The present study is concerned with the interaction of stress and corrosion in high strength aluminium a11oys in sheet form used in the aircraft industry. Many of these alloys are susceptible to exfoliation corrosion under service conditions. This type of corrosion was reproduced in the susceptible a11oys using the Exco test method. The fracture toughness of an A1 - Zn - Mg - Cu a11oy (7178 - T6) was assessed using R - curves. The interaction between exfoliation corrosion and the thinning effect of exfoliation on the fracture toughness are discussed. Fatigue crack growth behaviour in two naturally aged A1 - Cu ~ Mg alloys, 2024 - T351 and 3L7O (2014 - T4) was evaluated under conditions of low and high humidity using constant amplitude loading. A similar assessment of the effect of humidity on the propagation rates in the higher strength 7178 - T6 was carried out. Additionally, the effect of the presence of exfoliation corrosion on crack growth in 2024 - T351 and 7178 -- T6 was examined. Both humidity and exfoliation corrosion enhanced fatigue crack growth rates in the alloys, though to differing extents, with 7178 - T6 being affected more. The fatigue resistance of 7178 - T6 diminished even further in the presence of humidity and corrosia together. Finally, a mechanism to explain the way by which exfoliation corrosion affects the fatigue and fracture behaviour of thin aluminium sheets is proposed.

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The corrosion behaviour of a proprietary stainless steel ('Rex 734') was measured under conditions simulating those found in the human body, in order to assess its suitability for use as an orthopaedic implant material. The performance of this alloy was compared to that of another austenitic stainless steel implant alloy, AISI 316S12. A potentiostat, operating under an original system of computer control, was used to obtain corrosion, breakdown and protection potentials, and Dassive corrosion current densities. Testing was carried out in a variety of different solutions, with various material and surface preparation techniques. Corrosion fatigue and fretting experiments were also performed. Electrochemical corrosion measurements indicated that Rex 734 is more resistant to pitting and crevice attack than 316L in isotonic saline solution. Changing the austenitic grain size of both alloys on cold working Rex 734 did not produce any significant changes in corrosion behaviour. Electropolishing reduced the passive corrosion current on both materials. Torsional fatigue strengths of Rex 734 were higher than those of 316L in air, and were reduced by a smaller amount on exposure to physiological solution. Fretting experiments carried out on simulated bone plate and screw assemblies showed that Rex 734 had a higher initial electrochemical corrosion rate when fretting was initiated, but recovered more quickly after prolonged fretting, 316L specimens showed more fretting damage and wear than Rex 734 assemblies, with specimens containing a mixture of alloys having the worst fretting behaviour. Test samples containing welds exhibited electrochemical corrosion characteristics similar to those of unwelded alloy, but had a poorer fatigue performance. In conclusion, Rex 734 was recommended as a direct replacement for 316L in orthopaedic surgery.

The anharmonic character of indium-thallium alloys in the vicinity of the fee to fct elastic phase transition has been determined from the composition, temperature, hydrostatic pressure and uniaxial stress dependences of ultrasound wave velocities in single crystals grown by the zone levelling method and containing between 10at% and 30at% Tl. Both the pulse echo overlap and the pulse superposition ultrasonic techniques were employed. The hydrostatic pressure derivatives of second order elastic constants (SOEC), derived from the experimental results, have illuminated the effect of hydrostatic compression on the fcc-fct boundary of the In-Tl phase diagram and enabled the construction of an empirical, room temperature equation of state for fcc alloys. Mode Gruneisen parameters evaluated for these alloys strongly reflect the effects of increased levels of vibrational anharmonicity and the softening of the transverse acoustic phonon, wave vector q and polarised e, in that increasingly large negative parameters were recorded for this mode as the phase transition was approached. A small but significant thermal hysteresis was found to be associated with the structural transition in In-22at% Tl while the effect of temperature on the hydrostatic pressure derivatives of the SOEC of In-25at% Tl was observed to be negligible, at least for temperatures far from the transition temperature Tc. The hydrostatic and uniaxial stress derivatives of ultrasound velocities in five In-Tl alloys were used to evaluate five complete sets of independent, room temperature, third order elastic constants (TOEC). These results have allowed the evaluation of third order invariants in the free energy expansion with respect to irreducible strains and so for the first time permitted a critical, quantitative assessment of the applicability of Landau theory, in conjunction with the soft phonon concept, to a first-order elastic phase transition. This theory is found to fit experimental observations well. The relative magnitudes of third order invariants conform to the nearly second-order character of the phase transformation and to the negligible volume change associated with it. From trends in the third order invariants, Landau theory suggests that the second-order character of the elastic phase transition increases with the Tl content of the alloy, such that In-31at% Tl, for which Tc ≈ OK, should exhibit a structural transformation which is most nearly of second order.

The oxidation behaviour of a binary Fe-9Cr alloy and two ternary Fe - 9Cr - Si alloys containing 0.75wt% silicon has been studied at 500°C-600°C in CO2 based gas. The experimental approach has involved kinetic studies using a microgravimetric method together with detailed characterisation of the oxidation products using optical, X-ray and electron optical techniques. Initial experiments were to determine experimental conditions which allowed the early onset of breakaway oxidation on the Fe - 9Cr alloy. However, breakaway oxidation kinetics were not observed and protective duplex scales were formed under all the conditions investigated. Later experiments investigated the oxidation of Fe - 9Cr - Si alloys in CO2 - 13% CO gas. The oxidation process followed approximately a parabolic rate law for all the alloys, the oxidation resistance increasing with increase in silicon concentration. The lowest oxidation rates were associated with the presence of an internally oxidised zone beneath the duplex scale. Internal oxidation was favoured by an increase in either the oxidation temperature or the silicon concentration of the alloy. A model for the dependence of internal oxidation on the silicon concentration of the alloy has been proposed. Calculations showed that diffusion of oxygen was the dominant transport mode in internal oxidation and oxygen permeability increased with increase in temperature or silicon level. Outward diffusion of iron cations is the rate controlling process in scale growth, which is the dominant factor in the overall oxidation process. The reduction in oxidation rate associated with internal oxidation may be explained by an increase in the diffusion path length of iron cations outwards and a reduction of the iron cation concentration at the scale-metal interface. The model proposed for the internal oxidation of these alloys is also able to account for the differences observed in the structure of the initial thin oxide films.

The thermal oxidation of pure iron has been studied at 350-500°C in the oxygen pressure range 0.1 to 10-5 Torr. The oxidation of Fe-9% Cr alloy, self- and chromium-implanted iron was also studied at 400°C and 0.1 Torr. A novel oxidation apparatus based on a solid-state oxygen electrolyte was developed, which measured oxidation rates and was sensitive to transient oxidation effects. Oxidised and non-oxidised specimens were examined using optical and electron microscopy, secondary ion mass spectrometry and various x-ray techniques. Ion implantation was performed at 300 kV and theoretical (LSS) calculations of ion distributions were made. Initially iron oxidised rapidly to form magnetite, whilst a subsequent reduction in rate was accompanied by formation of haematite crystals on the specimen surface Oxidation mechanisms are elucidated using the pressure and temperature dependence of oxidation rate, and with the aid of a novel mathematical model. Haematite formation is slower at lower pressures, and this oxide is not observed below 10 -3 Torr due to continuous reduction by iron. Fe-9% Cr alloy oxidised parabolically at one tenth of the rate of pure iron, forming a protective M2O3 oxide containing chromium adjacent to the metal. Self-implanted iron oxidised parabolically, whereas chromium implantatation produced an initial low oxidation rate which increased markedly once the shallow implanted layer had been consumed. Radiation damage caused by implantation promoted more rapid oxidation, particularly with high ion doses; smooth fine-grained oxides were formed, containing an increased number of short-circuit diffusion paths and a high proportion of spinel. The benefits of chromium were short-lived because the implanted chromium readily dissolved in the growing oxide, and the initial high concentrations of chromium near the metal-oxide interface were not maintained. The behaviour of chromium implanted material is compared with 'breakaway oxidation' of conventional iron-chromium alloys.

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