<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)

<p>In considering the fabrication of engineering components from metals in the form of thin sheets (<6 mm in thickness), attention must be given to two important problems: a) the mechanisms by which plastic strain is distributed in the component, and b) the competition between continued deformation and fracture. The description of these processes from a theoretical viewpoint and the completion of an experimental program are complicated by the fact that both problems involve intrinsic properties of the sheet as well as external parameters such as temperature, strain rate and stress state. The objective of this thesis is to characterize the sequence of events which occur during the forming process by giving attention to both types of variables.</p> <p>The first portion of this work involves a macroscopic approach. An experimental program has been conducted to measure (by using grid techniques) the distribution of strain in a variety of aluminum alloys subject to loading trajectories which range from pure shear to biaxial tension. Equations have been derived to express the occurrence of localized necking and fracture in terms of strain and stress coordinates. These curves, generated by plotting the two events for a wide range of stress systems, may be termed failure maps. If one considers their position relative to the initial yield event, the maps are capable of representing the entire strain history of the sample. In addition, their shape and level can be used a) to establish the relevance of empirical failure criteria, b) to compare the behavior of materials (e.g to illustrate the competition between alloys to form a given component), and c) to examine the influence of stress state on the occurence of a well defined fracture mode.</p> <p>By varying the heat treatment of some of these alloys, it was possible to promote different fracture modes such as fibrous, shear or intergranular fracture and to examine these modes in terms of the resultant fracture map. The results indicate that the failure criteria applicable to sheet metal forming operations are in good accord with the fractographic evidence, and that one should not only consider the competition between continued deformation and fracture, but also between available fracture modes depending on heat treatment and stress state.</p> <p>Although the macroscopic approach outlined above gives a broad summary of the straining history of the sample, it is not sufficiently specific to enable a correlation with the microstructure. The second approach of this thesis, at a microscopic level, is concerned with local events occuring inside the material and attempts to elucidate the metallurgical factors which influence the conditions under which a well characterized alloy undergoes the transition from uniform deformation to some form of localized deformation and finally fracture.</p> <p>A model material (in the form of spheroidized carbon steels) was thus studied to obtain a quantitative description of the microvoiding process leading to fibrous fracture. Careful examinations of the fracture surfaces and the zones adjacent to them enabled a measurement of structural damage resulting from plastic deformation to be established. A detailed theoretical model was developed to relate the damage to continuum quantities such as strain and stress. The model was then extended to describe the process of void nucleation and growth at inclusions under more general stress systems.</p> <p>Finally, a new description of work hardening characteristics of materials at large strains was presented and related to the foregoing concepts of strain localization and fracture.</p> / Doctor of Philosophy (PhD)

<p>Scattering and radiation damage parameters are analytically evaluated for 14 MeV neutrons and 10-17 MeV protons on Fe, Ni, Cu, Zr, Nb and Au. Damage energies are computed for the interactions using both elastic and non-elastic data. The theoretical results show that proton encounters deposit a damage-energy greater by factor of two than that calculated for mono-energetic 14-MeV neutron events.</p> <p>To examine the theoretical results, electrical resistivity measurements are undertaken for Fe, Zr and Cu irradiated at 12 to 17.5K with 10 to 16 MeV protons. The resistivity change is measured as a function of dose. In addition, post-irradiation isochronal annealing is carried out in-situ using a closed-cycle helium-cooled cryostat.</p> <p>Values of the resistivity damage rate are determined from the experimental plots of Δρᵢ versus Φt, and compared with the values of dΔρᵢ/dΦt estimated from the theoretical damage-energy results. Also, the observed stage-I recovery is analysed in terms of the corresponding recovery reported for electron and fast-neutron irradiations. The data analysis also includes a discussion of the relation between the present 16-MeV proton data and published data estimated from a fusion-reactor spectrum.</p> <p>The results presented in this thesis indicate that 16 MeV protons approximate the anticipated damage due to a fusion-reactor spectrum. It is also shown that protons create a damage structure similar to a superposition of the damage structures generated by electrons and fast-neutrons. The sample state of imperfection is shown to influence the induced damage-state in proton irradiation.</p> / Doctor of Philosophy (PhD)

<p>The formation, structure and composition of anodic-oxide films on Be is investigated. According to the choice of electrolyte, porosity, dissolution and crystallinity can be varied almost at will for the formed anodic films. Also the procedure for the anodic sectioning of Be is described and applied to determining the depth distributions of energetic ⁸⁵Kr ions.</p> <p>The presence and the distribution of electrolyte components that are incorporated into the anodic films are investigated by the photon-emission technique. The depth profiles of the incorporated species are correlated with the mechanism of film formation.</p> <p>Finally, a criterion for the structure of anodic oxide films on metals is developed based on the kinetics of the formation and crystallization processes of the various oxides. The criterion is applied and found to work successfully with all systems for which information is available, whereas for unstudied systems it for which information is enables predictions to be made.</p> / Doctor of Philosophy (PhD)