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
Author(s)
Detail(s)
Awarding Institution | |
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Award date | 17 Feb 2010 |
Link(s)
Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(c68d270a-d2e5-4de1-bce5-b2e10b3b1264).html |
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Other link(s) | Links |
Abstract
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