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

先进高强度Al-Zn-Mg-Cu合金因其具有较好的强度密度比和较低的成本而被广泛应用于航空航天领域。在现代飞机设计中，这些材料经常用于由厚规格热轧板制造的大型整体部件中。然而，已经发现，它们在温暖潮湿的空气环境中容易受到“环境辅助开裂”（EAC）的影响。这是一种复杂的混合化学-机械损伤机制，其中高应力、水和化学反应与材料内的活性相的组合产生氢，氢进入材料并引起晶界脆化。然而，几个不同的断裂过程已被观察到，作为一个功能的材料，负载，和环境conditions.The项目的目的是利用最新的进展，在3D表征技术，以改善我们目前的了解不足，如何在一个胚胎裂纹控制的局部亚表面微观结构和环境从初始到可持续的自蔓延裂纹与不同的机械制度的过渡。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的连续切片电子显微镜，以探测胚胎裂纹内形成的局部环境以及与局部微观结构和化学的相互作用。特别是，通过解耦晶粒结构和沉淀物类型/化学的影响，以及暴露条件，该项目的目的是确定为什么一些裂纹继续成为自我维持和其他死亡，以及为什么有几个不同的裂纹扩展机制占主导地位的制度。

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.