The Effect Of Heterogeneous Microstructure On The Room Temperature Mechanical Properties In Bulk Metallic Glass

In this dissertation, the effect of heterogeneous microstructure on the room temperature mechanical properties of Zr-Cu-Ni-Al-Ta alloys has been systematically investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), differential thermal analysis (DTA), scanning electron microscope (SEM), mechanical testing machine and microhardness measurement.Alloys with nominal composition of (Zr_75+xCu_25-x)_78.5Ni₁₀Al_7.5Ta₄ (x=0, 5, 11) were fabricated by arc-melting and copper mould casting, three types of microstructures were obtained, i.e., a monolithic bulk metallic glass (BMG) with homogenous amorphous structure (x=0), a monolithic BMG with strong icosahedral medium-range order (IMRO) clusters (x=5) and amorphous/quasicrystalline composite (x=11). The room temperature compression tests were performed on the three types of alloys, and the results indicate that, compared to monolithic BMG with homogenous structure (x=0), the monolithic BMG with strong IMRO clusters (x=5) displays a distinct plasticity with plastic strain as large as 8.3%. However, the plastic deformation is dominated by a single major shear band, exhibiting a typical feature of cold shear.A"core/shell"structural model was proposed here to clarify how the IMRO clusters influence the shear band behaviors and improve the plasticity of BMG: the densely packed IMRO cluster acts as a core, and the core is surrounded by various other loosely packed clusters enriched with a great deal of free volume. It can be seen that significantly heterogeneous distribution of free volume is caused by the formation of a"core/shell"structure, which facilitate the initiation and propagation of shear bands and therefore improve the plasticity of BMG.The effect of cooling rate on internal microstructure and mechanical properties of (Zr₈₀Cu₂₀)_78.5Ni₁₀Al_7.5Ta₄ BMG was investigated. It was found that decreasing cooling rate would promote the formation of denser atomic configuration, which accounts for the higher hardness. Meanwhile, the free volume content would be reduced and the local IMRO clusters may be enhanced to nano-crystalline level (³nm) by the lower cooling rate, both of which would deteriorate the plasticity of BMG.The notch toughness of (Zr₇₅Cu₂₅)_78.5Ni₁₀Al_7.5Ta₄, (Zr₈₀Cu₂₀)_78.5Ni₁₀Al_7.5Ta₄ and (Zr₈₆Cu₁₄)_78.5Ni₁₀Al_7.5Ta₄ alloys was examined by three-point bending tests, and the values of 66.1±7.4MPa·m^1/2, 86.4±4.4MPa·m^1/2 and 24.5±3.2MPa·m^1/2 were obtained, respectively. Consistent with the compression test, the (Zr₈₀Cu₂₀)_78.5Ni₁₀Al_7.5Ta₄ BMG exhibits the highest notch toughness, which originates from the heterogeneous distribution of free volume.