Recently, Prof. Lu Ke and Li Xiuyan, researcher at the Institute of Metal Research of Chinese, Chinese Academy of Sciences, discovered the anomalous grain size effect of nano-metal mechinery stability. Related results were published online on March 29, 2019 in the Physical Review Letters.

The grain boundary of nano-metals is prone to grain boundary migration under mechanical deformation accompanying grain growth, which makes the nano-material soften. This phenomenon has a lot of experiments and related calculations under deformation conditions such as stretching, compression and indentation. Report of the simulation results. Mechanically driven grain boundary migration not only destroys the properties of the material, but also makes it difficult to prepare nano-crystals by plastic deformation.

Although the fundamental mechanism of mechanically driven grain boundary migration is still controversial, both related models and computational simulations indicate that mechanically driven grain boundary migration is accompanied by significant grain boundary region atomic recombination and dislocation motion, indicating the process and grain boundary state. Have a close relationship. It is generally believed that the grain boundary migration rate under the action of force is related to the grain boundary energy, the curvature of the grain boundary, the effective step on the grain boundary, and the like. The smaller the grain size, the larger the grain boundary curvature and the faster the migration rate.

Lu Ke and Li Xiuyan found that for the nano-crystalline Cu, Ag, Ni (copper, silver, nickel) samples prepared by plastic deformation, the quasi-static tensile deformation decreases with the grain size from submicron to nanometer. The boundary migration firstly increases, and when the grain size is smaller than the critical value, the grain boundary migration is gradually suppressed. This result subverts the conventional understanding, which is consistent with the correlation between the abnormal effects of the nano-crystal thermal stability grain size.

For Cu, Ag, and Ni, the critical grain sizes in the experiment were about 75, 80, and 38 nm, respectively. The results show that the grain boundaries of the nano-crystals below the critical size are prone to strain-induced grain boundary relaxation during plastic deformation, and this grain boundary relaxation inhibits the grain boundary migration behavior, which causes the nano-crystal deformation mechanism to gradually change from grain boundary migration to Incompletely dislocated motion forms deformed twins or stacking faults, and nano-crystalline mechanical stability is enhanced.

The study also found that the nano-crystals without mechanical relaxation near the critical dimension of Cu were heat-treated by a suitable annealing process to thermally relax the grain boundaries while maintaining the grain size substantially stable, and further tensile deformation process. The grain boundary migration is obviously suppressed, and the grains exhibit higher mechanical stability.

This finding shows that similar to the grain boundary segregation effect, the grain boundary relaxation effect related to grain size can significantly inhibit the mechanical drive grain boundary migration, which provides a new method for improving the mechanical stability of nanocrystals. It also provides an important reference for the development of nano-crystalline preparation processes. (By Shen Chunlei, Liu Yan)
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