Dislocation based modelling of deformation and fracture in real engineering alloys

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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.