<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>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>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)

<p>The nature of the dislocation substructure developed at large strains in single crystals of aluminum has been studied. The detailed characteristics of the substructure have been studied as functions of (a) strain, up to compressive strains of 1.0 and (b) crystal orientation. The crystal orientations have been selected to substructure developed during deformation.</p> <p>The Bishop-Hill and yield subsurface analyses have been adopted to predict the operative slip systems. Applicability of these methods to large strain deformation studies has been discussed.</p> <p>Results of the present study emphasize the influence exerted by crystal orientation via the nature of operative slip systems and the extent of homogeneity of slip on the extent of dynamic recovery and the resulting dislocation substructure. Development of high angle boundaries in the as-deformed condition in crystals deformed to large strains, is associated with inhomogeneity of slip. Origin of these high angle boundaries during deformation and their role in subsequent recrystallization process have been discussed.</p> / Doctor of Philosophy (PhD)

<p>The oxidation properties of Ni-2, Ni-6 and Ni-32wt.%Al alloys were investigated in one atm oxygen at the temperature range 1273 to 1573 K. The reaction kinetics were determined thermogravimetrically and by layer thickness measurements. The reacted specimens were analyzed using light microscopy, X-ray diffraction, X-ray topography and electron metallographic techniques (TEM, SEM, EPMA and AES). Particular interest was given to the growth of Al₂O₃ on these alloys.</p> <p>Ni-2wt.%Al alloy oxidizes parabolically, at a rate one order of magnitude larger than pure Ni, giving rise to a scale consisting of an outer AI-doped NiO layer, an inner NiO-NiAl₂O₄ layer and an Al₂O₃-alloy internal precipitation zone. The growth of the Al₂O₃ rodlike precipitates, which is interpreted by a NiO/Al(alloy) solid state displacement reaction, is controlled by oxygen diffusion through the Al-depleted alloy in the precipitation zone. The increase in the reaction rate is attributed to the doping effect of dissolved Al on NiO and the high affinity of Al for oxygen resulting in internal precipitation.</p> <p>The oxidation kinetics of Ni-6wt.%Al alloy are irreproducible due to the formation of an imperfect Al₂O₃ scale containing NiO nodules, localized at alloy grain boundaries, beneath which Al₂O₃ is precipitated internally. The transition from internal precipitation to continuous Al₂O₃ formation is interpreted by a model involving the impingement of favorably oriented rodlike precipitates and lateral diffusion of Al from the impingement sites to neighboring regions of the precipitation front. The steady oxidation stage is controlled by thickening of the Al₂O₃ film.</p> <p>The initial sub-microcrystalline film, which is formed on Ni-32wt.%Al alloy, "recrytallizes" subsequently to α-Al₂O₃ giving rise to a well oriented film containing regions of disarrayed polycrystalline oxide. Inert marker measurements indicated that the initial film grows by inward oxygen diffusion. The metallographic observations on the growth of the recrystallized α-Al₂O₃ scale are consistent with counter-current Al and oxygen boundary diffusion in the disarrayed oxide and outward Al lattice diffusion in the oriented film. Accordingly, the growth of this scale was interpreted by a short-circuit diffusion model involving simultaneous reactants lattice and boundary diffusion.</p> / Doctor of Philosophy (PhD)