<p>This research has been carried out to understand the relation between work hardening, deformation behaviour, dislocation substructure and crystallographic texture developed during plastic flow in pure Al and continuous cast and direct chill cast AA5754 AI-Mg alloys. Tensile and strain rate sensitivity tests are performed in the temperature range 4.2K-295K; details of the process of dislocation accumulation, the dislocation substructure development and texture evolution are followed using a range of techniques such as TEM, in-situ resistivity measurements and X-ray diffraction. The tensile deformation results show an unprecedented level of strength and unusual work hardening behaviour of AI-alloys at 4.2K. The electrical resistivity data suggest that fracture is initiated by the collapse of the dislocation network at places where dislocations develop a critical spacing for spontaneous annihilation. This spacing is estimated at approximately 8nm at 4.2K for both pure Al and Al alloys and rv12nm at 78K for pure AI. Strain rate sensitivity measurements suggest that deformation of high purity Al is governed by dislocation-dislocation interactions in a broad range of temperatures, whereas Al alloys exhibit a larger thermal component of flow stress due to the presence of solute atoms in the matrix. The texture studies show that, independently of initial texture, the tensile axis of deformed samples rotates to (111) stable end orientation, and the intensity of the (111) peak increases parabolically with the flow stress, also strongly affected by the temperature. The experiments carried out on samples oriented at different angles to the rolling direction indicate that the crystallographic texture is the major factor responsible for the anisotropy of the alloys' mechanical properties. This is attributed to the effect of the operating slip systems activated during tensile deformation. The Visco-Plastic Self Consistent modeling has been carried out to predict mechanical behaviours of AA5754 alloys and to provide insight into operating mechanisms of plastic flow as well as the origin of the anisotropy of mechanical properties. These results indicate that the more effective dislocation storage in the substructure at a low temperature significantly increases the work hardening rate at the later stage of deformation, and the crystallographic texture is the major cause of the anisotropy in the flow stress behaviour.</p> / Doctor of Philosophy (PhD)

<p>The influence of dynamic behaviour of materials (i.e. the response of materials to large strain, high strain rate, deformation under large hydrostatic pressure occurring during metal cutting) on machinability (i.e., chip morphology, tool wear and surface finish) has been investigated in ferrous alloys with a wide range of matrix and volume fraction of second phase particles. With the increase of cutting speed, there is change in the tribological phenomenon at the tool-chip interface from sliding to seizure. A physical model for seizure is proposed based on atomic contact at the tool-chip interface. The model predicts the critical cutting speed for onset of seizure from force measurements. Seizure is said to occur when the normal stress exceeds yield strength of asperities such that the true area of contact approaches the apparent area of contact. The tribological phenomenon of seizure is shown to cause thermoplastic shear localisation. In consequences, the temperature at the tool-chip interface rises, leading to dissolution wear of the tool into the chip by a diffusion mechanism which causes chemical wear of the tool. The technique of ICP-MS has been developed and used to separate the physical and chemical wear aspects of the tool by measurement of minute concentrations of tungsten present in the chips as WC as distinct from tungsten atomically dissolved in the chips. The results have confirmed that chemical crater wear dominates at high cutting speeds. The temperature distribution at the tool-chip interface has been predicted by finite element analysis and used to compute diffusion wear. A comparison of theoretical and experimental values of diffusion wear suggests that high diffusivity paths operate at the tool-chip interface to enhance the diffusivity by more than two orders of magnitude. The maximum depth of the measured crater depth profile has been found to coincide with the phase transformation temperature of the workpiece material rather than the maximum predicted temperature at the tool-chip interface. The amount of dissolution wear, as measured by the amount of tungsten transferred into the chips is attributed to dislocation pipe diffusion. It is further suggested that dislocations generated by deformation concomitant with phase transformation provide high diffusivity paths that contribute to enhanced diffusion wear. The implication is that dissolution crater wear of the tool is phase transformation coupled. Dissolution crater wear can be suppressed if the tribological phenomenon of seizure can be prevented. This can be achieved by in-situ lubrication at the tool-chip interface through inclusion engineering of the workpiece. Alternatively, the diffusion wear can be minimized by coating the tool with a compound which hasa the least solubility in the workpiece. The microstructural response to changes of metal cutting variables during the machining of a range of iron alloys with varying heat treatment condition and microstructural constituents of the matrix has been investigated to establish the inter-relationship among chip morphology, tool wear and surface finish. The microstructural changes in the chips have been analysed by optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffractiion techniques. The results from the chips formed during machining of martenistic Fe-28.9%Ni-0.1%C alloy confirmed the presence of austenite, exhibiting grains as fine as 40-100 nm in the white shear bands. This structure is attributed to a sequence of events: the reverse phase transformation of martensite to austenite, shear localisation, formation of the transformation shear bands, and probably dynamic recrystallisation. This it is demonstrated that thermal softening due to phase transformation causes shear localisation leading to chip segmentation in the primary shear zone. Phenomenological observations are presented to confirm that shear localisation is caused by (i) geometrical softening due to second phase particles (graphite inclusions in cast iron, inclusions in free cutting steels), (ii) thermal softening of the matrix due to phase transformation or recrystallisation and (iii) a combination of the above. Shear localisation causes step temperature rise in a narrow band, referred to as shear band. The interaction of the priary shear band with cutting edge of the tool is found to cause dissolution wear of the cutting edge of the tool. The loss of the cutting edge in turn is shown to impair surface finish. The effect of metal cutting variables, i.e., speed, feed, depth of cut, external lubricants on shear localisation, chip morphology and tool wear is investigated. It is shown that chip segmentation can be suppressed by decreasing the feed. This is analyzed in terms of damage concepts underlying chip fracture behaviour.</p> / Doctor of Philosophy (PhD)

<p>A number of perovskite oxides, typically, heavily doped SrTiO₃ samples, were synthesized and characterized with a view to establishing their potential as anode materials for solid oxide fuel cells (SOFCs). The structure, microstructure, electrical conductivity, reduction-oxidation behavior, phase stability, compatibility with electrolytes, and performance in SOFC operation were assessed.</p> <p>Ceramic samples were prepared with the formula (Sr₁ᵪRᵪ)(Ti₁_yTy)O₃ (R = rare earth elements, T = transition metals) and with charge balance achieved by A-site deficiency. Electrical conductivities were examined by the do four-probe method and impedance spectroscopy. It was found that yttrium is soluble in SrTiO₃ (SYT) up to 8 mol% and has marked effects on conductivity. Electrical conductivities were observed to increase with increasing donor-doping level, on reduction in low oxygen partial pressures. Electrical conductivity with values as high as 82 S/cm was achieved at 800°C and P(O₂) = 10ˉ¹⁹ atm. Electrical conductivities were reversible upon reduction and oxidation. The thermal expansion coefficient is compatible with electrolyte materials such as yttria-stabilized ZrO₂ and doped LaGaO₃. Cobalt-doped SYT, which showed a relatively high resistance to oxidation, was tested as the anode material in a fuel cell. Yttrium-doped SrTiO₃ meets the requirements for the anode in SOFCs to a substantial degree, and is a promising alternative anode material.</p> / Doctor of Philosophy (PhD)

<p>The cathodoluminescence of various thin films deposited on (111)Si substrates was systematically studied for the first time. The best dopants for red, green and blue (RGB) phosphor films are Eu, Tb and Ce, respectively. A program was developed for the calculation of chromaticity coordinates from the luminescent spectra. High luminescence can only be achieved by post-annealing thin films above 900°C for yttrium silicates or above 850°C for zinc silicates. The band-gaps of the thin films were measured by UV absorption spectroscopy.</p> <p>Rare earth (Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, E³⁺) doped Y₂SiO₅, Y₂Si₂O₇, Y₄.₆₇(SiO₄)₃O and Mn²⁺ doped Zn₂SiO₄ thin films were prepared by magnetron sputtering. Their compositions were studied by Rutherford backscattering and X-ray diffraction. Sputtering parameters such as magnetic field, pressure, target-substrate distance and substrate temperature were studied. The chromaticity coordinates and brightness of thin film phosphors were compared with their powder counterparts. These films show potential for practical applications.</p> <p>Electroluminescence was found only in Zn₂SiO₄:Mn²⁺ and Y₂SiO₅:Ce³⁺ thin film phosphors. The devices of ITO/Zn₂SiO₄:Mn²⁺ (or Y₂SiO₅:Ce³⁺)/BaTiO₃/Al and multi-insulated phosphor thin films were prepared and their electroluminescent (EL) response under different voltages, frequencies and pulse widths, and their decay curves and transferred charges were studied. With the same chromaticity coordinates as Sylvania powder CL samples, Zn₂SiO₄:Mn²⁺ (2% mole) thin films are shown to have very good EL performance with a brightness as high as 80 foot lambents (fL) excited by 400 Hz a.c. pulses, with a short decay time of 0.6 ms and with an efficiency of 0.78 lumin/watt.</p> / Doctor of Philosophy (PhD)

<p>A systematic study of the influence of oxygen partial pressure, temperature and time on 110K phase formation in the BiPbSrCaCuO system has been undertaken. The optimum nominal composition to promote the Bi₂Sr₂Ca₂Cu₃Ow (2223, 110K) phase, ie; Bi₁.₈₄Pb₀.₃₄Sr₁.₉₁Ca₂.₀₀Cu₃.₀₄Ow was studied. The relationships between 110K phase fraction, oxygen partial pressure, sintering temperature and time has been mapped for this composition via experimental data. The preferred conditions for 110K phase formation were identified as 0.01 atm≤Po₂≤0.35 atm, at 820≤T≤880°C for ≤36 hours with an intermediate grinding of the material every 6 hours. The optimum conditions for pure 110K phase were Po₂=0.08 atm at 845-865°C for 12 hours with grinding every 6 hours. The oxygen partial pressure range for single 110K phase stability was determined as 0.001 atm</p> / Doctor of Philosophy (PhD)

<p>Heavily deformed, two-phase materials have been shown to exhibit strengths well in excess of the rule-of-mixtures prediction using the bulk properties of the constituents. The work described in this thesis investigates a number of aspects regarding high-strength, high-conductivity, two-phase materials. Microstructural observations and mechanical testing were used to examine the process of co-deformation, the evolution of mechanical properties, and the thermal stability of these heavily deformed structures. Materials selection procedures for the design of high-field magnet coils were developed.</p> <p>The sustained co-deformation of the two-phases leads to the conclusion that the second phase behaves as a shearable obstacle to dislocation motion. The increase in strength displayed by the two-phase material over the bulk constituent strengths may be explained by existing models of particle shearing if increased interfacial energy due to residual stress is accounted for. The increased work hardening displayed by copper-silver alloys subjected to intermediate annealing treatments is explained by a continuum description of the evolution of slip line length.</p> <p>The combination of materials selection procedures and investigations into the strengthening and resistive mechanisms have led to a new approach to the design of non-uniform composite materials to optimize strength and resistivity in two-phase materials.</p> / Doctor of Philosophy (PhD)

<p>Two aspects of the electrical and light scattering properties of various epoxide based thermosets have been studied. In the first, the isothermal curing kinetics of the thermosets have been measured at several temperatures by dielectric spectroscopy and Brillouin scattering measurements. During the process of curing, the dc conductivity decreases according to a scaling law, δ₀ ∝ (tg-t)ˣ, or equivalently to a new equation, σ₀ ∝ exp[-B/(t₀-t)], and approaches zero on gelformation. Concomitantly, the time for the dipolar relaxation process becomes progressively longer and the dielectric permittivity becomes dominated by dipolar relaxation processes. The time dependence of the complex permittivity follows the formalism, ϕ(t)=exp-(t/τcure)ˠ with ɣ<0.4. ɣ decreases as the curing temperature is increased and tends towards a limiting value at a high curing temperature. The relaxation time increases on curing and the rate of its increase with respect to curing time first reaches a maximum and then decreases towards zero. This phenomenon is a manifestation of the rates of chemical processes which control the extent of cure. The initially broad Brillouin peak becomes narrow and shifts towards higher frequencies with the curing. The changes in the hypersonic velocity and attenuation during the curing of a thermoset correspond to the changes observed in the dielectric studies.</p> <p>In the second, both the sub-Tg and main relaxation processes of the thermosets were measured by dielectric spectrometry and their dependence on the curing and ageing were investigated. Amongst the two sub-Tg relaxation processes, the low temperature process is initially prominent and its strength decreases on both curing and ageing of a thermoset. The strength of the high temperature sub-Tg process initially increases, reaches a maximum value and then decreases on further ageing. A concept of accumulated equivalent curing time is introduced and theoretically justified for use in the investigation of the curing of thermosets, and a general method for obtaining the asymmetric distribution of relaxation times parameter from limited relaxation data is developed. For the sub-Tg relaxations, the calculated parameter remains constant during the curing process, but for the main relaxation it monotonically decreases towards a limiting value. The theoretical analysis developed here is generally applicable to phenomena where molecular diffusion allows a chemical reaction to occur, which in turn retards molecular diffusion which slows the chemical reactions, until a material reaches its vitreous state and both the diffusion and chemical reactions cease to occur over ones experimental time scale.</p> / Doctor of Philosophy (PhD)

<p>The intermetallic compound Al₁₃Fe₄ was rapidly solidified at a velocity in excess of 2 to 5m/s by a pulsed laser melting technique. Resolidification after laser melting occurred epitaxially by a stepwise growth process. The phase resulting from this growth process was determined by electron microscopy, to be a faulted slightly disordered variant of the stable compound. The chemical long-range order parameter of this phase was estimated to be 0.6 to 0.7 from the relative intensities of HOLZ spots in an electron diffraction pattern. A model relating order parameter to solidification velocity was developed to explain these results. The measured order parameter of the rapidly solidified Al₁₃Fe₄ was in agreement with the model predictions. A modification of the same model successfully explains the observed velocity at which the intermetallic compound ϒNi₃Al solidified without long-range order. An untested prediction of the general model is that above a critical velocity, estimated to be equal to between 0.5 and 3 times the liquid diffusion coefficient divided by the spacing of close-packed planes in the compound, a compound with long-range order will not form. This model is expected to apply to all types of intermetallic compounds except those in which size factor effects dominate the structure. The results of this work show that rapid solidification experiments on intermetallic compounds can be used to examine the melt-crystal interface kinetics.</p> <p>Experimental problems associated with pulsed laser melting experiments on intermetallic compounds were also examined in detail. To do this, artifacts from the starting material and from the TEM sample preparation processes were identified and characterized. From this characterization, three important things were learned. Firstly, laser melting experiments on thin films of intermetallic compounds are not equivalent to laser melting experiments on the bulk compound. This was determined by doing laser melting experiments on thin films of Al₁₃Fe₄ on rock salt, which resolidified by nucleation and growth from the melt, in contrast with the bulk compound, which resolidified epitaxially after laser melting. Secondly, the mechanism of laser induced topography developrrent on intermetallic compounds was examined, and found to be a spalling process caused by the thermal contraction on solidification. Thirdly, artifacts from ion thinning were characterized. The most important of these, 10nm grains of redeposited sputtered material on the sample, is difficult to separate from the solidification microstructure without the aid of cross section samples, and thus, may be misinterpreted as nucleation and growth in the melt.</p> / Doctor of Philosophy (PhD)

<p>The drive to produce materials with novel or beneficial combinations of properties has prompted research into a range of new materials and processing routes. In many applications one of the important design variables is the mechanical strength. Exceptional strengths can be achieved in certain materials consisting of two deformable phases when they are drawn into fine wires or rolled into thin sheets, the common example being pearlitic steel wire which can achieve strengths in excess of 5 GPa. The mechanisms that permit co-deformation and result in the observed strengthening are, however, not well understood. In this thesis an approach was adopted whereby co-deformation of a well characterized model material has been studied primarily using uniaxial tensile tests. Directional solidification of a Cu-1.56at%Cr eutectic alloy has been used to produce material consisting of submicron diameter single crystals of Cr embedded within a polycrystalline Cu matrix. It has been shown that these two phases exhibit preferred crystallographic orientation relationships, habit planes and growth directions the same as those found for solid state precipitates of Cr in Cu. On deforming this material it is found that the Cr fibres yield at stresses close to the theoretical limit. However, their are able to continue to co-deform with the Cu matrix to large plastic strains. This process of co-deformation is observed to cause a rate of nearly constant work hardening that results in both high strength and high ductility. This behaviour has been attributed to the fact that the Cr fibres continue to carry increasing elastic strain beyond their yield thereby contributing to an increasing level of internal stress in the material. It is suggested that this mechanism may play an important role in other co-deformed two phase materials. In particular, it is suggested that this may provide one mechanism for the continued high rate of work hardening in heavily co-deformed two phase materials.</p> / Doctor of Philosophy (PhD)

<p>Ceramics have material properties that make them useful for many industrial applications. They are strong, hard, and chemically inert. Their refractoriness gives them an advantage over metals and polymers for use at high temperature. Unfortunately, the inherent brittleness of ceramics limits their use in structural applications.</p> <p>One way to improve the toughness of ceramics is to combine them with other materials to make composites. The correct combination of materials can lead to synergism, and a significant improvement in properties. In this work, brittle laminates that contain weak interlayers are considered. The weak interlayers lead to crack deflection, and can result in non-catastrophic failure of the material. The requirements for consistent crack deflection and non-catastrophic failure are not fully understood.</p> <p>This work is an attempt to explain the observed fracture behaviour in brittle laminar composites that contain weak interlayers. A combination of experimental work, fracture mechanics modeling and finite element modeling has been used to predict the requirements necessary for non-catastrophic failure.</p> <p>The work shows the size of flaws in the surface of the composite, in the weak interlayer, and in subsequent strong layers in the material, all play an important role in the fracture behaviour. Control and understanding of the effect of the various flaw sizes can be used to achieve non-catastrophic failure and increased work of fracture in these composites.</p> / Doctor of Philosophy (PhD)