Stability of Ni-Nb-based and Zr-based bulk metallic glasses against environmental degradation
Ni-Nb 基及 Zr 基大塊非晶合金的抗氧化及抗腐蝕性能
Student thesis: Doctoral Thesis
The ternary (Ni8Nb5)99.5Sb0.5 bulk metallic glass (BMG) was synthesized in this thesis and the corrosion, oxidation and high temperature induced crystallization behaviors are investigated. The corrosion of the annealed counterpart and the component metals composed of glassy alloy was compared with the amorphous alloy. The oxidation property of the amorphous Ni-based alloy after the surface modification was compared with the amorphous precursor. The oxidation of (Zr65Al10Ni10Cu15)95Nb5 BMG was investigated in the amorphous and supercooled liquid state in synthetic air. The corrosion and oxidation mechanisms are analyzed and discussed. The corrosion behavior of bulk (Ni8Nb5)99.5Sb0.5 metallic glass was characterized using immersion test, potentiodynamic polarization tests in 1 N H2SO4, 3mass% NaCl, 1 N HCl and 6 N HCl solutions, respectively, potentiostatic measurements at anodic potentials and electrochemical impedance spectroscopy (EIS) investigation in 3% NaCl solution at room temperature. Potentiostatically formed surface layers in the passive region and the transpassive region were characterized with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The result indicates that the bulk (Ni8Nb5)99.5Sb0.5 metallic glass shows low corrosion rates in the above four kinds of solutions. The wide passive region and low passive current density were observed, even comparable with the valve metal Nb. In the chlorine mediums, the pits or cracks were observed presenting the dendrite structures due to the formation of intermetallics Ni3Nb, which may be mainly attributed to the structural and chemical heterogeneity formed during the rapid cooling. After annealing at the high temperature, the crystalline peaks were observed by the X-ray diffraction patterns (XRD). The grain boundary and dislocation accelerate the ion exchange from the electrolyte to the bulk alloy and promote the corrosion rate, therefore resulting in the increased current density in 1 N HCl solution. On the other hand, no corrosive pit was observed by scanning electron microscopy observation after long time annealing, however, a small number of dendritic pits were identified after short time annealing, which may be related to the elimination of chemical heterogeneity formed during casting. The electrochemical impedance spectroscopy for amorphous and crystalline alloy indicates that the impedance resistance of bulk metallic glass is higher than that of crystalline counterpart. Therefore the transfer of electron and ion is difficult for amorphous alloy, which verifies the improvement of the corrosion resistance for bulk metallic glass in further. The oxidation and high temperature induced crystallization of the (Ni8Nb5)99.5Sb0.5 bulk metallic glass were investigated in synthetic air within the temperature range of 460 - 700 ℃ using a thermogravimetric analyzer (TGA) and XPS. Isothermal thermogravimetric analysis reveals linear or parabolic oxidation kinetics depending on temperature. During the oxidation, the preferential migration of nickel forms the nickelrich regions on the surface due to the high diffusion rate. At lower temperature (less than Tg), the scale consists of Nb2O5 and a little pure nickel. However, at higher temperature (within the supercooled liquid region), NiO was also detected besides Nb2O5 and Ni. The oxidation of Nb is so fast that it might be oxidized before it could migrate and it was first oxidized to NbO, and then to Nb2O5. At the temperature of 700 ℃ (above Tx), similar oxidation mechanism was observed besides the formation of columbite NiNb2O6 on the surface from NiO and Nb2O5 as precursor oxides. Amorphous (Ni8Nb5)99.5Sb0.5 alloy after the surface modification by nitrogen ion bombardment the surface shows the favorable oxidation resistance at the temperature below Tg. The surface forms a thin layer of protective film composed of metallic Ni, Nioxide and Nb-oxide. However, when the oxidation temperature is above Tx, the surface layer after nitrogen ion bombardment is non-protective. The oxidation of the bulk (Zr65Al10Ni10Cu15)95Nb5 metallic glass was investigated in the synthetic air in the temperature range 380-430℃ using TGA and XPS. At 380℃ or lower temperatures (less than Tg), less mass gain was observed corresponding to very slow oxidation kinetics. The surface mainly keeps amorphous structure coupled with a small number of nanocrystal region with the largest grain size of about 5 nm. The oxidation kinetics at the temperature from 410℃ to 430℃ (corresponding to the supercooled liquid state) follows a linear law, thus suggesting the rate-determining step for the oxide growth. Zirconia (c-ZrO2) and CuO were identified as the dominant phase in the surface. The oxidation is dominated by the atomic diffusion and chemical affinity of component metal with the oxygen.
- Metallic glasses, Effect of high temperatures on, Bulk solids, Oxidation, Corrosion