Crystallization Behavior Of Gd-based And Zr-based Bulk Metallic Glasses And Effect Of High Magnetic Field On Crystallization

Metallic glass has attracted wide interest due to its special physical, chemical and mechanical properties. The study on crystallization mechanism is very important for nucleation theory and applications. Therefore, the crystallization kinetics of Gd-based and Zr-based bulk metallic glasses (BMGs) has been investigated in the thesis. Meanwhile, it has been demonstrated that high magnetic field has remarkable influence on the formation and crystallization of metallic glasses. Therefore, the effects of high magnetic field on several Zr-based bulk metallic glasses have been studied.Crystallization kinetics of the Gd53Al24Co20Zr3and Zr55Al10Ni5Cu30bulk metallic glasses has been performed using a continuous differential scanning calorimeter (DSC) technique. The glass transition temperatures, the onset of crystallization temperatures and crystallization peak temperatures of above two BMGs have been obtained from DSC traces at different heating rate. The Kissinger equation, Lasocka equation, Augis&Bennet equation, Ozawa equation and VFT equation were used to fit the characteristic temperatures of the BMGs. The results show that all the characteristic temperatures of the BMGs display significant dynamic effects. All of the five quations above well describe the relationship between the characteristic temperatures and the heating rate for the two BMGs when the heating rate is ranging from5to80K/min. The apparent activation energies of glass transition and crystallization derived from the Kissinger equations are smaller than those resulted from the Augis&Bennet equation and Ozawa equation for the two BMGs.Continuous heating transition (CHT) diagrams for the characteristic crystallization temperatures of the Gd53Al24Co20Zr3and Zr55Al10Ni5Cu30BMGs were constructed using the experimental data and the extension of the fitting equations above. The CHT diagrams and the experimental data at the heating rate of1K/min and120K/min suggest that the VFT equation is a better model to illustrate the relationship between the characteristic temperatures and the heating rate in a wider region.The relationship between the crystallized volume fraction and the apparent crystallization activation energy for the two BMGs was obtained using the Doyle-Ozawa method. It has been found that the apparent crystallization activation energies reach maximum at the crystalline volume fraction of about25%. Afterwards, they slowly decrease with the crystalline volum fraction increasing. Then, the apparent crystallization activation energies decrease rapidly when the crystallized volume fraction was larger than about90%, meanging that the crystallization is easier to happen. The crystallization mechanism curves of the Gd53Al24Co20Zr3and Zr55Al10Ni5Cu30BMGs have also been obtained by the fitting results from Johnson-Mehl-Avrami and Normal Grain Growth kinetic models which describe the nucleation and growth of solid-state transformation process.The isothermal crystallization behavior of Zr46.75Ti8.25Cu7.5Ni10Be27.5and Zr55Al10Ni5Cu30BMGs under high magnetic field has been investigated by X-ray diffraction (XRD) analysis and differential thermal analysis techniques. The effects of high magnetic field on crystalline phases, nucleation and growth have been obtained. The results show that: the effects of high magnetic field on the crystallization process of Zr-based BMGs are very complicated. So, it did not show a clear rule from XRD. However, the results from DSC show that high magnetic field can affect the apparent activation energy and reaction rate of crystallization.