The Effects Of Crystallization On The Correlated Properties In Some Bulk Metallic Glasses (BMGs)

Crystallization in amorphous alloys, including that occurs during solidification or during annealing treatment, has a big impact on the properties of these alloys. Understanding the relationship between the crystallization and the correlated properties of BMGs is helpful to the improvement of the comprehensive properties of these alloys. In this thesis, the effects of crystallization on the properties, such as yield stress, wear resistance and corrosion behavior, etc., in some bulk metallic glasses (BMGs), including Cu-, Ni-, Ti-, Zr-based BMGs, were systematically investigated. To do it, several bulk metallic glasses (BMGs) or partially crystallized BMGs were prepared by cold-copper mould suction-casting method. The thermal stability, crystallization process and microstructure were studied by advanced analysis techniques, such as differential scanning calorimeter(DSC), X-ray diffraction(XRD) and scanning electron microscope(SEM), etc.. On the basis of understanding of the crystallization behavior and microstructures of these alloys, the mechanical properties and corrosion behavior of these alloys were investigated, focusing on the effects of crystallization on these correlated properties. It is found that, Cu_52.5Ti₃₀Zr_11.5Ni₆ has strong glass forming ability(GFA), which exhibits a glass transition at 415℃. The crystallization occurs at 461℃. When the alloy is annealed at the temperature of 465℃and 650℃, the partially crystallized structure containing Cu₁₀Zr₇ phase and crystallized structure containing Cu₁₀Zr₇,Cu₃Ti,Ni₃Zr phases can be obtained. The crystallization has an important influence on the mechanical properties of Cu_52.5Ti₃₀Zr_11.5Ni₆ alloy. The partial crystallization slightly promotes the fracture strength without decreasing the plasticity of the amorphous alloy, but the full crystallized alloy showed a very low strength and a very poor ductility. The corrosion behavior of Cu_52.5Ti₃₀Zr_11.5Ni₆ BMG is also strongly dependent on the crystallization. Compared with the complete amorphous alloy, the partially crystallized alloy exhibits higher corrosion resistance both in 1N HCl and 6N NaOH solutions, but the fully crystallized alloy shows lower corrosion resistance. In addition, a preliminary study on the effect of the size of the crystalline phases on the corrosion behavior was also conducted. Similarly, the partial crystallization occurring during solidification also exhibits a big impact on the properties of BMGs. Taking the Ni-based BMG as an example, the alloy containing the crystalline phases shows higher corrosion resistant than that of the complete amorphous alloy. The corrosion resistance of the alloy can be further improved in the Ti-containing alloy with partially crystallized structure, because Ti addition can promote the formation of an oxide film. For the Ti-based BMG containing dendriteβ-Ti crystalline phases, its mechanical properties exhibits a strong temperature-dependency. The alloy exhibits a good combination of strength and ductility at the temperature of 650℃. At the end of the thesis, a preliminary exploration on the effect of the crystallization on the wear resistance was conducted on a Zr based BMG by using nano-indentation technique. The results indicated that partial crystallization can also improve the wear resistance of the BMG.