Study On Mechanical Behavior And Structural Evolution Of Metallic Glasses

Amorphous alloy （also known as metallic glass） is a kind of material withmetastable structure. Compared to traditional crystalline metallic materials, there areno typical crystalline defects, such as grain boundaries, dislocations, stacking faults,and so on in metallic glasses. Therefore, metallic glasses have many superiorcomprehensive performances. However, metallic glasses usually exhibit poor plasticdeformability at room temperature because of their shear localization. Effectivelyimproving plastic deformability of metallic glasses is a key point for using metallicglasses to replace many traditional crystalline materials. Up to date, the research workon the improvement of the plasticity of metallic glasses is still lack of a unified theory.Therefore, study on mechanical properties of metallic glasses, based on the plasticdeformation mechanism investigation, might bring important implications forunderstanding and improving various properties of metallic glasses. In this paper,compression test, elasticity test, digital scattering correlation method, high energyX-ray diffraction and in-situ tensile test under transmission electron microscope willbe used to investigate systematically the mechanical behavior and structural evolutionof metallic glasses in a wide temperature range.The compression tests suggest that the yield strength and the plasticity of aZr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5metallic glass are improved with the decreasing testtemperature. A statistics analysis and a dynamic analysis of the intermittent plasticflow in compression curve suggest that the dynamic behavior of serrated flowbehavior, which associates with shear banding behavior, changes from a chaotic stateto a self-organized critical state when the temperature decreased from roomtemperature to203K. The study finds that the volume of shear transformation zone,the activation energy of shear deformation, the behavior of the shear bands and thetemperature are closely correlated. These factors dominate the plastic flow of themetallic glass. The structural evolution of the Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5metallic glass withtemperature decreasing is investigated by in-situ high energy X-ray diffractiontechnique. When the temperature is higher than the glassy transition temperature, thecrystallization behavior occurs; its structure changes; and a non-linear variation canbe observed from the results of high-energy X-ray diffraction pattern. When thetemperature is less than approximately200K, the structure of the metallic glassshows more order, in which the energy state of the glass phase is reduced. Aconcordant shifting region model is introduced to describe the relationships amongtemperature-structure-properties.To reveal the relationship between shear band and deformation, in-situ tensiletests under transmission electron microscopy are carried out. According to the crackpropagation process in the Cu₅₀Zr₄₅Al₅metallic glass and its composite, twocompletely different crack propagation manners are discovered, i.e. void coalescencecrack propagation, and continuous crack propagation. The composite crackpropagation behaves the void coalescence propagation style, and the expansion of thepure metallic glass is the continuous expansion. They both occur in the plastic zone infront of crack tip, but are distinguished by their different heterogeneity state.