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Unknown types of grain boundary slip: How do Rachinger slip and Lifshitz slip differ?
Grain boundary sliding (GBS) is a material deformation mechanism in which grains slide against each other under the action of external forces, especially at high temperatures and low strain rates, and usually occurs in polycrystalline materials. . This phenomenon is intertwined with the creep process, and the shape of the grain boundary also affects the speed and extent of slip. At high temperatures, grain boundary sliding is a movement that prevents the formation of cracks between grains. For many materials, Rachinger slip and Lifshitz slip are the two most commonly mentioned types, but there are significant differences between them.
Rachinger slip is mainly elastic slip, and the grains almost retain their original shape; while Lifshitz slip involves a diffusion process, which causes the grain shape to change.
Comparison of Rachinger slip and Lifshitz slip
During high temperature creep, Rachinger slip is mainly manifested as the relative sliding of grains while maintaining their original shape under the application of external stress. During this process, the internal stress will continue to grow and eventually reach equilibrium with the externally applied stress. For example, when uniaxial tensile stress is applied, the grains slide to accommodate the stretching, and the number of grains increases along the direction of applied stress.
In contrast, Lifshitz slip is a process closely related to Nabarro-Herring and Coble creep. In this case, as stress is applied, the diffusion of vacancies will cause the grains to change shape, causing them to extend along the direction of the applied stress. This does not increase the number of grains along the direction of applied stress.
Through these two slip mechanisms, we can observe different deformation characteristics, which is crucial for understanding the behavior of materials at high temperatures.
Equilibrium mechanism and dislocation motion
When polycrystalline grains slide relative to each other, there must be a corresponding mechanism to help this slip occur and avoid overlap between grains. To this end, scholars have proposed a variety of equilibrium mechanisms, including dislocation movement, elastic deformation and diffusion adaptation mechanism. Especially under superplastic conditions, the role of dislocation movement and grain boundary diffusion is particularly significant.
For example, when a material is at superplastic temperature, dislocations in the material are rapidly emitted and absorbed at grain boundaries, which stabilizes the grain shape while supporting the material's flow at high strain rates.
Experimental evidence and the impact of nanomaterials
Experimentally, the phenomenon of grain boundary sliding has been observed in a variety of materials, including observations in NaCl and MgO twin crystals in 1962. These experiments revealed the slip behavior at grain boundaries using microscopic techniques. The emergence of nanocrystalline materials makes grain boundary sliding occur frequently during high-temperature operations, because its fine grain structure is more prone to slip at high and low temperatures compared to coarse grains.
Controlling grain size and shape can effectively reduce the degree of grain boundary sliding, which is crucial in the design of many materials.
Study on application in tungsten filament
In tungsten filaments, the main failure mechanism was found to be grain boundary sliding. As the operating temperature increases, diffusion between grain boundaries can lead to slip and eventually filament breakage. In order to extend the life of the filament, the researchers modified the tungsten by doping it with elements such as aluminum, silicon and potassium to reduce slip at high temperatures.
In conclusion, understanding the essential difference between Rachinger and Lifshitz slip is indispensable for the development of high-temperature materials, especially for extreme environments such as aerospace and automotive industries. This knowledge can help scientists and engineers design more durable materials to meet future challenges. Can we find the key solutions to these problems through the exploration of materials science?
