an acidic chloride containing solution.</p> <p>It is proposed that the improved resistance to corrosion of Ni-Mo alloys in naturally corroding systems is due to a sluggish anodic reaction imparted by a slow hydration of the metallic ions, while under anodic polarization in the presence of chloride ions it is due to the presence of a diffusion barrier film of MoO₃. The results obtained on the Ni-Mo alloys have been explained on the basis of a defect model for the passive film on the alloys.</p> / Doctor of Philosophy (PhD)

<p>A study was made of the fracture in a model composite containing nickel spheres in a glass matrix. The macroscopic fracture characteristics of the system were determined by fracture surface energy, strength and elastic modulus measurements. The microstructures of the composites were defined carefully using quantitative microscopy and the fracture processes were studied using fractographic techniques. In particular, the technique of ultrasonic fractography was used to study the crack-particle interactions in detail.</p> <p>It was found that for a non-bonded S glass-nicker system, the crack shape changes were similar to those observed for a glass-pore system. Theoretical models were used to explain the crack shape variations and their effect on properties. These models were based on the interaction of the stress field of the crack and the particles by local crack blunting and image stresses. Good agreement was obtained between theoretical predictions and experimental measurements of crack shape at breakaway and the fracture surface energy variations.</p> / Doctor of Philosophy (PhD)

<p>Several conflicting models have been proposed to describe the work hardening behaviour of alloys composed of a hard second phase in a plastically deforming matrix. Good agreement is reported when the models are compared with the results of unidirectional tests. To distinguish between the models, it is necessary to use tests which include deformation in both the forward and reverse directions.</p> <p>In this study the work hardening behaviour in aluminum copper alloys has been studied using deformation in compression immediately after deformation in tension. The large Bauschinger Effect obtained has been analysed to give the magnitude of the long range back stress present in the alloy due to the elastic deformation of the θ' particles in the plastically deforming matrix. Experiments were carried out on polycrystals, and on single crystal test pieces oriented for single and multiple slip over a wide range of temperatures. The results give excellent agreement with a model which calculates the long range back stress. The reported good agreement of other experimental work with an opposing model has been critically examined.</p> <p>The behaviour at large strains has been studied to determine the processes leading to plastic relaxation. The conditions at the onset of necking of the single crystal test pieces have been examined, and the formation of coarse shear bands and final fracture correlated with the work hardening rate.</p>

<p>In this thesis the growth and structure of nickel oxide scales formed on polycrystalline nickel during continuous oxidation and oxidation interrupted by vacuum anneals at 800° and PO₂ = 400 Torr were studied. The morphological development of nickel oxide formed commencing with early stages of oxidation to scale growth at very long times was studied by thermogravimetry, scanning electron microscopy and X-ray diffraction measurements.</p> <p>The results demonstrate the influence of the metal structure on the anisotropy of oxidation, the influence of the defect structure of the oxide set up by the growth process itself on the transport mechanism of the reactants, and the preferential growth of oxide grains.</p> <p>The nickel oxide scales exhibited initially a bilayer structure or a simple equiaxed structure; these structures could be transformed into a monolayer structure of columnar oxide grains by different methods. The early stage cellular structure of nickel oxide developed into inner equiaxed and outer columnar oxide layers by a combined transport of nickel, through the oxide lattice and through boundaries of the oxide grains. The structural development of the nickel oxide scale was correlated with the nonparabolic oxidation behaviour of nickel.</p> / Doctor of Philosophy (PhD)

<p>The Sulfidation kinetics of Ni-Al Alloys containing up to 13 a/o Al exposed in H₂S-H₂ atmospheres were investigated using a thermogravimetric technique. The scale morphologies were studied using scanning electron microscopy and eptical microscopy. The scales were analyzed using electron phone microanalyses, λ-ray and differential thermal analyses.</p> <p>It was observed that the sulfidation kinetics of these alloys are linear, that the rates decrease with increasing amounts of Al at constant PS₂ in the atmosphere and at a given alloy composition the rates increase with increasing PS₂. The scale consists of three regions. The outer scale consists of NiS when PS₂ is greater than 10ˉ⁴ atm and it consists of Ni₃S₂ otherwise. The external scale subscale boundary corresponds to the original alloy-gas interface. The subscale grows by inward diffusion of S whereas the external scale grows outward diffusion of Ni. The subscale consists of two layers having different morphologies, and compositions. The layer next to the external scale contains alternate dark and bright lamellae. The bright lamellae consist of Ni₃-S₂ whereas the dark lamellae consist of an Al rich sulfide phase, presumably Al₂S₃. The second layer of the subscale is liquid at the reaction temperature of 700°C, which contains Ni, Al and S.</p> <p>A model is advanced to account for the linear kinetics, assuming that a reaction at the outer scale-gas interface can be used to describe the sulfidation properties. In order to explain the formation mechanism of the Lamellar morphology of the subscale isothermally at the reaction temperature, a model is advanced based upon concepts of eutectic decomposition. A ternary Ni-Al-S phase diagram is constructed which is consistent with the mechanism and the observations on the Ni Al-S system reported in the literature and in the present work. The functional relationship between the growth velocity of the Lamellar front and the resulting Lamellar spacing is derived and verified. According to this model, segregation at the Lamellar front is not the rate controlling proceeds because the linear rate constant would be proportional to the sulfur partial pre-pressure in the atmosphere, which contradicts the observation.</p> / Doctor of Philosophy (PhD)

<p>A combination of high strength and good fracture resistance is obtained in high strength low alloy (HSLA) steels by the use of controlled rolling and addition of micro alloying elements to refine the scale of the microstructure. In these fine grained materials traditional property-structure relationships do not adequately describe the fracture behaviour.</p> <p>This thesis is concerned with the fracture properties of HSLA-steels at various temperatures and stress states. Three modes of failure are commonly observed. At low temperatures cleavage is the predominant fracture mode, whereas ductile failure by nucleation and growth of voids occurs at higher temperatures. In the intermediate temperature range delamination fracture on planes parallel to the rolling plane is observed. The various fracture mechanisms are discussed in terms of the detailed microstructure of the materials which has been characterized by the use of standard optical and electron metallography. In addition failure criteria for the most common fracture modes have been developed.</p> <p>It is found that the condition for cleavage failure is adequately described in terms of a Griffith equation where the crack length is determined by an effective grain size of the order of twice the ferrite grain size. Further it is argued that the low temperature fracture toughness can be expressed by the cleavage stress and the size of the process lone. For fine grained materials the process zone size is found to be independent of the scale of the microstructure.</p> <p>The resistance to ductile fracture has been characterized in terms of a critical crack opening displacement (COD). It is argued that the COD value is determined by the size of the process zone which is independent of the scale of plasticity. The process zone size is related to the inclusion spacing.</p> <p>Delamination is found to occur mainly by a grain boundary tearing mechanism. However, the presence of inclusion aggregates may reduce the fracture stress substantially. Delamination by the grain boundary tearing mechanism occurs at a critical value of the maximum shear stress indicating that crack nucleation is the critical event.</p> / Doctor of Philosophy (PhD)

<p>A study has been made of the initial stage sintering kinetics of nonstoichiometric MnO as a function of the oxygen partial pressure. The shrinkage and neck growth were measured for pairs of oxide spheres in the temperature range 900-1100°C and pressure range 10ˉ¹⁴</p> <p>The shrinkage was analyzed using both the classical single mechanism control approach and the more recent simultaneous mechanism approach. It was found that the sintering behavior as a function of oxygen pressure, hence the concentration of Mn vacancies, does not follow a simple relationship has been suggested. The kinetics indicate grain boundary diffusIon control at oxygen pressures less than 10ˉ¹² atm. and greater than 10ˉ⁹ atm. and volume diffusion control at pressures in between.</p> <p>Using the simultaneous mechanism approach, the predominance of grain boundary control at low and high oxygen pressures was verified; however, a substantial contribution from volume diffusion was present. The volume diffusion contribution reached a maximum at 1 x 10ˉ¹⁰ atm., the same pressure at which the maximum shrinkage and shrinkage rate was found.</p> <p>Diffusion coefficients calculated from both methods of analysis agree well with published values and with the published defect structure for MnO.</p> <p>The surface topography of the spheres was studied. It was found to influence the sintering behavior substantially and to be dependent on the atmosphere composition. A qualitative model for the effect of surface topography is presented.</p> / Doctor of Philosophy (PhD)

<p>The anodic oxide films of single phase Ni-Mo alloys (5-15 wt % Mo) formed in 0.15 N Na₂SO₄ (pH 2.8) have been investigated using electrochemical techniques, atomic absorption spectroscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The anodic oxide film of Ni formed under the same condition has been used as a reference in order to study the effect of molybdenum addition on the nature of the passive film of nickel. The electrochemical studies have indicated a negative effect of molybdenum addition to stability of the passive film and a different mechanism of the film growth on the alloys with respect to that on nickel. Dissolution of nickel and molybdenum during the film formation has been found to proceed in proportions other than their ratio in the alloy and which was also time dependent. Selective dissolution of molybdenum has been found during the initial stages of anodization and its enrichment in the film at longer anodization times. Chronoamperometric measurements combined with dissolution analysis suggested an increase of the film thickness with anodization time. Auger electron spectroscopy with ion sputtering confirmed the anodization time dependence of the film thickness and an enrichment of the film-solution interface in molybdenum. The thickness of the passive film of nickel has not been found to change significantly with adonization time. Characterization of nickel and molybdenum species in the film has been based on the findings of x-ray photoelectron spectroscopy, which combined with the evidence offered by structure analysis (RHEED), suggested the passive film to be a two-phase oxide film containing defective or hydrated NiO and probably amorphous MoO₃. A model which explains the growth of two-phase oxide coverage on Ni-Mo alloys has been proposed.</p> / Doctor of Philosophy (PhD)