Compared with ceramics and compounds with valence bonds or ionic bonds, the properties of interatomic bonds make the diffusion rate of atoms in metals higher. This high diffusivity makes the metal structure have great adjustability in different length scales, but it also brings structural instability and customization performance. This instability is the main bottleneck of the development of metal materials, which greatly limits its technical application at high temperature.

On August 6th, local time, the top academic journal Science published a research report entitled "Schwartz crystal structure in supersaturated al-mg alloy inhibits atomic diffusion", which provides a new method to solve the instability caused by high atomic diffusion rate in metals at high temperature.

The correspondent of this research report is Academician Lu Ke and Researcher Li Xiuyan, doctoral supervisor of Institute of Metals, Chinese Academy of Sciences.

Lu Ke has been devoted to the research of metal nanomaterials for more than 20 years, published more than 400 papers in academic journals and obtained more than 40 invention patents. He has won Acta Materialia Gold Award, Humboldt Research Award, Outstanding Young Scholar Award of the First Hong Kong Qiushi Foundation, ISMANAM Gold Award of the International Conference on Metastable and Nanomaterials, China Young Scientist Award, He Liang Heli Foundation Science and Technology Progress Award and TWNSO Science and Technology Award of the Third World Academy of Sciences. In 2020, Lu Ke won the "Material Science Award" for his pioneering discovery and utilization of nano-twin structure and gradient nanostructure to achieve high strength, high toughness and high conductivity of copper.

From 20 18 to 5438+00 in June, Lu Ke was appointed as the vice governor of Liaoning Provincial People's Government, responsible for science and technology, sports and other aspects. In charge of Liaoning Provincial Science and Technology Department (Foreign Experts Bureau), Sports Bureau, Major Technological Innovation and R&D Base Construction Engineering Center (Industrial Technology Research Institute).

It is written in the paper that due to the nature of interatomic bonds, the atomic diffusivity in metals is obviously higher than that in ceramics and compounds with valence bonds or ionic bonds. In the process of synthesis and subsequent treatment, by adjusting the diffusion control process, the structure has great adjustability in different length scales, thus making metal materials have a wide range of properties. For example, aluminum alloys are hardened by precipitation of intermetallic compounds near room temperature. In thermomechanical treatment, the strength and plasticity of steel can be widely adjusted by controlling diffusion phase transformation.

However, when the metal is exposed to high temperature or mechanical load, the high atomic diffusivity makes the structure and customization performance of the metal unstable. This instability is the main bottleneck of the development of metal materials, which greatly limits its technical application at high temperature.

The research team mentioned that it is a challenge to inhibit the diffusion of atoms in metals, especially at high temperatures. Interfaces or grain boundaries (GBs) associated with more open structures are considered as fast diffusion channels of atoms relative to the lattice. By optimizing the grain boundary segregation of other elements, the diffusion along the grain boundary can be slowed down. However, with the increase of alloying degree, the trend of second phase formation is enhanced and the interfacial alloying is limited.

Eliminating diffusion interface by forming single crystal is considered as a standard strategy to reduce diffusion rate, for example, the practice of manufacturing super alloys single crystal blades in high temperature applications of turbine engines. However, the research team believes that even in single crystal metals, its high diffusivity cannot be suppressed at higher temperatures. At higher homologous temperature, the equilibrium vacancy concentration in the lattice increases significantly, which will inevitably increase the diffusion rate of atoms.

In 2020, an important achievement of Lu Ke and others in Science showed that they found a metastable structure with very fine grains in pure copper, namely Schwartz crystal structure. The research team mentioned that although it contains extremely high density interface, this structure shows very high thermal stability at high temperature near the melting point to prevent grain coarsening.

Therefore, the research team believes that it is very meaningful to study whether this stable Schwarz crystal structure can inhibit the diffusion of atoms in the alloy at high temperature.

Structural characterization of sample SC-8.

In this latest research, the research team used a high-pressure twisting device to deform the single-phase supersaturated Al-Mg (Al-Mg) alloy under the hydrostatic pressure of 77K and 10GPa. When the applied strain exceeds ~20GPa, the microstructure of the alloy sample is refined to nanometer level, and randomly oriented nanocrystals with approximately equal axes are formed in the sample. The particle size distribution is uniform, and the average particle size is 8nm (sample SC-8). Through a series of analysis and tests, it is shown that the supersaturated Mg atoms are uniformly distributed in the nanocrystalline structure, instead of gathering or dispersing on GB like other Al-Mg alloys deformed at room temperature.

Aluminum is a highly diffusive metal, and magnesium is one of its most diffusive alloy elements. The research group observed the diffusion behavior of supersaturated Al-Mg alloy with Schwarz crystal structure. The diffusion process of intermetallic compounds at different temperatures, such as precipitation, grain coarsening and melting, was studied.

Structural evolution of samples during annealing.

It is found that this minimum interface structure can not only reduce the apparent cross-border diffusion rate of atoms in supersaturated Al-Mg alloy by about 7 orders of magnitude, but also keep the alloy structure unchanged at a temperature higher than the melting point.

It is considered that the observed results in this supersaturated Al-Mg alloy are consistent with those observed by our team in pure copper Schwarz crystal samples, and it is a self-diffusion control process.

Stability of lattice constant and grain size of samples.

It is mentioned that the non-diffusion characteristics of Schwarz crystal structure in metals are of great significance for understanding the basic diffusion process in the interface and the mechanics of solid transport, especially the diffusion process at high temperature. Schwartz crystal seems to provide a solid barrier to prevent the diffusion of atoms in metals and substitute alloys, and improve the stability of melting point temperature, which is much higher than that of traditional alloys.

Lu Ke and others believe that using Schwartz crystal structure to develop advanced aluminum and other alloys will make these materials have beneficial characteristics in high temperature applications.

Element distribution of annealed samples.

It is worth noting that this is the 13 article published by Lu Ke in Science since 2000. In addition, he published 1 articles in another top journal Nature in 20 10. Lu Ke, aged 56, graduated from Nanjing University of Science and Technology with a major in metal materials and heat treatment 1985, and received a doctor's degree in metal engineering 1990. In 2003, he was elected as an academician of China Academy of Sciences (the youngest academician), in 2005, in 2006, he was appointed as a review editor of American Science Weekly, and on 20/0/8, he was elected as a foreign academician of the National Academy of Sciences.

Proofreading: Zhang