Nanometals have poor stability due to the introduction of a large number of grain boundaries. Generally speaking, the crystal grain growth temperature of nanocrystals is much lower than the recrystallization temperature of coarse crystals, and some nanocrystal pure metals grow up even at room temperature. Poor stability has become the main bottleneck restricting the preparation and application of nano-metals. The traditional method of stabilizing nanocrystals is mainly through alloying to reduce the interface energy or form a drag effect on the migration of grain boundaries.

In recent years, the Nanometal Scientist Studio of the Shenyang National Research Center for Materials Science, Institute of Metal Research, Chinese Academy of Sciences has carried out systematic research on the stability of nanometals. In 2018, studio researchers found an abnormal grain size effect on the thermal stability of nanocrystals in nanocrystalline pure copper and pure aluminum prepared by plastic deformation, that is, smaller than the critical size, as the grain size decreases, the material deforms The mechanism changes from full dislocation dominance to incomplete dislocation dominance, the grain boundary relaxation mechanism is activated, and the stability of nanocrystals does not decrease but rises (Science, 360, 2018). Later, they found that although the internal mechanism of grain boundary migration under heating is different, the mechanical stability of nanocrystals under stress also has this abnormal grain size effect (Phys Rev Lett, 122, 2019). This research was selected as a highlight work by Science with the topic "A size limit for softening" (Science, 364, 2019).

It is understood that in some nano-metals, such as pure copper, nano-crystalline grains grow up even at room temperature. This inherent instability, on the one hand, brings difficulties to the preparation of nano-metal materials, on the other hand, it also limits the practical application of nano-metal materials.

The study also found that the abnormal stability of nanocrystals not only occurs in metals with low and medium stacking fault energy such as pure copper, but also occurs in pure nickel with high fault energy. The discovery of ultra-high stability nanocrystals is not only very important for us to understand the deformation mechanism of nanocrystals and the behavior of grain boundaries at nanometer sizes, but also shows the possibility of developing nanocrystals used at high temperatures.

However, the grain size of pure metals prepared by the commonly used severe plastic deformation methods (such as equal channel extrusion, stack rolling, etc.) is usually on the sub-micron scale, and it is difficult to activate the grain boundary relaxation mechanism during processing. For example, the grain size of pure copper prepared by severe plastic deformation is mostly in the range of 100-200 nm, with poor stability, and its grain growth temperature is much lower than that of coarse crystals. Recently, research by studio Li Xiuyan and others found that rapid heating can introduce annealing twins in nanocrystalline copper, thereby achieving "thermal relaxation" of nanocrystalline grain boundaries and improving the thermal stability of nanocrystalline. One of the difficulties in introducing annealing twins in nanocrystalline copper is: unstable nanocrystals, whose grain growth stability is only 393 K, which is much lower than the temperature of annealing twins (-523 K). Before the twins are produced during the heating process, the crystal grains have grown up first. Based on the Kissinger effect, the researchers proposed that increasing the heating rate can increase the grain growth temperature without affecting the twin growth temperature. Therefore, adopting rapid temperature rise not only avoids grain growth, but also produces growth twins. The pure Cu with a grain size of about 80 nm was rapidly heated to 523 K at a rate of 160 K/min and kept at 523 K for 15 minutes and then cooled. The grain size of the material did not change significantly, but the number of twins increased significantly. Like deformation twins, these growth twins can also relax grain boundaries and enhance the thermal stability of nanocrystals. After heat treatment, the apparent growth temperature of nanocrystals increased from below 393 K to above 773 K.

The thermal relaxation method of rapid heating to improve the stability of nanocrystals can be used to improve the stability of submicron and nanocrystals obtained by general severe plastic deformation, which is of great significance for the development of highly stable nanomaterials and the promotion of the application of nanometals.

This work is supported by the Key R&D Program of the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.

Nanometals have poor stability due to the introduction of a large number of grain boundaries. Generally speaking, the crystal grain growth temperature of nanocrystals is much lower than the recrystallization temperature of coarse crystals, and some nanocrystal pure metals grow up even at room temperature. Poor stability has become the main bottleneck restricting the preparation and application of nano-metals. The traditional method of stabilizing nanocrystals is mainly through alloying to reduce the interface energy or form a drag effect on the migration of grain boundaries.

In recent years, the Nanometal Scientist Studio of the Shenyang National Research Center for Materials Science, Institute of Metal Research, Chinese Academy of Sciences has carried out systematic research on the stability of nanometals. In 2018, studio researchers found an abnormal grain size effect on the thermal stability of nanocrystals in nanocrystalline pure copper and pure aluminum prepared by plastic deformation, that is, smaller than the critical size, as the grain size decreases, the material deforms The mechanism changes from full dislocation dominance to incomplete dislocation dominance, the grain boundary relaxation mechanism is activated, and the stability of nanocrystals does not decrease but rises (Science, 360, 2018). Later, they found that although the internal mechanism of grain boundary migration under heating is different, the mechanical stability of nanocrystals under stress also has this abnormal grain size effect (Phys Rev Lett, 122, 2019). This research was selected as a highlight work by Science with the topic "A size limit for softening" (Science, 364, 2019).

It is understood that in some nano-metals, such as pure copper, nano-crystalline grains grow up even at room temperature. This inherent instability, on the one hand, brings difficulties to the preparation of nano-metal materials, on the other hand, it also limits the practical application of nano-metal materials.

The study also found that the abnormal stability of nanocrystals not only occurs in metals with low and medium stacking fault energy such as pure copper, but also occurs in pure nickel with high fault energy. The discovery of ultra-high stability nanocrystals is not only very important for us to understand the deformation mechanism of nanocrystals and the behavior of grain boundaries at nanometer sizes, but also shows the possibility of developing nanocrystals used at high temperatures.

However, the grain size of pure metals prepared by the commonly used severe plastic deformation methods (such as equal channel extrusion, stack rolling, etc.) is usually on the sub-micron scale, and it is difficult to activate the grain boundary relaxation mechanism during processing. For example, the grain size of pure copper prepared by severe plastic deformation is mostly in the range of 100-200 nm, with poor stability, and its grain growth temperature is much lower than that of coarse crystals. Recently, research by studio Li Xiuyan and others found that rapid heating can introduce annealing twins in nanocrystalline copper, thereby achieving "thermal relaxation" of nanocrystalline grain boundaries and improving the thermal stability of nanocrystalline. One of the difficulties in introducing annealing twins in nanocrystalline copper is: unstable nanocrystals, whose grain growth stability is only 393 K, which is much lower than the temperature of annealing twins (-523 K). Before the twins are produced during the heating process, the crystal grains have grown up first. Based on the Kissinger effect, the researchers proposed that increasing the heating rate can increase the grain growth temperature without affecting the twin growth temperature. Therefore, adopting rapid temperature rise not only avoids grain growth, but also produces growth twins. The pure Cu with a grain size of about 80 nm was rapidly heated to 523 K at a rate of 160 K/min and kept at 523 K for 15 minutes and then cooled. The grain size of the material did not change significantly, but the number of twins increased significantly. Like deformation twins, these growth twins can also relax grain boundaries and enhance the thermal stability of nanocrystals. After heat treatment, the apparent growth temperature of nanocrystals increased from below 393 K to above 773 K.

The thermal relaxation method of rapid heating to improve the stability of nanocrystals can be used to improve the stability of submicron and nanocrystals obtained by general severe plastic deformation, which is of great significance for the development of highly stable nanomaterials and the promotion of the application of nanometals.

This work is supported by the Key R&D Program of the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.

Link to this article：Ultra-high stability nanocrystals found in metals 

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