Simulations on the interaction between grain boundaries and precipitates

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The goal of the project is to develop a very detailed understanding and analytical model for the interaction between grain boundaries and precipitates of secondary chemical phases, i.e. the pinning force or "Zenger drag". This force is of utmost importance in the production of materials because it controls the microstructure evolution and hence the material properties during processing steps involving recrystallization and grain growth. Still, in spite of the importance, the current physical understanding of the pinning force is rather incomplete, as can be seen from the fact that the existing models involve only very little detail about the specific precipitates and grain boundaries. For instance, current models involve no information at all about the interface between the precipitate and the grain boundary, or the triple line where the grain boundary touches the precipitate. Likewise, the current models do not account for the fact that a curved grain boundary will touch and interact with more precipitates concurrently than a flat one. This tendency is known to be the principal element in the success of the Friedel model for dislocation pinning. In comparison to the highly detailed dislocation pinning (strengthening) models in use today, the effect of grain boundary pinning has only been addressed in rudimentary ways. This means that in spite of their importance, the current Zener drag models are probably rather inaccurate. The proposed project is aiming to develop comprehensive new models for the Zener drag that consider a multitude of potentially important influences. A multi-scale approach is used in which atomistic detail is combined with a larger scale model to derive homogenized results. Firstly, the details of a grain boundary interacting with one precipitate is investigated on an atomistic basis. Using molecular dynamics, the detachment of various grain boundaries from various concrete types of precipitates is simulated. The very same depinning processes is also simulated using a vertex model, in which the grain boundary and the precipitate interface are represented by abstract, facetted planes. The direct comparison of these models, e.g. by adjusting the vertex model to the atomistic one, allows to understand all detail about the depinning process, so that the most important influences can be identified, quantified and judged in respect of importance. Secondly, the vertex model can then be utilized to simulate the interaction of a grain boundary with realistic arrays of precipitates. Hence, highly accurate models for the Zener drag are derived, both for recrystallization and grain growth. These can later be used to improve manufacturing processes and materials properties.