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Workplace hardening in bulk amorphous materials
[ Instrument network instrument R & D ] Work hardening or deformation hardening, that is, the behavior of metal materials that causes strength to increase with plastic deformation, reflecting the material's ability to resist further deformation during uniform plastic deformation. It is the most important phenomenon in the mechanical behavior of engineering materials, and it is also an important basis for the widespread use of metals as structural materials. Amorphous alloys (also called metallic glasses) have many excellent mechanical properties (high yield stress, high toughness, and record-breaking "damage tolerance"), but strain softening is their Achilles heel. Unlike traditional crystalline materials, their deformations are highly localized and manifest as non-uniform deformations dominated by shear bands. This directly leads to its brittleness at room temperature, which becomes the bottleneck of amorphous alloys. Therefore, achieving the work hardening behavior of bulk amorphous alloys is considered to be the core scientific issue in the field of amorphous alloys and even all amorphous materials.
Recently, Li Yi (corresponding author), associate researcher Pan Jie (first author), and Ph.D. student Zhou Weihua from the Institute of Metal Research, Shenyang National Research Center for Materials Science, Chinese Academy of Sciences, and AL Greer (communicator of the Department of Materials, Cambridge University, UK) (Author), Dr. YP Ivanov cooperated for the first time to achieve work hardening in bulk amorphous materials, subverting people's inherent understanding of deformation and softening behavior of amorphous materials, in order to develop amorphous alloys with uniform plastic deformation ability and its industry Applications provide new ideas and directions. Related results were published in Nature on February 26.
Researchers first used a three-dimensional compressive stress method to produce a large range and high degree of rejuvenation in bulk amorphous alloys, and developed an amorphous alloy with an energy state equivalent to a cooling rate of 1010K / s (Nature Communications 2018). On this basis, it was found through uniaxial tensile or compression tests that the bulk amorphous alloy in the high energy state (rejuvenated state) exhibits work hardening and excellent plastic deformation ability when deformed. During the work hardening stage, no shear band was observed, indicating that the alloy has undergone uniform rheology, which is completely different from the deformation behavior of traditional amorphous alloys that rely on shear bands. In addition, the hardening rate of amorphous alloys is much higher than any common crystalline metal system. Comparing the changes in the structure and energy state of the rejuvenated and traditional as-cast bulk amorphous alloys before and after deformation, it was found that the hardness of the rejuvenated amorphous alloy increased significantly during work hardening, but the energy decreased significantly. The results of the radial distribution function of the amorphous alloy show that the structure of the rejuvenated bulk amorphous alloy after work hardening is more orderly (increased in density), which is completely opposite to the deformation process of deformation softening and energy increase of the traditional as-cast amorphous alloy.
The principle of work hardening of crystalline metal is that dislocation proliferation and interaction during the deformation process hinder each other's movement. This micromechanism was first proposed by GI Taylor in 1934. Although other factors such as microstructure can also affect the work hardening behavior of the material, the most basic principle remains unchanged, and it is still the process of defect multiplication and the increase of material energy. However, the results of this study show that the work hardening of bulk amorphous alloys is accompanied by the annihilation and reduction of material defects (more relaxed states), and is a transition process from a high energy state to a low energy state. This is completely contrary to the traditional work hardening process of crystalline materials, indicating that amorphous alloys have completely different work hardening mechanisms. This research is not only a re-understanding of the material hardening mechanism in the past 85 years, but also laid a solid theoretical foundation for the application of amorphous materials as structural materials.
The research was supported by the National Natural Science Foundation of China, the National Research Center for Materials Science of Shenyang, the Institute of Metals, and the Chinese Academy of Sciences.