Aluminium alloys suffer from localised attack termed exfoliation corrosion which is intergranular in nature where the attack proceeds around the elongated grains present in the microstructure. The formation of a more voluminous corrosion product than the original material results in the lifting of grains giving the characteristic of exfoliation corrosion - lamellar attack. This results in surface degredation in the form of pitting, flaking or blistering. The precipitates present within the microstructure and the grain shape can alter how susceptible the material is to exfoliation corrosion. The distribution of precipitates present can be altered by heat treatment which affects the exfoliation corrosion susceptibility. A study on the effects of heat treatment and grain shape on exfoliation corrosion has been completed for two AlCuMg alloys (2014 and 2024) and two aluminium - lithium alloys (8090 and 2091). A MASTMAASIS salt - spray cabinet was used to determine the exfoliation corrosion susceptibility of all the alloys. A four - point bend test was developed to determine the depth of penetration and compared to depths measured metallographically. The results for the 2XXX series alloys suggest that as the grain aspect ratio increases the exfoliation susceptibility increases. Although the aspect ratio did not differ greatly between each alloy, the different phases present in the two alloys seem to contribute more markedly to the susceptibility. Heat treatment in the 2024 alloy suggested that the peak - aged condition gave a higher susceptibility to corrosion than the under - aged and over - aged conditions. Results indicated that the heat treatment has a greater effect on exfoliation corrosion than the grain aspect ratio, showing that the grain boundary microstructure is an important factor in the exfoliation process. The four - point bend test tended to underestimate the depth of attack by virtue of measuring the whole area of the specimen whereas metallographic measurements only took into account the attacked areas. End grain testing suggested that the attack can not only proceed longitudinally but also "short - circuit" down the transverse path of the elongated grains in the 2XXX series alloys. For the aluminium - lithium alloys the over - aged condition showed more resistance to attack than the under - aged and peak - aged conditions but the under - aged was more resistant to attack for the 2091 alloy. Stress corrosion cracking showed trends similar to exfoliation corrosion.

As current aerospace materials are subjected in service to increasingly onerous conditions of stress and temperature, the hazard of fatigue failure becomes more acute. Engineers utilise the methodology of fracture mechanics to estimate fatigue crack growth rates but fatigue crack initiation, which involves the interplay of many microprocesses, is only investigated empirically. The aim of this study was to investigate the fatigue damage accumulation mechanisms in the titanium alloy IMI 834 in order to develop a fundamental understanding of the controlling physical processes and the micromechanisms which occur at the dislocation level. Load controlled four point bend test specimens of IMI 834 were cyclically fatigued to failure with an R ratio of 0.1 over a range of maximum stress levels and the fatigue and fracture surfaces were examined by optical and scanning electron microscopy. The examination of cross-sectional foils prepared from the fatigue surface enabled the fatigue damage to be examined in the T.E.N. as a function of orientation and depth below the specimen surface. The distribution, orientation and type of slip bands were identified in the primary-a and the transformed-fJ grains, and their interaction with secondary phases, precipitates and grain boundaries was determined. The results show that fatigue damage accumulation in INI 834 occurs primarily on basal slip bands in the primary-a phase and on basal and prismatic slip bands in the transformed-fJ phase. The segregation of a-stabilising elements to the primary-a phase during alloy processing allows the formation of an ordered phase which increases the propensity for planar slip on the basal plane. A mechanism for fatigue crack initiation along this plane is proposed. In addition, the occurrence and identification of an interface phase is discussed in the light of current theories regarding this phase.

No description available.

A pin on disc wear machine has been used to study the oxidational wear of low alloy steel in a series of experiments which were carried out under dry wear sliding conditions at range of loads from 11.28 to 49.05 N and three sliding speeds of 2 m/s, 3.5 m/s and 5 m/s, in atmosphere of air, Ar, CO2, 100% O2, 20% O2-80% Ar and 2% O2-98% Ar. Also, the experiments were conducted to study frictional force, surface and contact temperatures and surface parameters of the wearing pins. The wear debris was examined using x-ray diffraction technique for the identification of compounds produced by the wear process. Scanning electron microscopy was employed to study the topographical features of worn pins and to measure the thickness of the oxide films. Microhardness tests were carried out to investigate the influence of the sub-surface microhardness in tribological conditions. Under all loads, speeds and atmospheres parabolic oxidation growth was observed on worn surface, although such growth is dependent on the concentration of oxygen in the atmospheres employed. These atmospheres are shown to influence wear rate and coefficient of friction with change in applied load. The nature of the atmosphere also has influence on surface and contact temperatures as determined from heat flow analysis. Unlubricated wear debris was found to be a mixture of Fe_2O_3, Fe_3O_4 and FeO oxide. A model has been proposed for tribo-oxide growth demonstrating the importance of diffusion rate and oxygen partial pressure, in the oxidation processes and thus in determination of wear rates.

An investigation was undertaken to study the effect of poor curing simulating hot climatic conditions and remedies on the durability of steel in concrete. Three different curing environments were used i.e. (1) Saturated Ca(OH)2 solution at 20oC, (2) Saturated Ca(OH)2 solution at 50oC and (3) Air at 50oC at 30% relative humidity. The third curing condition corresponding to the temperature and relative humidity typical of Middle Eastern Countries. The nature of the hardened cement paste matrix, cured under the above conditions was studied by means of Mercury Intrusion Porosimetry for measuring pore size distribution. The results were represented as total pore volume and initial pore entry diameter. The Scanning Electron Microscope was used to look at morphological changes during hydration, which were compared to the Mercury Intrusion Porosimetry results. X-ray defraction and Differential Thermal Analysis techniques were also employed for looking at any phase transformations. Polymer impregnation was used to reduce the porosity of the hardened cement pastes, especially in the case of the poorly cured samples. Carbonation rates of unimpregnated and impregnated cements were determined. Chloride diffusion studies were also undertaken to establish the effect of polymer impregnation and blending of the cements. Finally the corrosion behaviour of embedded steel bars was determined by the technique of Linear Polarisation. The steel was embedded in both untreated and polymer impregnated hardened cement pastes placed in either a solution containing NaCl or an environmental cabinet which provided carbonation at 40oC and 50% relative humidity.

No description available.

Carbonated cement paste surfaces were characterised prior to application of surface treatments. Their chemical and physical properties varied with distance from the surface and method of carbonation. From the surface inwards the pH of expressed pore solutions and porosity were observed to increase. Hardness increased after natural carbonation, but decreased after accelerated carbonation. Generally, accelerated carbonation caused more extreme changes. Investigations were carried out on four concrete surface hardening treatments; two sodium silicates and two silicofluorides. These treatments penetrated and hardened the surface of naturally dried uncarbonated cement paste to a depth fo 250m. Silicofluorides reacted with uncarbonated and carbonated cement pastes to form calcium fluoride. The question of how sodium silicates harden the surface remains unanswered. Surface hardeners do not significantly affect the rate of carbonation, and are unsuitable for re-alkalising carbonated cement paste. Water repellent treatments studied include a silane, a siloxane and a silicone. The silane exhibited the maximum penetration, up to 24mm under favourable conditions, but penetration in all cases was limited by moisture in the substrate. Water repellent treatments slow down water vapour diffusion but, with time, internal moisture levels should reflect external relative humidities. Water repellents may be used to reduce carbonation-induced corrosion where ingress of moisture from intermittent wetting may be slowed. However, treatment with water repellents can temporarily push the carbonation front deeper into the concrete.

No description available.

The work described in this thesis is an attempt to provide improved understanding of the effects of several factors affecting diffusion in hydrated cement pastes and to aid the prediction of ionic diffusion processes in cement-based materials. Effect of pore structure on diffusion was examined by means of comparative diffusion studies of quaternary ammonium ions with different ionic radii. Diffusivities of these ions in hydrated pastes of ordinary portland cement with or without addition of fly ash were determined by a quasi-steady state technique. The restriction of the pore geometry on diffusion was evaluated from the change of diffusivity in response to the change of ionic radius. The pastes were prepared at three water-cement ratios, 0.35, 0.50 and 0.65. Attempts were made to study the effect of surface charge or the electrochemical double layer at the pore/solution interface on ionic diffusion. An approach was to evaluate the zeta potentials of hydrated cement pastes through streaming potential measurements. Another approach was the comparative studies of the diffusion kinetics of chloride and dissolved oxygen in hydrated pastes of ordinary portland cement with addition of 0 and 20% fly ash. An electrochemical technique for the determination of oxygen diffusivity was also developed. Non-steady state diffusion of sodium potassium, chloride and hydroxyl ions in hydrated ordinary portland cement paste of water-cement ratio 0.5 was studied with the aid of computer-modelling. The kinetics of both diffusion and ionic binding were considered for the characterization of the concentration profiles by Fick's first and second laws. The effect of the electrostatic interactions between ions on the overall diffusion rates was also considered. A general model concerning the prediction of ionic diffusion processes in cement-based materials has been proposed.

This thesis describes an experimental study of the abrasion resistance of concrete at both the macro and micro levels. This is preceded by a review related to friction and wear, methods of test for assessing abrasion resistance, and factors influencing the abrasion resistance of concrete. A versatile test apparatus was developed to assess the abrasion resistance of concrete. This could be operated in three modes and a standardised procedure was established for all tests. A laboratory programme was undertaken to investigate the influence, on abrasion resistance, of three major factors - finishing techniques, curing regimes and surface treatments. The results clearly show that abrasion resistance was significantly affected by these factors, and tentative mechanisms were postulated to explain these observations. To substantiate these mechanisms, the concrete specimens from the macro-study were subjected to micro-structural investigation, using such techniques as 'Mercury Intrusion Forosimetry, Microhardness, Scanning Electron Microscopy, Petrography and Differential Thermal Analysis. The results of this programme clearly demonstrated that the abrasion resistance of concrete is primarily dependent on the microstructure of the concrete nearest to the surface. The viability of indirectly assessing the abrasion resistance was investigated using three non-destructive techniques - Ultrasonic Pulse Velocity, Schmidt Rebound Hardness, and the Initial Surface Absorption Test. The Initial Surface Absorption was found to be most sensitive to factors which were shown to have influenced the abrasion resistance of concrete. An extensive field investigation was also undertaken. The results were used to compare site and laboratorypractices, and the performance in the accelerated abrasion test with the service wear. From this study, criteria were developed for assessing the quality of concrete floor slabs in terms of abrasion resistance.