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From www.imm.rwth-aachen.de: Grain Boundary Mechanics 已有6人參與
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Grain Boundary Mechanics The reason for the multifarious use of metals and alloys as constructional or functional materials is founded by their outstanding properties. They are commonly used under conditions with high temperatures or mechanical strains for example in aerospace, energy industry or combustion engineering, which requires particular physical and mechanical properties to meet the necessary demands. The knowledge of the material's behaviour and the product life of a module is therefore the important issue. In fact, the properties of metal arise from their crystallinity. In this connection, construction faults at manufacturing have the most important impact on material, and not as may assumed the perfect periodic crystal build-up. Generally, the number, different kinds of crystal faults and their allocation is what we call Microstructure. If one can realize to influence the Microstructure in a purposeful way, it is possible to create new materials, for example gradient materials or composites. Additionally, materials, which already exist, e. g. nano crystal or high temperature components, can be improved and one is able to optimise the properties according to the purpose. Grain boundaries are one of the most important parts of Microstructure, being available in almost every crystal material. The only exception is build by a special cultured single crystal. Usually, grain boundaries arise by solidification of crystals. Macroscopically, they are interior interfaces of materials, in which place the crystallographic orientation changes in a discontinuously way. Hence, they are part of crystal solid bodies but with different properties to the rest of the crystal, having a direct impact on the mechanically and electronically characteristics of ones crystal. Especially, the capability of moving, dissolving or regenerating under suitable conditions leads to the fact, that grain boundaries are the most important structural lattice imperfections in crystal materials. The grain boundary movement depends on a great number of parameters. Two of them are temperature and stress which have the greatest bearing on the mobility, because they are responsible for activating the movement. That's why they are called the driving force. Particularly, high temperatures can make grain boundaries moving very quickly, so that their movement alters the Microstructure and properties of the material. Creating models, which simulate the grain boundary movement and describe the Microstructure development during a technical process, for example at deformation or recrystallisation, can help to control the changes of a material's properties and Microstructure development during the production process. Furthermore, these models deliver information about the behaviour of a certain module at its usage. Grain boundary engineering, in general, is the survey of grain boundary movement, which has become a very important part in research in recent time. It means that one is able to impact the behaviour of material and alter the Microstructure by activating grain boundary movements. Some materials, like gradient materials are defined different in various parts. So the knowledge of grain boundary movement helps to adjust them more precisely, getting a better calibration between the material and the requirements of its use. In recent years, there has been the presumption that grain boundary movements, which have been distinguished between mobile and immobile ones, are not impacted by mechanical strains. Material's behaviour under mechanical strain is not constrained by grain boundaries, except a non-mechanical grain boundary movement would be activated by high temperatures. Eventually, it is our aim to investigate, if grain boundaries are really independent from mechanical strains, and if they can be still distinguished between mobile and immobile ones. For this purpose, we produce so called bi-crystals with only one single grain boundary and try to activate the movement by a mechanical stress field. The result is that grain boundaries and their movements actually are able to impact by external fields which abandon most of the physical theories about interface dynamics. Even the distinction between mobile and immobile grain boundaries does not seem to be sensible any longer. For example, low-angle grain boundaries are supposed to be immobile, but actually, they are able to move much faster than mobile high-angled grain boundaries under appropriate conditions. In fact, there are several mechanisms with different parameters which are able to actuate a grain boundary movement, depending on the grain boundaries structure and their driving force. The knowledge of the different mechanisms of moving is quite important, cause we've done a step towards materials design by grain boundary engineering. Right now we're in the position to intervene in the kinetic of grain boundaries, while using the appropriate test conditions. |
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