Nanocrystallization Behavior At High Temperature Of Zr-Based Bulk Metallic Glass

In this thesis, the compressive deformation and nanocrystallization behaviors of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG) were investigated in the supercooled liquid region on different conditions. The effect of the nanocrystallization on the mechanical properties of the BMG at room temperature was studied as well.The compressive tests of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG were performed on the Gleeble1500D. The experiment result shows that the compressive deformation behavior of the BMG in the supercooled liquid region is sensitive to deformation temperature and strain rate. Flow stress decreases with the increase of the deformation temperature or with the decrease of the strain rate. But when temperature is too high or strain rate is too low, flow stress increases with the increase of the strain. The BMG exhibits strain softening at low temperature or high strain rate, and strain strengthening at high temperature or low strain rate.By means of X-ray diffraction (XRD), the nanocrystallization behavior of the deformed Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG on different conditions was analyzed. The microstructure and component of nanocrystals in the BMG were further observed by Transmission Electronic Microscopy (TEM). Both amount and size of the nanocrystals increase with the increase of the deformation temperature and the decrease of the strain rate. Initially the spherical nanocrystals disperse in the amorphous matrix, and then aggregate one another. The main crystallization products consist of ZrBe2,Zr2Cu,ZrCu, and the unidentified phases.In order to investigate the effect of nanocrystallization behavior in the supercooled liquid on the mechanical properties, the uniaxial compression tests of constant strain rate at room temperature were performed. Young's Modulus (E) increases with the increase of the deformation temperature and the decrease of the strain rate. Compared with the as-cast BMG, there is almost no change of the plasticity deformation properties of the relaxed samples, but the strength decreases a little. When the deformation temperature is in the range of 385℃³95℃, the mechanical properties of the BMG at room temperature can be improved. But both plasticity and strength become worse when the temperature is above 395℃. When the strain rate is in the range of 1×10-2 s-1¹Ã—10-3 s-1, the plasticity of the BMG at room temperature is better, while the strength is higher during 1×10-3 s-1⁵×10-4 s-1. Finally the compressive fracture at room temperature was observed by Scanning Electron Microscopy (SEM), and the fracture mechanics was analyzed preliminary.