The arrangement of atoms in metallic glasses is disorder, resulting in their novel mechanical properties and exhibiting deformation and fracture behaviors different from those in convenitional crystalline materials. Due to their high fracture strength and hardness, metallic glasses have been regarded as potential engineering materials since its birth in 1960. Though their glass-forming ability (GFA) has been studied for a long time, and many alloy systems with good GFA have been developed, the mechanical properties and deformation mechanisms of metallic glasses were not well understood so far. In the present thesis, the deformation and fracture behaviors of a series of metallic glasses and their composites were investigated systematically under different loading modes.
The tensile and compressive fracture of metallic glasses follows different yielding criteria, leading to the asymmetry between the tensile and compressive deformation and fracture in metallic glasses. The shear-cleavage coefficient is regarded as important parameter influencing the plastic deformation and fracture modes of different materials. The shear-cleavage coefficient is relatively small for the as-cast and low-temperature annealed Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 metallic glass composites, thus shear fracture and certain plasticity were produced in them under tension. However, the shear-cleavage coefficient is obviously large for the Ti60Cu14Ni12Sn4Nb10 nanostructured composite and the high-temperature annealed Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 metallic glass composites, resulting in nearly zero plasticity and normal fracture with the fracture angle of in them under tension.
The tensile fracture of metallic glass can be divided into three stages: the multiplication of free volume, the coalescence of free volume and the formation of void, and the fast propagation of shear crack. The concept of the critical shear offset can well explain the size effect on the tensile plasticity: when the specimen size is larger than the equivalent critical shear offset, the shear deformation is unstable, leading to the global brittle fracture in metallic glasses; when the specimen size is smaller than the equivalent critical shear offset, the shear deformation is stable, resulting in some tensile plasticity in metallic glasses.
The aspect ratio can also influence the compressive plasticity of metallic glasses. The high compressive plasticity in the specimens with small aspect ratio results from the multiple shear bands induced by the constraining stress. Macroscopically brittle or ductile deformation behavior can occur in chemically and structurally identical metallic glass. The concept of critical shear offset can well explain the strong size effect on the enhanced plasticity of metallic glass by taking the shear fracture energy density into account.
There are two compressive fracture modes, i.e. shear fracture and fragmentation, which are related to the surface energy of metallic glasses. The fragmentation coefficient can be regarded as an external parameter characterizing the brittleness of metallic glasses. The dynamic instability of metallic glasses can be divided into two stages, i.e. mirror region and hackle region. The mirror region is characterized by the periodic nanoscale steps produced by the low-velocity dynamic crack propagation, and the hackle region is characterized by the micron crack bifurcation during the high-velocity dynamic crack propagation.
Under small punch loading, regular multiple shear bands were formed in the Zr52.5Ni14.6Al10Cu17.9Ti5 metallic glass, which results from the complex biaxial tension condition of the test. With the annealing temperature increasing, the failure modes of the metallic glasses change from the circumferential shear fracture to radial shear fracture, and finally to radial normal fracture. The propagation of shear bands can be well controlled under the small punch loading. Therefore, the small punch technique is a potential method to investigate the evolution of shear band and evaluate the ductility and brittleness of metallic glasses.
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