<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)

<p>The influence of superimposed pressure on the damage accumulation processes leading to various tensile fracture modes has been studied on axisymmetric samples of different materials.</p> <p>For ductile fracture damage accumulated in both smooth and notched tensile samples had been characterized in a variety of spheroidized steels tested under superimposed pressures ranging from 0.1 MPa to 1100 MPa by counting the number of voids existing at the cementite-iron interfaces and measuring their area fraction. A quantitative model accounting for the size dependence of void nucleation has been developed to describe these results.</p> <p>The shear fraction mode has been investigated by performing tensile tests on an Al-Zn-Mg alloy under superimposed pressures up to 1100 MPa. The pressure does not affect the onset of shear bands, but changes their growth as it opposes and dilational damage created in the bands. This allows the occurrence of the further plastic deformation process to take place. The microscopic observation of the shear bands, but changes their growth as it opposes any dilational damage created in the bands. This allows the occurrence of the further plastic deformation to take place. The microscopic observation of the shear bands has revealed that they are created by structural softening events, and that they involve a complex spatial correlation of crystallographic slip events over the entire cross-section.</p> <p>Brittle fracture has been studied in both Al-Au and Fe-P alloys, displaying intergranular fracture and transgranular cleavage. The pattern of microcrack development and the changes in fracture modes have been characterized for these two alloys.</p> <p>Finally, two types of fracture maps in stress space are presented, where the various fracture mechanisms are represented by lines, and which can also be used as damage contour maps. These maps are constructed either by considering a complete model of damage accumulation and fracture criterion, or simplified analytical relationships to express damage development and occurrence of failure.</p> / Doctor of Philosophy (PhD)

<p>The oxidation properties of Fe-Al alloys containing 1.5,3,4,5,7,8,12,16,20 atomic percent aluminum were investigated in dry oxygen, wet oxygen, laboratory air and alloy/(Fe+FeAl₂O₄+Al₂O₃) diffusion couples at 1173K. The reaction kinematics were determined thermogravimetrically and by layer thickness measurements. The reacted specimens were analyzed using light microscopy, electron metallography, x-ray diffraction, Auger electron spectroscopy and Mössbauer spectroscopy. Particular interest was given to the growth of Al₂O₃on these alloys. A duplex external scale consisting of Fe₂O₃at gas/scale interface, (Fe,Al)₃O₄ at scale/subscale interface and a subscale of FeAl₂O₄ platelets with Al₂O₃tips grew on alloys containing up to 5 a/o Al oxidized in dry oxygen. The growth of the FeAl₂O₄-Al₂O₃ platelet precipitates, which is interpreted by a (Fe,Al)₃O₄/Al (alloy) solid state displacement reaction, is controlled by oxygen diffusion torugh the subscale in the very early stage of oxidation. A oxidation continues, the oxygen pressre at the scale/subscale interface decreases due to increase of aluminum concentration of (Fe,Al)₃O₄ by dissolution of FeAl₂O₄ precipitates. The diffusion outward from the bulk alloy plays and important role as the velocity of precipitation front decreased by decreasing oxygen activity which leads to the development of a continuous Al₂O₃film at the subscale front in Fe-5 a/o Al alloy and isolation of the subscale from the alloy substrate. Multilayered scale consisting of Fe₂O, Fe₃O, FeO+Fe₃O, FeO+FeAl₂O₄ and subscale grew on alloys containing up to 5 a/o Al alloy oxidized in air or wet oxygen. The FeO+FeAl₂O₄ layer and subscale grew by inward oxygen migration generated by dissociation of wustite. The high rate of oxygen transfer inward with hydrogen generated by dissociation of water vapour, accelerated the oxidation processes and did not allow aluminum diffusion outward from the bulk of the alloy to affect the oxidation behavior. For Fe-7 and 8 a/o A alloys, a thin duplex scale was formed consisting of Fe₂O₃ layer at oxygen/scale interface and Al₂O₃ layer at scale/alloy interface. An oxidation model was advanced to describe the scale, development involving iron diffusion through leakage paths in the Al₂O₃ formed initially to form Fe₂O₃ layer at the scale/gas interface. Only the Al₂O₃ is formed on Fe-12, 16 and 20 a/o Al alloys. An oxidation mechanism is proposed based on inward oxygen diffusion down the grain boundaries and outward aluminum diffusion to account for the formation of hollow nodules in the scale. Alloys containing 3, 5, 7 and 8 a/o Al oxidized internally when coupled with (Fe+FeAl₂O₄+Al₂O₃) mixture. The Fe-3 a/o Al alloy oxidized linearly but the Fe-5, 7, and 8 a/o Al alloys at internal oxidation zone/alloy interface. However, an Al₂O₃ film was formed at (Fe+FeAl₂O₄+Al₂O₃)/alloy interface with no internal oxidation in Fe-12 a/o Al alloy diffusion couple. Alloy composition at which transition from internal to external scale formation of Al₂O₃ occurs, is calculated using available models and compared with the experimental results.</p> / Doctor of Philosophy (PhD)

<p>This thesis treats the discontinuous mode of precipitation in Al-22 and 28 at .8 Zn. High resolution STEM x-ray microanalysis has been used to measure the composition profiles in individual depleted lamella formed under isothermal conditions. These data, combined with local values of cell velocity and interlamellar spacing, have been used to evaluatethe kinetic and thermodynamic quantities of main concern. It is shown that the reaction is grain boundary-diffusion controlled and depending of individual grain boundary characteristics. The driving force for grain boundary migration and possible free-energy sinks are discussed. The results are compared with predictions of existing theories.</p> <p>The problem of the optimal interlamellar spacing is addressed, mainly from the experimental point of view. Experimental data for Al-Zn and Mg-Al alloys are analyzed and related to the discussion.</p> <p>The problem of the optimal interlamellar spacing is addressed, mainly from the experimental point of view. Experimental data for Al-Zn and Mg-Al alloys are analyzed and related to the discussion.</p> <p>It is demonstrated that initiation involves diffusion induced grain boundary displacements, and that during the growth process a significant amount of elastic energy is stored in the product phases. The dissolution of the reheated lamellar structure is volume diffusion-controlled.</p> / Doctor of Philosophy (PhD)

<p>A theoretical and experimental study has been made which indicates the importance of the spatial distribution of damage and a complete physical description of the operative damage processes in providing a fracture condition. A continuum damage model is presented which investigates the influence of void coalescence in accelerating damage levels, for a random array of voids growing via a ductile hole growth mechanism. A simple geometric coalescence condition was assumed. The dependence of fracture strain upon initial void volume fraction and stress state history is predicted. This approach is extended in a simulation to predict the void size distribution which develops. The Dirichlet tessellation was evaluated as a method for characterizing dispersions of points or particles. Point dispersions were generated, ranging from strongly periodic to strongly clustered, and properties of their associated tessellations were evaluated. This suggested parameters which were sensitive indicators of periodicity or clustering in a dispersion. this approach was extended to the characterization of inclusion and particle distributions in steels and several other alloys. The influence of temperature and stress on damage and fracture was studied in several continuously-cast HSLA steels. Fracture was controlled by the inclusions, the centre-line transformation products, or the interaction between these two sources of damage. Observations also suggested that the distribution of the banded transformation products could influence damage levels in the ferritic regions. This study concludes with an investigation of the creep fracture in a Ni-Sn alloy, which contained an inhomogeneous tin distribution. The regions with high tin content accumulated damage rapidly, however, ductility was promoted by the material low in tin which did not cavitate as readily. The influence of stress level on the strain dependence of damage accumulation rate, and the shift to surface crack-controlled fracture was interpreted as an effect of the enhanced relative contribution of grain boundary sliding to total strain at lower stresses.</p> / Doctor of Philosophy (PhD)