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

先进的高强Al-Zn-Mg-Cu合金因其具有竞争力的成本和较高的强度密度比，在航空航天工业中得到了广泛的应用。在现代飞机设计中，这些材料经常被部署在由厚规格热轧钢板制造的大型整体部件中。然而，人们已经发现，在温暖潮湿的空气环境中，它们可能容易受到环境辅助开裂(EAC)的影响。这是一种复杂的化学-机械混合损伤机制，高应力、水和化学反应与材料中的活性相结合，产生氢进入材料并导致晶界脆化。然而，随着材料、载荷和环境条件的变化，已经观察到了几种不同的断裂过程。本项目的目的是利用3D表征技术的最新进展来改善我们目前对胚胎裂纹内局部亚表面微结构和环境如何控制从萌生到具有不同机制的持续自扩展裂纹的转变的了解。这将包括使用原位和多尺度3D成像技术，通过结合非常高分辨率的断层扫描和破坏性连续切片电子显微镜，来探测胚胎裂缝内形成的局部环境以及与局部微观结构和化学物质的相互作用。特别是，通过分离颗粒结构和沉淀物类型/化学以及暴露条件的影响，该项目将致力于确定为什么一些裂纹继续自我维持而另一些裂纹死亡，以及为什么存在几个不同的裂纹扩展机制主导的区域。

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.