Metals are generally crystalline, and it is usually difficult to become amorphous. Metallic glass, as an emerging amorphous metal material, exhibits many advantages such as high strength, high elasticity and wear resistance, which has aroused great interest. For example, Apple has signed an agreement with Liquidmetal, a well-known amorphous alloy research company, to jointly develop amorphous metal materials and technologies for the communications market, which has attracted widespread attention. As a potential structural material, the law of fatigue fracture under fluctuating stress in industrial applications is a question that must be answered. However, due to the limitations of experimental conditions and understanding, there are few studies and reports on the mechanical behavior of metallic glass under cyclic loading, and many basic key scientific questions need to be answered urgently.
The Micro-nano Scale Materials Research Center of the State Key Laboratory of Metal Material Strength of Xi’an Jiaotong University has conducted a long-term exploration in metal mechanical behavior. In a research paper published online in the Proceedings of the National Academy of Sciences, Professor Shan Zhiwei of the center and his collaborators published their latest research results. With the aid of the transmission electron microscope quantitative in-situ nanomechanical testing device designed by themselves, they selected aluminum-based amorphous samples as the research object, and carried out fixed strain cyclic loading of amorphous metals within the elastic limit. After a certain number of cycles, micro-cracks appeared in the amorphous material and eventually formed several tiny grains in the crack tip area. Surprisingly, some of these tiny grains will evolve into a large grain as the loading progresses, and this large grain will prevent the growth of microcracks, thereby effectively increasing the damage resistance of the material.
In bulk metallic glass, because the shear deformation zone (STZ) forms a correlation effect in space and time, plastic deformation usually occurs locally in the shear zone. However, the molecular dynamics simulation results in this paper show that under the action of cyclic loading, STZ, which is the basic unit of plastic deformation carrying metal glass, not only undergoes shear deformation under stress, but also has some relative diffusion between the constituent atoms of STZ. . During a single load, due to the rapid expansion of the shear band, the diffusion effect is usually not obvious; however, under the action of cyclic loading, the atom diffusion in STZ will have sufficient time to gradually accumulate and eventually make some atoms form a long-range ordered arrangement Crystallization occurs in this way. These findings are of great significance for the study of the intrinsic fatigue and fracture behavior of amorphous metals, and have laid experiments and theories for the application of amorphous metals in micro / nanoelectromechanical systems (M / NEMS) and structural performance optimization. basis.
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