The effects of strain rate and testing temperature on the mechanical response of a near equi-atomic NiTi alloy have been investigated. All experiments have been conducted in compression, at testing temperatures of room .temperature (-20°C), 30°C, 40°C and 50°C. Quasi-static experiments were performed using a Hounsfield HK50 universal testing machine, and high strain rate measurements were obtained using the split Hopkinson pressure bar technique. The primary differences in the behaviour of the material within these deformation rate regimes appeared to be the presence of a possible transformation inhibition mechanism that occurs for high rates of strain, which manifests itself as an accommodation of applied load after the onset of transformation, increased strain rate sensitivity at high rates, and temperature dependence not evident at low rates. Initial material characterisation was achieved' through microhardness testing, DSC, DMTA, X-ray and electron diffraction, resulting in clarification of the transformation temperatures, martensitic volume fraction and microstructure of the alloy. A post experiment X -ray investigation was also performed in order to establish the microstructural response of the material to deformation. From the stress-strain data collected, the strain rate sensitivity and entropy of transformation of the alloy have been calculated. The application of a standard Arrhenius type equation has also been attempted, in order to estimate the material parameters of activation volume, and the free energy of transformation in the absence of stress. This model was found to be reasonably representative of the response of the alloy, although the results calculated demonstrated a high degree of intrinsic error.

Legislative and environmental demands are increasing the pressure on transport industries to introduce vehicles with lower weight, greater efficiency and reduced emissions while remaining competitive. For these reason the aerospace, defence and automotive industries worldwide are unanimous in the call for the development of new alloys with a step-change in performance. In the present thesis the results of the thorough microstructural and mechanical properties investigation of a 19 kg spraycast billet of the normally nanoquasicrystalline containing composition Al93Fe3Cr2Ti2 produced at Oxford University along with three binary Al intermetallics: Al3Ti, Al13Cr2 an Al13Fe4 produced by suction casting in the form of 5 mm rods at Sheffield University are presented and discussed.

After the review of the predictive methods in the high temperature fatigue field the characterization of the low cycle fatigue behaviour of a nickel superalloy, MARM002, is carried out using one of those predictive methods, the Strainranqe Partitioning method. The basic concepts and procedures of this method are reviewed and applied to MARM002 over the range 750° - 1040°C. These procedures involved the determination of the four basic life relationships at 850°C and 1040°C and their use in conjunction with a damage rule were then employed to assess the degree of insensitivity of the four basic strainrange versus life relationships to test temperature. Three damage rules and the assumptions behind them are discussed within the context of their application with the Strainrange Partitioning approach. An alternative analysis based on Ostergren's method is studied in a separate chapter and proved to be an easy and helpful method in predicting high temperature low cycle fatigue of several superalloys. The characterization of the creep-fatigue behaviour of MARM002 was completed by separation of initiation and propagation periods within two temperature intervals. A situation of early formation of cracks at the higher temperature interval 950°- l040°C with accelerated growth at lower tennperatures 750° - 250°C showed that great care is necessary when applying MARM002 isothermal results to cases where temperature varies.

Conventional power plants are major emitters of CO2 gases, which are believed to be contributing to global warming. An efficient, co-firing biomass-coal power plant with oxy-firing combustion system (running at high steam temperature and pressure), can play a vital role in CO2 emission reduction. However, these techniques will further worsen the issue of fireside corrosion of heat exchangers. An increase in fireside corrosion rates can cause short component lives and unexpected failures if not dealt with appropriately. The aim of this PhD study was to use laboratory-based testing to assess the performance of alloy materials under superheater conditions in simulated co-fired (biomass and coal) air and oxy-fired combustion. In this PhD project five different alloys were used. Synthetic deposits were also prepared to simulate superheater deposit compositions. Tests were carried out at temperatures appropriate for metal temperatures in superheaters/reheaters of future power plants. The performance of samples was determined using: mass change data, advanced microscopy techniques, x-ray diffraction and dimensional metrology. Additional tests were carried out to investigate deposit stability and the effect of high concentrations of salts. The results achieved have confirmed the hypothesis that increased fireside corrosion rates are due to the combined effect of extreme environment: high temperatures, SO2 and HCl gases, aggressive deposits. Corrosion damage follows trends that resembles ‘bell-shaped’ curve in both air and oxy-fired conditions. Alloy corrosion damage in novel oxy-firing compared to air-firing conditions was significantly higher at 700C. The peak of the curve shifts from 650 to 700C in oxy-fired conditions. The alloys with higher chromium content clearly showed better corrosion resistance. The work on deposit chemistry and exposure to high salt concentrations has improved the understanding of corrosion reaction mechanisms. Corrosion damage data have been used to produce basic fireside corrosion mathematical model; which can be used as a stepping stone towards further development of fireside corrosion models.

No description available.

The most commonly used alloy in the electronics industry has been the ubiquitous tinlead alloy. As the demand for electronic devices continues to increase, there have been concerns about the continued use of lead and its long term environmental impact. In the last decade there has been a push to ban the use of lead in electronic products. Legislation from various governments around the world limiting the use of lead has given rise to the drive to find suitable lead-free alternatives. The aim of this research project was to establish a systematic approach for the selection of electrochemical parameters for the electrodeposition of tin-rich copper-tin alloys from a single electroplating bath. By studying and understanding a model system such as copper-tin, one can then use the information obtained as a basis to successfully deposit various other tin binary alloys in the future. Tin-rich deposits were enabled by employing various strategies such as maintaining a high Sn to Cu ratio in the electrolyte and by using surface active agents that have been known to facilitate alloy co-deposition. The effect of surfactants on the tin content in the deposit was initially examined with the aid of a rotating cylinder Hull cell. It was found that the presence of fluorosurfactant was crucial in eliminating metal oxide formation. Cyclic voltammetry at a rotating disk electrode showed that inclusion of surfactant in the electrolyte had no effect on the reduction potential of tin which remained at -0.45 V vs SCE. However, the reduction potential for copper shifted from approximately -0.13 to -0.18 V vs SCE, thereby facilitating alloy co-deposition. Chronoamperometry and anodic stripping voltammetry showed that current efficiency for copper-tin deposition ranged from 55-92% depending on the deposition time and deposit composition. Results from voltammetry experiments were used in the next galvanostatic electrodeposition experiments at vitreous carbon electrodes. Deposits containing up to 96 wt.% tin were obtained from both direct current and pulse plating modes. It was found that an optimal current density of 22 mA cm-2 was needed to obtain desirable deposits. For pulse plating the peak current density should be set to 100 mA cm-2 with a duty cycle of 0.2. Cu-Sn alloys obtained consisted of two phases, tetragonal tin and a hexagonal Cu6Sn5 intermetallic compound. Deposit annealing showed that the Cu3Sn intermetallic was not formed.

The effects of microstructure, mechanical properties and hydrogen content on the hydrogen induced cold cracking (HICC) resistance of high strength low alloy (HSLA) steel weld metals were studied in this investigation. The weld metals were designed in a previous work. Their microstructures were characterised by optical and electron microscopy (FEG-SEM and TEM). Microphases, such as non metallic inclusions (NMI) and martensite-austenitecarbide constituent (MAC), were studied in some detail due to their hydrogen trapping capacity. Fractographic studies of hydrogen charged tensile samples were carried out to study the effect of microstructure and hydrogen content on the fracture micromechanisms. A critical hydrogen content (Ck) was estimated for each weld metal. The trapping capacity 'of each weld metal was studied using an electrochemical double pulse technique to measure the hydrogen trapping constant (k). The weld metals were classified based on composition, microstructure and micro-phase characteristics. NMI number density, size and spatial distribution were determined and thermodynamic calculations were proposed to identify their type. MAC morphology and distribution were qualitatively assessed. Retention of austenite was estimated, considering chemical and size stabilisation of remaining austenite. It was found that a continuous network of grain boundary ferrite (PF(G)), in combination with the presence of retained austenite in the MAC particle and certain NMI characteristics were beneficial to increase Ck. In weld metals without PF(G), retained austenite proportion and NMI distribution and size play a critical role in maintaining tolerance to hydrogen. From fractographic observations, it was proposed a phenomenological model that correlates microstructure, hydrogen content and the stress intensity factor, with the activation of different fracture micromechanisms: micro-void coalescence (MVC), quasicleavage (QC) and intergranular (IG). The trapping capacity of the weld metals was evaluated and results indicate that this capacity is the result of a complex combination of factors such a NMI inclusion size and distribution, presence of retained austenite and microstructure. The value of k takes into account these effects.

No description available.

No description available.

No description available.