Study On The Effect Of Nb And Ta Addition On Structure, Thermal Stability, Crystallization Behavior And Properties In Zr-Cu-Al-Ni Bulk Metallic Glass

In this dissertation, the effect of substitution of Zr with Nb and Ta on structure, thermal stability, crystallization behavior, compressive mechanical and corrosion properties of the as-cast Zr₆₅Cu_17.5Ni₁₀Al_7.5 bulk metallic glass were investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray detector (EDX), differential scanning calorimetry (DSC), differential thermal analysis (DTA), mechanical testing system, high resolution transmission electron microscopy (HRTEM) and electrochemical polarization.. Bulk Zr_65-xCu_17.5Al_7.5Ni₁₀M_x (M=Nb, Ta) (x=0¹0at%) alloys with a diameter of 3mm were prepared by copper mold casting. XRD patterns and SEM observation show that the substitution of Zr with a small amount of Nb or Ta does not change the amorphous nature of obtained alloys. However, if Nb or Ta content reaches a certain lever, bulk metallic glass matrix composites containing crystalline phases will be formed. The morphologies and amount of the precipitates in the amorphous matrix varies with the content of Nb or Ta. DSC and DTA analyses reveal that the thermal stability of Zr₆₅Cu_17.5Al_7.5Ni₁₀M_x (M=Nb, Ta) decreases with the increase of Nb or Ta content. For the same content of Ta and Nb, the Ta-bearing alloy exhibits a higher thermal stability as compared to the Nb-bearing alloy. Moreover, the addition of Nb or Ta changes the exothermic mode of the base BMG from single stage to multiple stages. In addition, the enthalpy ?H for crystallization decreases with the increasing Nb or Ta concentration. It is verified in the first time that the first-stage exothermic reaction for alloys containing 2at% and 5at% Nb or Ta corresponds to the formation of icosahedral quasicrystals, and the following ones to the phase transformation of the quasicrystals into the stable phase Zr₂Cu compound. Besides, composition analysis on partially quasicrystallized samples demonstrates that Nb mainly located in quasicrystals, indicating Nb promote the formation of quasicrystals. Furthermore, the quasilattice distance of the quasicrystal varied with different Nb or Ta contents. Study on the compressive mechanical behavior and fracture feature reveal that the plasticity and strength of the base alloy are enhanced by the addition of appropriate amount of Nb and Ta. Among the alloys obtained, the Zr₆₀Cu_17.5Al_7.5Ni₁₀Nb₅ bulk metallic glass exhibits the largest plasticity with a value of 6% and superior strength of about 1741MPa. On the other hand, the Zr55Cu17.5Al7.5Ni10Ta10 bulk metallic glass matrix composite exhibits the highest strength of 1920MPa and distinct plasticity of about 4%. It is believed that the ductile bulk metallic glasses (for example, Zr60Cu17.5Al7.5Ni10Nb2) may follow a different deformation mechanism from the Zr55Cu17.5Al7.5Ni10Ta10 bulk metallic glass composite as far as the improvement in general plasticity is concerned. In some cases, the crystalline particles gained in the metallic glass matrix composite are too small for efficient interaction with shear bands, and ther is no improvement of the mechanical properties compared to the monolithic base glass. The corrossion behavior of Zr-Cu-Al-Ni-Nb/Ta alloys was examined by electrochemical polarization measurements in 0.3N NaCl solution at 20℃. It is found that the addition of Nb or Ta could effectively increase the pitting potentials of the base bulk metallic glass. However, the pitting potential decreases once crystalline phases are formed when Nb or Ta has reached a certain level due to that the interfaces between crystalline phase and amorphous matrix can be easily attacked during anodic polarization. It is particularly pointed out that the Zr60Cu17.5Al7.5Ni10Nb5 bulk metallic glass exhibits the highest critical pitting potential and lowest passive current. The corrosion resistance of the BMGs in the present study is much better than 1Cr18Ni9Ti stainless steel.