Environmentally Assisted Cracking of High Strength Al Alloys - Transition from Initiation to Sustainable Cracking

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Advanced high strength Al-Zn-Mg-Cu alloys are widely employed in the aerospace industry, owing to their competitive cost and high strength-to-density ratio. In modern aircraft design these materials are frequently deployed in large integral components manufactured from thick-gauge hot-rolled plates. However, it has been discovered that they can be susceptible to 'Environmentally Assisted Cracking' (EAC) in warm humid air environments. This is a complex hybrid chemical-mechanical damage mechanism whereby a combination of a high stress, water, and chemical reaction, with active phases within the material, generates hydrogen which enters the material and causes grain boundary embrittlement. However, several different fracture processes have been observed, as a function of the material, load, and environmental conditions.The aim of this project is to exploit recent advances in 3D characterization techniques to improve our current poor understanding of how the local sub-surface microstructure and the environment within an embryo crack controls the transition from initiation to sustainable self-propagating cracks with different mechanistic regimes. This will include using in-situ and multi-scale 3D imaging techniques, by combing very high resolution tomography and destructive serial sectioning electron microscopy, to probe both the local environment formed within an embryonic crack and the interaction with the local microstructure and chemistry. In particular, by decoupling the effects of grain structure and precipitate type/chemistry, and the exposure conditions, the project will aim to determine why some cracks go on to become self-sustaining and others die, and why there are several regimes where different crack propagation mechanisms dominate.