No description available.

The microstructural chemistry of the commercial aluminium alloy 7150, containing Al, Zn, Mg, Cu and some trace impurities, was investigated in detail. This alloy is a precipitation hardening alloy, deriving most of its strength from the fine distribution of solute rich precipitates formed during thermal processing. At peak strength this alloy suffers from the common problem of stress corrosion cracking, leading to unpredictable premature failure in the presence of a corrosive environment. Failure is mainly intergranular, thus the structure and chemistry of the grain boundary regions is of interest. A large number of previous investigations have failed to correlate any individual parameter with the stress corrosion cracking behaviour. As the analytical techniques have improved over the last three decades, more complex investigations of the microstructure and the microchemistry have been attempted, in order to more fully characterise the development of this alloy during thermal processing. This thesis presents the results of two of the highest resolution techniques available for microchemical analysis. Scanning transmission electron microscopy X-ray analysis, using a VG-HB501 dedicated scanning transmission electron microscope, enables chemical analysis with a 2nm electron probe, while atom probe analysis, using a VG-FIM100 atom probe with an additional position sensitive detector, enables single atom chemical identification with sub-nanometre spatial resolution. However, both of these techniques have their own experimental limitations which restrict the accuracy of the results obtainable. A detailed description of the many factors limiting both techniques is presented. Combining these techniques has enabled chemical analysis of all the microstructural features present in this alloy on the nanometre scale. A description of the chemical changes occurring during age hardening of this alloy is given in summary.

Stress corrosion cracking (SCC) susceptibility of the phosphorus de-oxidized copper has been evaluated in synthetic seawater polluted by sulfides using slow strain rate test (SSRT). The effect of concentration of sulfide, temperature, and applied cathodic and anodic potentials on the final strain values and maximum stress were also studied. No cracks were found under the tested conditions. The final strain and maximum stress values decreased but not significantly, with increase in the temperature, applied anodic potential and sulfide concentration. The observed effect is due to the section reduction by uniform corrosion. Lateral cross section and microscopic examination of the fractured specimen ruled out the existence of the localized corrosion. Electrochemical measurements showed that the Cu2S film is not a protective film and also exhibits a mass transfer limitation to the inward diffusion of the sulfides. Based on these results the reasons for the absence of cracking are also discussed.

Stress corrosion cracking (SCC) susceptibility of the phosphorus de-oxidized copper has been evaluated in synthetic seawater polluted by sulfides using slow strain rate test (SSRT). The effect of concentration of sulfide, temperature, and applied cathodic and anodic potentials on the final strain values and maximum stress were also studied. No cracks were found under the tested conditions. The final strain and maximum stress values decreased but not significantly, with increase in the temperature, applied anodic potential and sulfide concentration. The observed effect is due to the section reduction by uniform corrosion. Lateral cross section and microscopic examination of the fractured specimen ruled out the existence of the localized corrosion. Electrochemical measurements showed that the Cu2S film is not a protective film and also exhibits a mass transfer limitation to the inward diffusion of the sulfides. Based on these results the reasons for the absence of cracking are also discussed.

Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1968. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Although the phenomena of stress corrosion cracking is known to occur in 316L stainless steel in chloride environments, there is little knowledge regarding the incubation stage, i.e. when the cracks are very short and about a few grains in length scale. The project aimed to determine whether Grain Boundary Engineering GBE, through thermo-mechanical processing, could improve resistance to transgranular chloride stress corrosion crack nucleation in austenitic stainless steels. The material studied is type 316L austenitic stainless steel, with cracking developing in conditions of controlled humidity at 80°C under saturated MgCl2 salt deposits. Three batches were used; as received (AR), Heat treated at 950ºC designated as TM950ºC, treated at 1075ºC; designated as TM1075ºC and also treated at 1150ºC; designated as TM1150ºC for modifying the structure. The thermo mechanical treatment resulted in increase of ∑(3-29) and ∑3 in both fractions by almost 20% as the annealing temperature increased between 950ºC to TM1150ºC, on the other hand (AR) has sustained on 10% improvement over the TM950ºC in both fractions CSL% = ∑(3-29) and ∑3.In-situ observations show that the short cracks may grow to larger scale length in the (AR), TM950ºCand TM1150ºC than can grow in TM1075ºC.DIC analysis was not suitable for testing in this environment might be due to many factors, such as salt content residual effects, evaporation of the salt liquid, surface detritions and lateral movements due to tensile testing. The interaction between short stress corrosion cracks and microstructure was characterised by Electron backscatter diffraction (EBSD). The high angle grain boundaries (HAGB) are shown to act as barriers to cracking resulting in hindering or deviating the crack tip, which slows the overall crack growth rate. The type of microstructure that would have superior TGSCC resistance would be that, possess much higher fraction of both fractions CSL%= ∑(3-29) and ∑3.

Stress corrosion cracking of a 14 wt% Cr martensitic stainless steel, with commercial names PH-15Cr5Ni, FV520B or X4CrNiCuMo15-5, used for the manufacture of low pressure turbine blades, has been studied with the intention of gaining a better understanding of the processes involved, how they occur and why. Industrially this is very important as stress corrosion cracking is considered to be a delayed failure process, whereby microscopic cracks can potentially propagate through a metal undetected until catastrophic failure occurs. The aim of this work is to establish links between crack length and external factors, such as exposure time, in order to devise a method of dating stress corrosion cracks and therefore predicting their possible occurrence in-service.

Lifetime prediction of components in Alloy 600 is a major concern for nuclear power plants. Alloy 600 components have been shown to be susceptible to stress corrosion cracking (SCC). In the 1990’s, an engineering model was developed in order to predict the life time as a function of the main macroscopic parameters (stress, environment, material), based on laboratory results. This model has since been used to predict the ranking of various Alloy 600 components, using the knowledges of the manufacturing and service conditions for each component. It was applied successfully in the case of forged control rod drive mechanism (CRDM) nozzles. However, it was found necessary to improve this model to account for the strain history of the different components. Predictions using the model, investigated from an array of test results on Alloy 600 in laboratory primary water, have demonstrated that the time for initiation differed significantly according to the strain path applied to the specimen. The present work is dedicated to assess SCC results from samples with different strain paths and different level of cold work in order to better understand the manufacturing conditions on SCC. The samples are machined in three different directions and tested at different durations in order to model the time for transition (transition between slow and fast propagation) as a function of cold work, strain path and stress. Thermomechanical treatments are also applied on two different heats of Alloy 600: forged WF675 (very susceptible to SCC in as received conditions) and rolled 78456/337 (non susceptible to SCC in as-received conditions) in order to transform the forged microstructure into a microstructure close to the rolled microstructure and vice-versa. These microstructures are then tested in primary conditions and the results are compared to the results obtained on as-received material in order to get a better understanding of manufacturing process and microstructure parameters regarding SCC behaviour.

No description available.