Mechanical Behaviors Of Zr-and Hf-based Bulk Metallic Glasses Over Quasi-static To Dynamic Compression

In this thesis, the isothermal crystallization behaviors and glass forming ability ofZHf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀bulk metallic glass （BMG） were investigated by differentialscanning calorimetry （DSC）, transmission electron microscopy （TEM） and X-raydiffraction （XRD）. The dynamic compression response characteristics, quasi-staticcompression response characteristics and fracture characteristics ofZr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5and ZHf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀bulk metallic glasses wereinvestigated by split Hopkinson pressure bar（SHPB）, two-stage light gas gun andMTS810.The study of isothermal crystallication behaviors showed that ZHf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀BMG had obvious kinetic feature. Its crystallization activation energy was 655 kJ/mol andit showed a higher thermal stability. The analyses of TEM, HRTEM and XRD showed thefollowing phase transition process during isothermal annealing. The primary cubiccrystallization phase Al₁₆Hf₆Ni₇could be found after isothermal annealing for 3600 s at723 K. The crystallization phases of Al₁₆Hf₆Ni₇（Cubic）, Ti₂Cu₃（Tetragonal） andCu3Ti（Orthorhombic） could be found after isothermal annealing for 3600 s at 763 K.While the main crystallization products were CuTi2and Al₁₆Hf₆Ni₇after isothermalannealing at 783 K.Under the quasi-static compression condition, the experimental results showed thatthe Hf-based and Zr-based BMGs were all not sensitive to strain rate. The fracturestrength of Hf-based BMG was higher than Zr-based BMG. The mode of fracture wasbrittleness fracture. The fracture strength increased with the separation of CuTi2. But thefracture strength decreased with the separation of CuHf₂. A large number of nanoscaledimple structure observed on the fracture surface revealed the performance of the localplasticity, which indicated that the normal stress played an important role in the fractureprocess. Meanwhile, the periodic stripes structure could be observed on the fracturesurface of Hf-based BMG, its spacing about 60 nm. The produce of periodic stripes wasthe result of the interaction of elastic wave and the extension of dynamic crack. While theyield strength significantly decreased with the increase of strain rate in conditions of thedynamic compression. The metallic glass presented negative strain rate sensitivity. Thefracture morphology of the vein-like patterns and branch-like patterns, while a partial melting phenomenon, could be found. The results showed that the adiabatic heating wasled because a large number of elastic strain energy accumulated and instantly released atthe condition of high strain rate. And thus its local viscosity decreased rapidly, thenviscous rheological layer formed. In addition, the rapid crack initiation and propagationaccompany with the generation of multiple shear bands and fracture, which increasesadiabatic heating. The further local softening of metallic glass in a wider range led to thesharp decline in the strength of the material.At lower strain rate conditions, Zr based BMG showed the ductile fracturecharacteristics. The vein-like patterns and local molten phenomenon could be found on thefracture surface. But the periodic stripe structure was not found. While at higher strain rateonditions, the periodic corrugations could be observed on the fracture surface of Zr-basedBMG. Its spacing was about 100 nm. The relationship of the spacing of the periodiccorrugations couldn’t be observed with the strain rate. Under shock wave at the higherimpact velocities, obvious traces of laminated spallation at the back （free） surface andmelting （liquid droplets） at impact point were observed. The changes of angles to theshocking direction were observed because of the interaction between the compressiveshock waves and the rarefaction waves. Evidences of melting （liquid droplets） due to highstrain rate deformation were observed at the fracture surface. The coupling of mechanismand heat were presented at the dynamic damage. The compressive normal stress inducedthe consequent temperature rise in the core of a shear band. The temperature and thesoftening effected the formation of vein and dimple structure. The multiplication of thefree volume led to the decrease of viscosity and strength. Under loads, the crack wouldextend and broken along the shear zone of low viscosity and micro-cracks. The lightemission lasted from 200μs to 1500μs and the intensity increased from 44 to 900W/（Sr.μm） with the increase of velocity. The deformation of the sample was more severeand the stored elastic strain energy was much high with the increase of impact velocity. Itwas believed that the interaction between the compressive shock wave and the rarefactionwave from the back and lateral surface induced severe deformation and fracture of thesamples. The light was emitted initially due to thermal energy as temperature increasedseverely, and then excess energy motivated the hot particles so that they could cross thedetector’s field of view.