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Why does the shape of the grain boundaries determine the strength of the material? What is the secret behind this?
In the field of materials science, the study of grain boundary shape and its sliding behavior can help us unlock the key to material strength. Grain boundaries are the boundaries between different grains in polycrystalline materials, and the shape of these boundaries not only affects the mechanical properties of the material, but also determines the deformation behavior of the material in a high temperature environment.
Grain boundary sliding (GBS) is one of the main mechanisms of material deformation, especially at high temperatures, this phenomenon is more common.
As external stresses act, the grains may begin to slide against each other, a behavior that occurs at high temperatures and low strain rates. The study found that the two main forms of grain boundary sliding include Rachinger sliding and Lifshitz sliding, and that the layout and shape essentially determine the rates of these sliding.
During high temperature creep, grain boundary sliding is almost always associated with lattice diffusion. When the grain boundaries exhibit a wavy morphology, their shape can be simulated by a sine curve. The ratio of the wavelength to the amplitude of the grain (λ/h) has a significant effect on the creep rate. When this ratio increases, the sliding rate increases, and grain boundary diffusion can promote this process.
A high ratio of λ/h may hinder the diffusion flow, eventually leading to the formation of voids and initiating material fracture.
In studies of different materials, grain boundary sliding has been shown to be particularly important for fine-grained materials. It has been shown that Lifshitz sliding contributes about 50-60% of the deformation strain during Nabarro-Herring diffusion creep. This also shows that grain boundaries are not only the weakness of the material, but to some extent, they are also the source of its strength.
From the perspective of different forms of grain boundary sliding, Rachinger sliding is an elastic deformation, and the grains mostly maintain their original shape. However, when uniaxial stress is applied, the bonds between the grains will be relative, allowing the grains to rearrange along the stress direction. Lifshitz sliding, on the other hand, relies on diffusion processes, meaning that when stress is applied, the shape of the grains changes, leading to a completely different deformation behavior.
This makes the study of grain boundary sliding and its related mechanisms an important topic in materials science. As the temperature increases, many complex processes occur simultaneously, and the relationship between grain boundary sliding and other deformation mechanisms such as dislocation motion and diffusion becomes increasingly interesting.
We can use some experimental methods to estimate the contribution of grain boundary sliding to the total deformation, which is of great significance in the strength design of structural materials.
In superplastic deformation technology, the mechanism of sliding via grain boundaries is frequently used. Moreover, in different metal and ceramic materials, grain boundary sliding also leads to different degrees of microstructural changes and destructive behaviors. Future research may further reveal the underlying principles of grain boundary shape and its decisive influence on the mechanical properties of materials, and provide a more reliable theoretical basis for material design.
In summary, the influence of grain boundary shape and its sliding behavior does play an important role in the strength of the material, which leads to an important question to think about: in future material design, how can we more effectively use this phenomenon to improve the performance and life of the material?
