Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of "flow unit" in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.%非晶合金是一类具有诸多优异性能的先进金属材料,同时也是研究非晶态物质的模型体系.最近,大量的实验和模拟证据显示,在非晶合金中可能存在类似晶体中缺陷的"流变单元",这些动力学单元和非晶合金的的流变、物理、力学性能密切关联.本文主要综述了流变单元提出的背景、实验证据、流变单元的特征、激活与演化过程、相互作用以及相关的理论.文中提供了大量实验证据证明流变单元模型不仅可以帮助理解非晶态物质中如形变、玻璃转变、弛豫动力学以及非晶结构和性能的关系等重要的基本物理问题,而且可以指导非晶合金性能的调控和设计,获得性能优异的非晶合金材料.
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