Dislocation based modelling of deformation and fracture in real engineering alloys

如果载荷很小，金属就会弹性变形，当载荷移除时，会恢复到原来的形状。然而，如果载荷超过某个值，则会发生永久变形，即所谓的塑性。塑性远比弹性复杂得多，因为它涉及到金属中原子键的断裂。这些断裂的原子键称为位错。这类似于毛毛虫的运动：它不会试图同时向前移动整个身体；相反，它会随着横扫毛毛虫身体的一波运动而递增地向前移动身体。金属含有大量的位错：这些线扫过金属，使原子面滑过每个面，导致金属永久变形。当金属被加载时，新的位错成核，一些位错被困在障碍物中。然而，如果负载施加得太快或金属太冷，位错线就没有时间形核和移动：相反，整个原子平面被撕裂，使金属破裂。在核反应堆中，燃料棒被包裹在锆合金中：随着时间的推移，用于冷却燃料棒的水中的氢扩散到锆中，并被位错线和金属上的任何小裂缝或缺口所吸引。如果氢浓度过高，氢原子会聚集在一起形成沉淀，阻碍位错的运动，很容易断裂。正是这种复杂的相互作用，位错，扩散，沉淀形成和断裂，我的目标是在计算机上模拟。通过利用现代显卡(为玩视频游戏而开发的)的能力，这是可能的，它允许以很小的成本轻松地执行大规模并行模拟。即使这样，也只能模拟非常小体积的材质。传统的力学测试(弯曲或压缩金属块)总是在几毫米大小的大样品上进行，这意味着根本不可能显式地模拟样品中的所有位错。在过去的十年里，对仅有几微米大小的样品进行机械测试已经成为可能。样本如此之小，以至于通过利用现代图形卡的能力，将有可能显式地模拟包括材料中的每个位错在内的实验，并观察它们如何相互作用以及如何与多个沉淀物相互作用。能够在这种细节水平上模拟整个实验是史无前例的，它将为了解金属在塑性变形和断裂时到底发生了什么提供了新的细节。在该项目中获得的基本新见解将用于制定更准确的工业工程设计规则，并涉及与加利福尼亚州劳伦斯利弗莫尔国家实验室、伦敦帝国理工学院、牛津郡库勒姆聚变能源中心、特丁顿国家物理实验室和德比劳斯莱斯的科学家和工程师密切合作。

If loaded a small amount, a metal will deform elastically, returning to its original shape when the load is removed. However if the load exceeds some value, then permanent deformation occurs, known as plasticity. Plasticity is far more complex to understand than elasticity as it involves breaking lines of atomic bonds in the metal. These lines of broken atomic bonds are called dislocations. This is analogous to the motion of a caterpillar: which does not attempt to move its whole body forward simultaneously; instead it incrementally moves its body forward in a wave of motion sweeping through the caterpillar's body. Metals contains a huge number of dislocations: these lines sweep through the metal allowing atomic planes to slip over each over, causing the metal to be permanently deformed. When metal is loaded, new dislocations are nucleated and some become trapped at obstacles. However, if the load is applied too quickly or the metal is too cold, then the dislocation lines do not have time to nucleate and move: instead whole planes of atoms are ripped apart, fracturing the metal.In a nuclear reactor, the fuel rods are cladded in a zirconium alloy: over time, hydrogen from water used to cool the fuel rods, diffuses into the zirconium and is attracted to dislocation lines and to any small cracks or notches in the metal. If the hydrogen concentration becomes too high, hydrogen atoms will clump together to form precipitates which block dislocation motion and can easily fracture. It is this complex interaction between, dislocations, diffusion, precipitate formation and fracture which I aim to simulate on a computer. This is possible by utilising the power of modern graphics cards (developed to play video games) which allow massively parallel simulations to be performed easily and at little cost. Even then it is only possible to simulate a very small volume of material. Traditional mechanical tests (bending or compressing pieces of metal) were always performed on large specimens, several millimetres in size, meaning it was simply not possible to simulate all the dislocations in the sample explicitly. In the last decade it has become possible to perform mechanical tests on samples that are only a few microns in size. The samples are so small, that by utilizing the power of modern graphics cards, it will be possible to simulate the experiment including every dislocation in the material explicitly, and watch how they interact with each other and with multiple precipitates. Being able to simulate an entire experiment at this level of detail is unprecedented and it will provide new insights into the details of what exactly goes on when metal deforms plastically and fractures. The fundamental new insights gained during the project will be used to develop more accurate engineering design rules for industry and involves close collaboration with scientists and engineers at Lawrence Livermore National Laboratory in California, Imperial College London, Culham Centre for Fusion Energy in Oxfordshire, The National Physical Laboratory in Teddington and Rolls-Royce in Derby.