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What is grain boundary sliding and why is it so important at high temperatures?
In the field of materials science, grain boundary sliding (GBS) is a mechanism closely related to material deformation, especially under high temperature conditions. When polycrystalline materials are subjected to external stress and are at a high homogenous temperature (approximately above 0.4 lattice melting point), slip between grains begins to occur, which is a natural response of the material to deformation. Through grain boundary sliding, the material can prevent cracks caused by stress concentration between internal grains.
Grain boundary sliding is usually intertwined with creep phenomena and plays a key role in the stress deformation of materials under high temperature environments.
Two types of grain slip
According to the different mechanisms, grain boundary sliding can be mainly divided into two types: Rachinger sliding and Lifshitz sliding. Rachinger slip is a purely elastic deformation. The grains retain most of their original shape during the slip process, and the internal stress gradually accumulates to balance with the external stress. Lifshitz slip is related to Nabarro-Herring and Coble creep, which involves the diffusion of defects inside the grains and the change of grain shape.
In Rachinger slip, the applied uniaxial tensile stress causes the grains to slip along the stress direction, followed by an increase in the number of grains along the stress direction.
Adaptation mechanisms and rheological behavior
In polycrystalline materials, grain boundary sliding requires some coordinated mechanisms to avoid overlap between grains, which is usually achieved through dislocation motion, elastic deformation, and diffusion adaptation. Under superplastic conditions, grain boundary sliding is accompanied by diffusion flow, which is crucial in promoting the deformation of the material.
For superplastic deformation, the rate of grain boundary sliding and its deformation mechanism can be adjusted according to the conditions of stress and strain rate to promote the deformation and ductility of the material.
Deformation rate at high temperature
As the temperature increases and the time increases, grain boundary sliding will have an important impact on the creep process of the material. By measuring different slip rates in metals, ceramics, or other materials, scientists can estimate the contribution of grain boundary sliding to the overall deformation of the material.
Experimental evidence and the impact of nanomaterials
Since 1962, grain boundary sliding has been observed in multiple experiments, and its results have led researchers to rethink the properties of nanostructured materials. Nanocrystalline materials, due to their fine grains, help reduce creep effects under normal conditions, but may become disadvantageous in high-temperature environments due to grain boundary sliding.
Precautions and Applications
Controlling the size and shape of grains is an important strategy to reduce grain boundary sliding. Coarse-grained materials usually delay the onset of slip, while single crystals can even completely suppress this phenomenon. In addition, by adding small precipitates at the grain boundaries, the grain boundaries can be effectively strengthened and unnecessary slip can be reduced.
Steel Simulation Effect
The application of high-strength steel is ubiquitous in the engineering world, and simulation research on this type of material is crucial for actual construction. By inputting parameters such as elastic modulus, yield strength and temperature, the cycle and behavior of steel during deformation can be predicted, especially the strength performance of grain boundary sliding at high temperature.
Application cases of titanium wire
The tungsten filament used in light bulbs can operate at temperatures ranging from 2000K to 3200K. Understanding and preventing creep mechanisms is critical to extending their service life. The study found that slip in tungsten wire is mainly due to the diffusivity of grain boundary flow. By improving the coating, such as germanium or a sodium-potassium mixture of germanium, this grain boundary sliding can be significantly reduced, thereby extending the life of the tungsten filament to more than 440 hours.
As we gain a deeper understanding of grain boundary sliding, we can't help but ask, how can we further exploit this mechanism to improve and extend the service life of high-performance materials in the future?
