Deformation Behavior Of Zr-Based Bulk Metallic Glasses

Metallic glasses can be used as structural and advanced functional materials in many fields and have extremely important applications due to their excellent properties. However, the practical applications of metallic glasses are severely constrained because of their limited glass forming ability (GFA) and plasticity. In the dissertation, we studied the effects of minor addition on the GFA and deformability of a Zr-based bulk metallic glass (BMG). Additionally, a series of experiments were designed to study the deformation behavior of BMGs.The effects of alloying elements Al, Fe and Ag on the GFA of Zr₃5Ti₃₀Cu_7.5Be_27.5 alloy were discusssed. It was found that 1at.% Al (or Ag) or 2, 3 , 5 and 7at.% Fe addition can greatly improve the GFA of Zr₃5Ti₃₀Cu_7.5Be_27.5 alloy and the critical size were increased from 15 mm to larger than 20 mm. Meanwhile, the thermodynamic criterion (ΔT_x orγor T_rg) were effective in evaluating the GFA of Zr_35-xTi₃₀Cu_7.5Be_27.5M_x(M=Al, Fe and Ag) glass forming systems.The uniaxial compression performance of Zr_35-xTi₃₀Cu_7.5Be_27.5M_x (M=Al, Fe and Ag) BMGs were studied. The results indicated that BMG (3 mm in diameter) containing 1.5at.% Al showed a irreversible strain of 15.3% and excellent deformability.The morphologies of the fractured samples were examined by scanning electron microscopy (SEM). Different from conventional vein-like pattern, the fracture morphology of BMGs containing 10at.% Fe, 7 and 10at.% Ag was similar to that of quasi-cleavage fracture. The microstructures of Zr_35-xTi₃₀Cu_7.5Be_27.5M_x (M=Al, Fe and Ag) BMGs were investigated by using transmission electron microscopy (TEM). It was found that TEM bright field images of BMGs of Zr_35-xTi₃₀Cu_7.5Be_27.5Al_x (x=1.5 and 2), Zr_35-xTi₃₀Cu_7.5Be_27.5Fe_x (x=1 and 2) and Zr_35-xTi₃₀Cu_7.5Be_27.5Ag_x (x=1, 2, 3, 5 and 7) were composed of phases with brighter and darker contrasts, indicating phase separation occurred during sodification.Nanoindentation experiments were carried out to study the influence of annealing temperatures on deformation behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG. The results showed that the deformability of the sample annealed at 553 K was significantly better than that of the as-cast samples and the ones annealed at 693 K.The deformation of metallic glasses was mainly caused by the formation and propagation of shear bands. In the current work, the sliding displacements of shear bands under compression were measured precisely by field emission scanning electron microscopy (FESEM). From the FESEM observations, the sliding displacements of shear bands accounts for 95.32% and 93.89% of the total irreversible strain for Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG and Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG, respectively.For further understanding the role of shear bands played during deformation in BMGs, innovative and reliable compression-tension experiment was designed. We comparatively studied the influence of the shear bands formed during compression on the tensile strength of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG and the 304 stainless steeel. It was found that while the tensile strength of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 BMG decreased remarkably after precompression, the yielding and tensile strength of the 304 stainless steel remain nearly the same as that without pre-compression. The evolution of shear bands in BMGs was also discussed.