Thermal Stability And Mechanical Behaviors Of Hf-Based Bulk Metallic Glasses

In this thesis, the isothermal crystallization kinetics and crystallization behaviors of Hf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀ bulk metallic glass were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The quasi-static compression behavior, dynamic compression behavior and mechanical properties of bulk metallic glass were studied by MTS810, split Hopkinson pressure bar(SHPB) and scanning electron microscopy (SEM).The results of isothermal crystallication kenetics revealed that Hf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀ bulk metallic glass has obvious kinetic character. The crystallization activation energy is as high as 655.70 KJ/mol, which represtents a higher thermal stability. The metastable primary crystallization phase Al₁₆Hf₆Ni₇ (Cubic) could be observed after annealing for 3600 s at 723 K. After annealing at 763 K, the crystallization phase consisted of Ti₂Cu₃ (Tetragonal), Al₁₆Hf₆Ni₇(Cubic), and Cu₃Ti(Orthorhombic). The main crystallization products after annealing at 783 K were Al₁₆Hf₆Ni₇ and CuTi₂. The analysis results of TEM, HRTEM and XRD also proved the phase transition process during annealing.The experiment results show that the Hf_44.5Cu₂₇Ni_13.5Ti₅Al₁₀ bulk metallic glass isn't sensitivity of strain rate under the quasi-static compression condition. The fracture strength is 2400 MPa. The dimple structure and periodic corrugations (about 60 nm spacing) could be observed on the fracture surface. The fracture mechanism attributed to ductile fracture in the range of micro-scale. In addition, interaction of elastic waves and the dynamic crack front plays a important role during the formation of periodic corrugations.The yield strength rapidly declines with the strain rate rising under the dynamic compression. The metallic glass presents sensitivity of strain rate. The branch-like patterns, vein-like patterns and local molten texture could be observed on the fracture surface. The analysis shows that internal strain energy accumulated and it instantly dissipated at high strain rate, which lead to adiabatic heating. And then the viscosity of amorphous fracture surface drops rapidly, a plastic flow layer formed ultimately. In addition, the rapid crack initiation and propagation give rise to form multiple fracture and increases adiabatic heating. It makes local softening of amorphous alloy in wide range, which leads to sharp reduction of strength.