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Recently, a new generation of 7xxx alloys used for aircraft structural components has been found to be sensitive to intergranular hydrogen environmentally assisted cracking (HEAC) in moist air environments. HEAC can cause subcritical crack growth at stress levels significantly below yield. Thus, some components have been found to be at risk of in-service cracking. The higher sensitivity of these novel alloys has been ascribed to their composition; notably a high Zn/Mg ratio and a low Cu content. There has been substantial research regarding the environmentally assisted cracking behaviour of 7xxx alloys exposed to moist air and other aqueous solutions. However, there is a paucity of data for the cracking behaviour of these novel 7xxx alloys. Additionally, most research has been performed on large pre-cracked fracture mechanics samples. These neglect crack nucleation and microstructurally short cracking, despite these stages dominating the lifetime of components. The work done in this study can be divided up into five different sections, including three different experiments and the development of two Abaqus models. The first experiment involved the use of static 4-point bending to analyse the propensity to form cracks from smooth samples of AA7075-T651 and AA7449-T7651 exposed to warm moist air. AA7449-T7651 was found to be much more prone to crack formation. For the second experiment, fracture mechanics samples were used to estimate the HEAC crack growth rates of AA7449-T7651 in moist air for range of temperatures. Increasing the temperature was found to increase the crack growth rates and decrease the threshold stress intensity factor for cracking. The third experiment involved in-situ X-ray tomography imaging of small tensile samples of AA7449-T7651 during HEAC. This technique was used to correlate the microstructure with the HEAC crack growth behaviour. Finally, two cohesive zone models (CZM) were developed to be able to simulate the HEAC behaviour for the DCB and in-situ tensile tests. These models were useful to understand how hydrogen was distributed within the samples during the tests.

Crack initiation mechanism changes induced by environmentally driven carburisation are poorly understood. Physical restrictions on practical sample production volumes and a historical focus on macroscopic phenomena has limited the data available for practical use in mechanistic investigations. Following preliminary studies conducted by EDF Energy, experiments are presented that were undertaken to address this knowledge gap. In this thesis microstructural changes caused by oxidation in a simulated reactor gas, previously developed by EDF Energy, are presented for two 316H stainless steels formed via different processing routes. The oxidation process leads to the formation of a thick duplex oxide layer and carburisation of the steel behind the oxidation front. Because of the complicated interactions and property variations introduced at the surface of the steel, focus is given to chemical distributions in the oxides formed on a cast steel along with grain boundary morphology changes in the carburised layer on a sample taken from a forged bar. Auger electron spectroscopy measurements conducted on a transverse section through the oxidised surface compliment previous observations of carburisation in Type 316H. Observations on the cast steel show the complex chemistry and structure of the spinel oxide along with the elevated levels of dissolved carbon in the near surface. Similar oxide depths are identified in samples from the cast and forged materials, however, measurements of carbide morphology changes in the forged material indicate that it is thermal ageing, a consequence of prolonged exposure to elevated temperature during oxidation, that drives these changes rather than the carburisation itself. Laboratory and synchrotron X-ray diffraction measurements are utilised to interrogate stresses generated during thermal cycling and mechanical loading of conditioned samples. During thermal cycling stresses measured in the oxide show disagreement with simple models, that are based on estimated oxide properties. Synchrotron measurements appear to indicate the presence of a compressive triaxial stress field in the carburised layer behind the oxide. Currently, it is not known if this stress state persists at elevated temperatures. Understanding of the stress state in the carburised layer is important for making meaningful predictions of component life and critical to the understanding of potential cracking mechanisms operating in the near surface of a component during operation. The diffraction results are supported by post-test microscopic investigations conducted on the cracked surfaces of the carburised layer. During repeated thermal cycling cracks appear at defects in the oxide layer are thought to be initiated from tensile stresses generated from local creep of the substrate. From room-temperature mechanical tests, oxide fracture initiates at an applied bulk strain between 0:45% to 0:9%. The cracks generated being identified as intergranular cracks that propagate to a depth just beyond the carburised zone. The results of a micro-tensile test conducted at 500 °C, with in-situ observations conducted on the outer oxide, confirm that a critical strain must be applied to the sample before cracking occurs. The value of the critical strain reported at this temperature is between 0:06% to 0:1%, i ii much lower than at ambient temperature, presumably due to the absence of high compressive stresses that protect the oxide at lower temperatures. Uncertainty on the initiation strain values are expected to be high because of complications posed by the experimental apparatus. The combination of high temperature micro-scale testing, in conjunction with other analytical techniques, appears the logical future avenue for investigating the mechanical properties of the carburised layer in conditioned components.