Nanocrystalline materials have attracted more and more attention due to their unique microstructure and excellent properties (such as high strength, high hardness, high toughness, superplasticity, etc.). In the past few decades, most of the research work has focused on how to obtain a small average grain size, while the microstructure uniformity, which is critical to engineering reliability, has rarely been studied. Since the failure of macroscopic materials usually occurs in the weakest link of the microstructure, reducing the grain size distribution and obtaining a uniform microstructure is extremely important for the development and application of polycrystalline materials including nanocrystalline materials.
Recently, researchers from the Massachusetts Institute of Technology, Lanzhou University, University of Pennsylvania and other institutions have made important progress in the field of bulk nanocrystalline and ultra-fine crystalline materials. The concept of ultra-uniform nanocrystalline materials has been proposed for the first time and a systematic theoretical analysis has been carried out. And experimental demonstration. Relevant research results were published in Advanced Functional Materials, the top international journal of materials science, with the title "Ultra-Uniform Nanocrystalline Materials via Two-Step Sintering". Dr. Dong Yanhao from the Massachusetts Institute of Technology is the first author of the paper. Professor Li Ju from MIT, Professor Li Jiangong from Lanzhou University and Professor Chen Yiwei from the University of Pennsylvania are the co-corresponding authors of the paper. The main collaborators of the paper also include PhD student Yang Hongbing from Lanzhou University, Professor Zhang Lin from University of Science and Technology Beijing and PhD student Li Xingyu, Dr. Dong Ding from the Idaho National Laboratory in the United States and Professor Wang Xiaohui from Tsinghua University.
Paper link:
https://doi.org/10.1002/adfm.202007750
People's understanding of particle growth is usually based on the Ostwald ripening process described by the Lifshitz-Slyozov-Wagner (LSW) model. For the grain growth of dense materials, Hillert proposed in 1965 the normalized grain size distribution in ideal materials under steady state. The standard deviation of the size distribution is 0.354, which can be regarded as the Hillert limit of the uniformity of grain size in polycrystalline materials. Due to the complex grain boundary composition and structure in actual materials, the uniformity of the grain size distribution in the dense bulk materials prepared by people is far from reaching this theoretical limit.
Based on the Ostwald maturation mechanism, the author gives an analytical solution to the generalized LSW-Hillert grain growth theory, and analyzes and finds that a larger grain growth index can give a narrower normalized grain size distribution. This indicates that if a large grain growth index can be maintained during the sintering and densification process of bulk materials, then an ultra-uniform polycrystalline material that breaks the Hillert limit may be obtained. This theoretical prediction was verified in partially sintered porous alumina samples. The study found that in the samples with a relative density of 65-70%, the normalized grain size distribution is the narrowest, and the standard deviation of the normalized size distribution is only 0.25. As the subsequent densification process progresses, the grain size distribution gradually widens, and its standard deviation gradually increases. Eventually, with the rapid grain growth at the end of sintering, the grain size uniformity in the dense sample deteriorates rapidly. This trend also appears in yttria stabilized zirconia in tetragonal and cubic phases.
Therefore, in order to obtain ultra-uniform dense polycrystalline materials, a new sintering scheme needs to be designed to retain the uniformity of the microstructure in the middle of sintering. The two-step sintering method proposed by Professor Yiwei Chen from the University of Pennsylvania and Professor Xiaohui Wang from Tsinghua University can just achieve this. One goal. This is because the first sintering in the two-step sintering method can obtain a porous sample with a relative density of 70-85%. At this time, the grain growth index is high and the microstructure uniformity is good; in the second sintering process, due to the grain boundary Migration is not activated, and the sintered body is densified only through grain boundary diffusion, without grain growth. This allows the material to achieve complete densification while retaining uniformity, that is, perfect balance between densification and uniformity.
In general, the super-uniform concept and its system theory and experimental demonstrations proposed in this paper have important reference value and guiding significance for the development and application of nanocrystalline materials. At the same time, the ultra-uniform microstructure obtained in alumina nanocrystalline ceramics shows that if nanoparticles with a narrower or uniform size distribution can be prepared, it is expected that a two-step sintering method can be used to obtain nanostructures with more uniform microstructure and better performance crystal material.