Abstract: metallic glasses feature high strength, high hardness, high elastic strain and satisfactory corrosion resistance. However, suffering from small dimension and high brittleness, metallic glasses can hardly realize widespread application. Free volume plays a crucial role in affecting the glass transition, plastic deformation, structure and properties of metallic glasses. Accordingly, it is of great significance to straighten out the deve-lopment of free volume theory. In this work, we firstly introduce the concept of free volume, then discuss the relationship of free volume with glass transition, plastic deformation and properties, finally summarize the quantitative methods for free volume evaluation. Free volume is involved in basic processes of glass transition and plastic deformation of metallic glass. Due to low coordination number and weak mechanical bonding with the surrounding matrix, structural disturbance is prone to occur at free volume sites of metallic glasses under the actions of temperatures and stresses. Glass transition is demonstrated to be a dynamic process which is largely depended on free volume. Glass transition occurs when the reduced free volume content reaches a certain critical value. Besides, the plastic deformation mechanism of metallic glass can be also explained by free volume theory. The atomic transition and further plastic deformation can be realized with the assistance of free volume. moreover, when the free volume content reaches a certain critical value, the localization of macroscopic deformation appears, namely, shear bands generate. It has been proved by molecular dynamics simulation, theoretical calculation, transmission electron microscopy, acoustic emission monitoring, enthalpy measurement and density measurement that there exist a large amount of free volume in shear bands. Generally speaking, free volume can be produced by microalloying, plastic deformation, ion beam irradiation, increasing the cooling rate, and decreasing the arc power. It is commonly believed that higher free volume content results in more sites for nucleation of shear bands, and further leads to the formation of multiple shear bands. meanwhile, with the increasing free volume content, the viscosity decreases, yet the atomic mobility and plasticity increase. Besides, the loose arrangement of atoms and low elastic modulus at free volume sites cause the weak deformation resistance of metallic glass. Consequently, the strength and hardness of metallic glass decrease with the increase of free volume content. moreover, free volume also exert notable effects on the atomic diffusion rate and thermal stability of metallic glasses. The major approaches for determine the free volume content are as follows. (1) Thermal analysis: the free volume content is determined by calculating the exothermic enthalpy during the structural relaxation process. (2) Positron annihilation spectrum: free volume content is determined by detecting positron lifetime. (3) Density measurement: the introduction of free volume will cause the density variation, hence the free volume content can be determined by density measurement. (4) X-ray diffraction: free volume content is determined by measuring the variation of the maximum diffraction wave vector Qmax in the X-ray diffraction spectrum.
时博, 王金辉, 魏福安. 金属玻璃自由体积理论的研究概述[J]. 材料导报, 2019, 33(7): 1221-1226.
SHI Bo, WANG Jinhui, WEI Fuan. A State-of-the-art Review on Study of Free Volume Theory in metallic Glasses. Materials Reports, 2019, 33(7): 1221-1226.
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