The diffusion rate of atoms in metals is very high. By adjusting the diffusion process, their structures and properties can be greatly adjusted, but this leads to their unstable properties at high temperature. Eliminating diffusion interface by making single crystal or a large amount of alloying is helpful to solve this problem, but atomic diffusion can not be inhibited at higher homologous temperature.

Here, Li Xiuyan of the Institute of Metals, Chinese Academy of Sciences &; Academician Lu Ke and other researchers found that Schwartz crystal structure can effectively inhibit the atomic diffusion of ultra-fine grains in supersaturated Al-Mg alloy. The related paper was published in the latest issue of Science, entitled "Using Schwartz crystal structure to ensure that Al-Mg alloy inhibits atomic diffusion".

Paper link:

https://science.sciencemag.org/content/373/6555/683

Because of the nature of bonds between atoms, the atomic diffusivity in metals is obviously higher than that in ceramics and compounds with covalent or ionic bonds. By customizing the diffusion control process in the process of synthesis and subsequent treatment, this feature makes the structure have great adjustability in different length scales, thus making metal materials have a wide range of properties and characteristics. For example, aluminum alloys can be hardened by aging at near room temperature to precipitate intermetallic compounds. 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 has become the main bottleneck of the development of metal materials, which greatly limits its technical application at high temperature.

Preventing the diffusion of atoms in metals is a challenge, especially at high temperatures. In high entropy alloys, several different metal elements are mixed in a lattice, so the alloying of a large number of metals with foreign elements will be limited by inhibiting lattice diffusion. 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 increases, which limits the development of GB alloying.

Eliminating diffusion interface by forming single crystal is a common strategy to reduce diffusion rate, such as the practice of manufacturing superalloy single crystal blades in high temperature applications of turbine engines. However, even in single crystal metals, high diffusion coefficients cannot be suppressed at higher temperatures. Substitution diffusion and self-diffusion are controlled by vacancy diffusion mechanism. At higher temperature, the equilibrium vacancy concentration in the lattice increases significantly, which will inevitably increase the diffusivity of atoms.

Recently, researchers have found a metastable structure with extremely fine grains in pure copper: Schwartz crystal structure, and its interface is least limited by twin grain boundaries. Although it contains very high density interface, this structure shows very high thermal stability at high temperature near the melting point to prevent grain coarsening. Therefore, it is very interesting to explore whether this stable Schwarz crystal structure can inhibit the diffusion of atoms in the alloy at high temperature.

Aluminum is a highly diffusible metal, and magnesium is one of its most diffused alloy elements. Here, the researchers 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. It is found that Schwartz crystal structure can effectively inhibit atomic diffusion before the structure melts. By forming these stable structures, the precipitation of diffusion-controlled intermetallic compounds from nanocrystals and their coarsening are inhibited until the equilibrium melting temperature, and the apparent cross-border diffusion rate decreases by about 7 orders of magnitude near the equilibrium melting temperature. Using Schwarz crystal structure to develop advanced engineering alloys may bring useful properties for high temperature applications.

Fig. 1 SC-8 structural features.

Structural evolution after annealing.

Stability of lattice constant and grain size.

Fig. 4 Distribution of elements after annealing.

Here, the results observed by researchers in this supersaturated Al-Mg alloy are consistent with those previously observed in pure Cu Schwarz crystal samples, which is a self-diffusion control process. The non-diffusion characteristics of Schwarz crystal structure in metals are of great significance for understanding the basic interfacial diffusion process and solid transport mechanics, especially the interfacial diffusion process at high temperature. Schwarz 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. This stability is much higher than that of traditional alloys. Using Schwartz crystal structure to develop advanced aluminum and other alloys will make materials have useful characteristics in high temperature applications. (Text: Aquatic)

This article comes from WeChat official account "Materials Science and Engineering". Welcome to reprint, please contact, refuse to reprint to other websites without permission.