<p>An improved theory of vacancy formation energy and volume, based on local pseudopotential theory, is presented and applied to vacancies in the alkali metals and in aluminum. Good agreement with the existing experimental results is obtained.</p> <p>The solubility of hydrogen in magnesium has been measured in the temperature interval 400°C - 600°C and for pressures 50 - 760 mmHg, from which the heat of solution of hydrogen has been determined.</p> <p>A theory of the heat of solution of hydrogen is developed. The host lattice is treated with the local pseudopotential theory.</p> <p>Nonlinear theory is used for the screening of the proton and a method for an approximate self-consistent solution proposed. The theoretical results for the heats of solution in aluminum and magnesium are in good agreement with the experimental values, particularly when one considers the magnitude of the numbers involved.</p> / Doctor of Philosophy (PhD)

<p>The temperature and time dependence of the thermoelastic martensitic transformation in Cu-Zn-Al shape memory alloys have been studied by means of internal friction, DSC (Differential Scanning Calorimetry), X-ray diffraction and metallography. A physical model and mathematical treatment have been developed to derive an expression for internal friction associated with the thermoelastic martensitic transformation. This expression has been able to explain the amplitude dependence of the internal friction in various ranges of stress amplitude, where other theories have failed. In the special case where the applied stress amplitude is much higher than the critical stresses for the stress-induced martensitic transformation, the internal friction is found to be linearly proportional to the reciprocal of the stress amplitude, in good agreement with the experimental results. The time evolution of internal friction and shape change has been measured simultaneously in different microstructural states of the shape memory alloys. In the single-martensite phase state, the monotonic decrease of internal friction with time, without any accompanying shape change, has been attributed to the progressive pinning of the martensite/martensite interfaces. When the parent phase is present, the appearance of an internal friction peak, accompanied by an evident shape change, is found to be associated with the stress-induced martensitic transformation and the subsequent pinning of the martensite/parent and martensite/martensite interfaces.</p> <p>The effects of aging in both martensite and parent phases have been studied. Aging in parent phase caused the precipitation of $\alpha$-phase which hindered the subsequent martensitic transformation and resulted in a suppressed internal friction peak and a deteriorated DSC peak. Aging in the martensite phase resulted in the pinning of the martensite/martensite interfaces by vacancies, which significantly obstructed the subsequent transformations upon heating and cooling. This has been evidenced by both internal friction and DSC measurements.</p> / Doctor of Philosophy (PhD)

<p>SnO₂ films have been prepared by reactive sputtering in an oxygen-containing glow discharge, by ion-beam sputtering in an accelerator, and by anodizing with an ethylen-glycol-based electrolyte. Preparation of SnO₂ films by bombarding metallic tin with oxygen ions has also been attempted. The sputter-deposited films were amorphous if deposited on KCl or Ta at temperatures from -100° to +200°C, though they were crystalline if deposited on SnO₂ at 200°C. The anodic films were variously crystalline or microcrystalline, though clearly not amorphous. Films formed by bombarding metallic tin with oxygen ions turned out to be crystalline α-SnO at all doses from 1 x 10¹⁵ to 3 x 10¹⁷ ions/cm². The crystallization temperatures ranged from <200°C for crystalline SnO₂ substrates, to 250°-300°C for KCl substrates, to 400°-450°C for Ta substrates, to 475°-550°C for unsupported films. The crystallization product was consistently cassiterite, i.e., normal SnO₂. Once crystallinity was attained in unsupported sputtered films, the grain size remained in the vicinity of 400 Å to a temperature of about 1000°C. This is in good agreement with the behaviour of anodic films; which retained their microcrystalline structure in heat treatment again until a temperature of about 1000°C was reached. The thickness of the reactively sputtered films was estimated from the observation of their interference colours when deposited on a Ta substrate, while the thickness of the anodic films was estimated by sputtering the films with 20-keV Kr ions until metal was exposed and noting the weight change. The anodization of Sn has a rather low efficiency (4-21%), and this could be shown to be due to electronic conduction rather than dissolution. Kr-ion bombardment of SnO₂ thin films has shown that this oxide presents a very high value of the sputtering coefficient (21.5 ± 1.5 atoms/ion for 10-keV Kr impact), a result which can be attributed to thermal sputtering (i.e., bombardment induced vaporization). As far as the structure of the target material after bombardment is concerned, the experimental evidence was that SnO₂ undergoes amorphization. The effects of ion bombardment of bulk samples (either SnO₂ sintered powder or natural cassaterite) were investigated using the following techniques: (I) reflection electron diffraction, (II) marker-release spectrometry, (III) dissolution measurements, and (IV) resistivity measurements. Through the use of techniques I and II we have established that the annealing (to the original singlecrystalline phase) of bombardment induced amorphousness occurs in two stages: (i) a homogeneous transformation to a polycrystalline phase showing the normal cassiterite structure; (ii) epitaxial recrystallization to single-crystal cassiterite. Dissolution measurements have established the depth of amorphization due to ion bombardment, while resistivity measurements have indicated that bombardment does not cause SnO₂ to change its stoichiometry. Finally the various results allowed us to expand the formalism on which marker-release spectrometry is based, namely we have identified a previously overlooked release process (Stage IC) due to epitaxial crystallization.</p> / Doctor of Philosophy (PhD)

<p>The mechanical behaviour and the microstructural changes preceeding fatigue fracture have been investigated in ferrous systems subjected to controlled cyclic straining. Attention has been focussed on three basic aspects of the fatigue process:</p> <p>(1) The Bauschinger Effect and the Polarisation of Plastic Flow in Single- and Two-Phase Iron-Based Alloys: the investigation involved the study of the influence of dislocation distribution on the magnitude of the Bauschinger effect, and the study of the Bauschinger effect in two-phase materials as a function of both forward strain and volume fraction of particles. The results are interpreted in terms of some of the currently available theories for the deformation in plastically inhomogeneous materials.</p> <p>(2) Fatigue Softening in Single-Phase Low-Carbon Iron: the investigation involved the study of the microstructural changes occurring in prestrained specimens during cyclic softening, and the influence of initial dislocation arrangement on the rate and extent of fatigue softening. Simple models were developed to rationalize the kinetics of fatigue softening.</p> <p>(3) Instability of Plastic Flow in Quenched-Aged Armco Iron: the investigation involved the study of the influence of various particle dispersions on the stability of plastic flow in fatigue, and the detailed microstructural features developing in fatigue which lead to the localization of plastic flow and onset of fatigue cracks. The problem of instability has been rationalized, macroscopically, in terms of the mechanical equation of state, and microscopically, in terms of the detailed structure of the localized regions.</p> <p>Structural changes were monitored by Transmission Electron Microscopy and ancillary techniques in order to provide a detailed characterisation of the fatigue state.</p> / Doctor of Philosophy (PhD)

<p>The present work has examined the role of shear bands, distribution of second-phase particles, and changes in strain path on deformation and recrystallization processes in single phase and two-phase Al alloys.</p> <p>The development of shear bands and their influence on the overall recrystallization kinetics in an Al-4.8% Mg alloy was studied at both the macroscopic and microscopic level. Shear bands which contained high angle boundaries acted as preferential nucleation sites for recrystallization nucleation, and influenced the early states of recrystallization kinetics.</p> <p>The evolution of the substructure around non-deformable silicon particles was studied in Al-Si-Cu single crystals which were oriented for single and mutiple slips. The relationship of the local substructure around particles was related to the subsequent recrystallization behaviour by using electron diffraction. The influence of the spatial distribution of second-phase particles was correlated to the occurrence of recrystallization nuclei in particle-stimulated nucleation (PSN) by using the Dirichlet cell technique. PSN appears to involve only a small fraction of particles in the distribution, indicating that recrystallization nucleation depends on particle clustering as well as particle size and the level of imposed strain.</p> <p>The influence of change in strain path on mechanical response (deformation and strain hardening) and microstructural evolution during deformation and recrystallization processes were examined. The rate of dynamic recovery appears to be enhanced by the change in strain path. The change in strain path is reflected in changes in recrystallization behaviour and can be rationalized in terms of the energy stored by various strain paths. An attempt has been made to correlate the experimental findings with phenomenological models of dynamic recovery available in the literature.</p> / Doctor of Philosophy (PhD)

<p>Measurements of the internal, friction of several aluminum alloys were made over the temperature range from 80 to 300 K using the two component resonator technique. The technique was improved, by using a metallic bond between the two components, so that measurements in the Qˉ¹ ≃ 10ˉ⁶ range could be made at three frequencies. Preliminary experiments using quartz crystals as specimens were performed in order to check the properties of the metallic joint. A method of checking the bond, by measuring the resonator properties at about half the fundamental frequency of the quartz transducer alone, was developed.</p> <p>Comparisons of the internal friction spectra of hydrogen charged samples with spectra of vacuum annealed samples were used, in conjunction with a simple model of the distribution of hydrogen, to determine upper limits on the binding energy between hydrogen and substitutional solutes in the aluminum matrix.</p> / Doctor of Philosophy (PhD)

<p>Diffusion of nitrogen in zirconium nitride was studied using the l5N gas-solid exchange technique.</p> <p>Two kinds of samples were used: ZrN 0.71-0.79 spheres, 60u and 90u diameter and ZrN 0.93 slabs 254u thick. The spheres were prepared by dropping 99% powdered zirconium nitride through an electric arc while the slabs were obtained by direct nitridation of 99.995% zirconium sheets at 1150°C in very pure nitrogen at 1 atm. These samples were carefully characterized by chemical analysis, density and lattice parameter measurements, electron microanalysis, X-ray diffraction and scanning electron microscopy techniques.</p> <p>The diffusion annealings were performed at a pressure of 200 Torr and in the temperature range 1000-1200°C. The rate of depletion of 15N in the isotopically enriched gas phase was measured by a mass spectrometer and used to calculate the self-diffusion coefficient.</p> <p>The diffusion coefficients for spheres and slabs could be respectively represented by</p> <p>Dspheres(cm^2/sec) = (2.97+18.76-2.56)x 10^-10 exp (-(23,000+/-5,400)/RT)</p> <p>and</p> <p>Dslabs(cm^2/sec) = (4.08+88.23-3.90) x 10^-6 exp(-(36,000+/-4,500)/RT)</p> <p>These results are in good agreement with the chemical diffusion coefficients available in the literature. It is shown that the rather low activation energies, in particular in the case of spheres, do not preclude a vacancy mechanism for nitrogen diffusion.</p> / Master of Science (MS)

<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)