<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)

<p>In this thesis the scattering of hydrogen and helium ions, in the incident particle energy range 5-15 keV, from silicon targets is studied both theoretically and experimentally. In the theoretical treatment, electronic and nuclear energy losses are included in a computer simulation model. The computer simulation technique has been developed to study the interaction of the incident ions and target atoms for an incident reduced energy range 0.05<ε₀<20 of interest in fusion research. The theoretical model is based on a Thomas-Fermi interatomic potential and experimental values for the electronic energy loss coefficient. The backscattered energy and angular distributions are calculated by following the histories of 5,000 to 10,000 particles. An experimental system using the time-of-flight technique has been developed to measure the energy spectra of light ions and neutrals backscattered from solid surfaces thus permitting a comparison of the theory with experimental data for a well characterised target. To test the time of-flight technique, ions and neutrals, were scattered from thin silver films (20-40 Å) evaporated on silicon substrates. From the measurements it was found that the sensitivity of the technique is better than 1/20 of a monolayer for silver on silicon. The energy resolution for helium scattered from these targets was found to be 10-15% which is consistent with the time-of-flight resolution and the electronic-loss energy broadening. To measure the energy of the backscattered particles, time-of-flight spectra for hydrogen and helium scattered from silicon were recorded. These spectra were then transformed to energy spectra. The comparison between the experimental and theoretical results showed good agreement down to a backscattered energy of 500 eV (cut-off energy due to detector calibration limits). A number of experiments were done to measure the charge fractions of the backscattered H and He particles from a silicon target. For hydrogen scattering no surface peak were observed and also there was no penetration effect on the charge fractions. This is an indication that hydrogen neutralization occurs at the surface. On the other hand, in the helium case peaks were observed in the charge fractions corresponding to surface scattering. This indicates that helium bound states occur inside the target. Finally, the effect of surface cleaning was studied. The results showed that a factor of 2 increase in ion yield was observed after cleaning the surface by 5 keV neon bombardment. The reduction in the charge fraction for the practical surface was mainly due to carbon and oxygen impurities on the surface.</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)

<p>This thesis is concerned with the cooperative growth problem of the discontinuous precipitation reaction of rod-like particles and their subsequent spheroidization into rows. Previous theories are examined, and the details of a model for the transformation, which assumes the existence of local equilibrium between the product phases,are derived. The steady state diffusion equation is solved under the assumption of boundary diffusion controlled growth in the case of rod symmetry. The available kinetic data and auxiliary information are used to test the model. The results are consistent with the premise that a boundary friction term determines the rate of interface migration. The electron microscopy is used to determine the precipitate morphology and their size and distribution. The results are interpreted in the light of existing knowledge and concepts especially developed for the purpose of this investigation.</p> / Doctor of Philosophy (PhD)

<p>The influence of the elastic interaction energy of plate-shaped precipitates causing tetragonal distortion on nucleation, growth and coarsening is considered.</p> <p>The analysis shows that the elastic interaction energy may be minimized by the formation of regular three-dimensional arrays, which emphasize the "edge-face" configuration. Algebraic conditions of stability against coarsening are developed, and the results of the detailed numerical test of stability against coarsening for a particularly simple and symmetric array are reported. Experimental observations on θ" (Al-3%Cu alloy) microstructures show strong evidence of short-range ordering, with two characteristic configurations (edge-face and "parallel-step"), both energetically favourable.</p> <p>It is shown that the elastic interaction energy may have a decisive role in the nucleation stage of θ' preoipitates. The θ' microstructure, in a relatively early stage of development, is often inhomogeneous and consists mainly of linear (parallel inclined and cross-like) stacks.</p> <p>Numerical calculations of the elastic interaction energy show that the experimentally observed stacks are energetically favourable, and can be generated in an auto-catalytic way. Numerical and preliminary experimental results on the stability against growth and coarsening of elastically-locked linear arrays are reported.</p> <p>Finally, the chemical driving force and elastic retarding force acting on moving ledges on a planar interphase boundary are considered. It is shown that elastic interaction may cause a departure from the local equilibrium at a moving growth ledge, and may dictate the location of "homogeneous" nucleation of growth ledges. Some experimental results concerning ledge nucleation and ledge interaction are reported.</p> / Doctor of Philosophy (PhD)

<p>The sulfidation properties of the Fe-Al alloys containing 6, 9, 18 and 28 atomic percent Al were investigated in alloy/FeS diffusion couples, in sulfur vapour at the dissociation pressure of the FeS and in H₂S+H atmosphereat 1173K. The reaction kinetics were determined thermogravitmetrically and by layer thickness. measurements. The reaction specimens were analyzed using light microscopy, X-ray diffraction and electron metallographic techniques (SEM, EPMA) along with EDAX spectrometry. Particular interest was given to the mode of precipitation and growth of sulfide phases. Diffusion coupling of Fe-Al alloys with FeS in the form of compacts results in periodic precipitation of sulfide (FeS+FeAl₂S₄) bands in the alloy. The thickness of sulfide bands increases with depth satisfying the Jablczynski's relationship for Liesegang type of precipitation process. The initial stage of sulfide band formation is explained by a model involving precipitation of Al₂S₃which is subsequently converted to FeAl₂S₄. Growth of each sulfide band or layer is supported by raid diffusion of iron and aluminum in the FeS phase. It is suggested that aluminum depletion from the alloy in the front of a sulfide layer ultimately leads to its cessation of growth. A new sulfide band begins to form at a distance ahead of the sulfide layer. where the concentration product for the precipitation of Al₂S₃ is satisfied. The mode of precipitation of sulfide changes from bands parallel to the original alloy surface to the platelets normal to alloy surface when Fe-6, 9 and 18 Al alloys are sulfidized in sulfur vapour supplied by a mixture of Fe and FeS. The acicular internal sulfide precipitates of FeAl₂S₄ and Al₂S₃ in these alloys are elongated in the growth direction. Their growth is interpreted by a model involving enhanced sulfur diffusion along the incoherent interfaces between the internal sulfides and alloy matrix. The alloy composition at which transition from the internal to external scale formation of Al₂S₃occurs, is calculated using available models and compared with the experimental results. Fe-9 Al alloy sulfidizes parabolically in H₂S+H atmospheres giving rise to a scale consisting of an outer Al-doped FeS layer, an inner FeS+FeAl₂S₄ layer and Al₂S₃+alloy internal precipitation zone. the Fe-18 Al alloy is sulfidized by a two-stage kinetics. In the initial stage, growth of the FeS+FeAl₂S₄ nodules is observed accompanied by the internal sulfidation beneath these nodules. The final stage of the reaction curve commences when an inner film of Al₂S₃ forms at the external scale/alloy interface. Models based upon diffusion and the thermochemistry of the sulfidation reactions are advanced to account for the reaction kinetics and sulfide morphologies. The ternary Fe-Al-S isothem is determined experimentally and it is used to interpret the scale microstructures which grew by parabolic kinetics.</p> / Doctor of Philosophy (PhD)