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

In this thesis, the isothermal crystallization kinetics and crystallization behaviors of Hf44.5Cu27Ni13.5Ti5Al10bulk metallic glass were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively.The quasi-static compression behavior, dynamic compression behavior and mechanical properties of bulk metallic glass were studied by MTS810and scanning electron microscopy (SEM).The process of heat treatment were studied by high vacuum oven.There is only one crystallization peak on the DSC curve, temperature of glass transition is775K.The following is the crystallization process of amorphous: amorphous^amorphous’+Al16Hf6Ni7â†'amorphous"+HfAl3â†'amorphous’"+HfAl3+CuTi3.Quasi-static compression tests were carried out at room temperature, compressive strength of cylindrical specimens of Hf44.5Cu27Ni13.5Ti5Al10can reach2030MPa, Amorphous typical fracture morphology including dimple, partial melting, amorphous fractograph crack occurrence and propagation and dissemination were observed. After heat treatment, the compressive strength of cylindrical specimens has experienced the process:from large to small, reaches the maximum2330MPa after780K heat treatment; Compared with the case above,the compressive strength is sharply reduced after800K heat treatment. In the range of temperature from780K to800K, the internal structure of amorphous alloy is the most sensitive to temperature.The results of hardness test for different regions show that the hardness has close relationship with the distribution of free volume.The marginal region of the melt has the fastest cooling rate and the most free volume,therefore,has the lowest hardness. The center region of the melt has the lowest cooling rate and the fewest free volume,thus has the highest hardness.The intermediate section of the melt had a hardness between that of the edge and centrality.