Air Oxidation Kinetics Study of

Defect and Diffusion Forum Vols. 283-286 (2009) pp 209-213
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© (2009) Trans Tech Publications, Switzerland
Online available since 2009/Mar/02
Air Oxidation Kinetics Study of Zr58Nb3Cu16Ni13Al10
Bulk Metallic Glass
Dongya Huang1,a, Xiangjin Zhao2,a,Tao Zhang3,a, Vincent.Ji4,b
a
Dept. Mat. Sci. & Engine., Beijing Univ. of Aeronautics and Astronautics, Beijing 100083, China
b
ICMMO/LEMHE, UMR CNRS 8182, Université deParis-Sud 11,Orsay,91405, France
1
[email protected]; [email protected]; [email protected]; [email protected]
Keywords: Zr-based bulk metallic glass, oxidation, diffusion, GIXRD
Abstract. The isothermal oxidation behavior of Zr58Nb3Cu16Ni13Al10 bulk metallic glass (BMG)
under dry air in the glassy state and the supercooled liquid state (SLS) was studied by the
thermogravimetric method. The oxidation rate and thickness growth speed in the SLS were both
hugely higher than in the glassy state. The oxidation kinetics of BMG in both states for 1.5 hours
was different, the parabolic law was followed in the glassy state at 300°C and 350°C, contrarily the
linear law was followed in SLS at 400 °C. After the oxidation for 126 hours in SLS, the oxidation
kinetics possessed two stages, the linear stage and the parabolic stage. The diffusion of the Cu2+ ion
and CuZr intermetallic alloys were detected by GIXRD.
Introduction
Zr-based bulk metallic glasses have been considered for use in many mechanical components
because of their good forming ability, excellent strength, better physical and chemical properties,
compared to traditional crystalline alloys[1]. With high thermal [2-5] stability, Zr-based amorphous
alloys are one of the most popular systems, which are potential engineering and structural materials
for the future manifold applications.
There have been several studies of oxidation kinetics of Zr-based amorphous alloys [6-11]. They
helped to reveal the influence of the microstructure and annealing temperature on the micro
mechanism of the oxidation. Uwe et al. [10] have shown that nano-crystalline and quasi-crystalline
alloys have better oxidation resistance than the amorphous alloys in the case of Zr-Cu-Ni-Al alloys.
Kai et al. [8] has also shown the crystalline Zr55Cu30N5Al10 alloy has a higher oxidation resistance
than the amorphous alloy. The oxidation kinetics follows a linear law at 300 °C with the formation
of single tetragonal ZrO2. However, the oxidation process followed a parabolic law at temperatures
higher than 350 °C, with the formation of both tetragonal ZrO2 (t-ZrO2) and monoclinic ZrO2
(m-ZrO2). Liu et al. [11] have revealed that both t-ZrO2 and m-ZrO2 were formed after oxidation in
the different states. t-ZrO2 was preferentially found in the outer layer of the oxide scale and m-ZrO2
in the inner layer of the oxide scale.
Although many important results have been found on the oxidation of Zr-based amorphous alloys,
there were paradoxical results in the case of Zr-based amorphous alloys. In the following study, the
oxidation micro-mechanism in Zr58Nb3Cu16Ni13Al10 amorphous alloy has been achieved in the
isothermal oxidation process, which involved calculation of diffusion parameters. As well as its
oxidation kinetics, the thickness growth of oxide scales and phase transition are studied.
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210
Diffusion in Solids and Liquids IV
Experiments
Zr58Nb3Cu16Ni13Al10 bulk metallic glass was prepared by casting the melt into a copper mold in an
argon atmosphere. The samples for oxidation were sliced into 10×10×1mm3 shapes and then were
mechanically polished to a mirror finish. The isothermal oxidation of Zr58Nb3Cu16Ni13Al10 bulk
metallic glass was carried out under dry air by the thermogravimetry analyzer (TGA, SETARAM
Model TG92). The net flow-rate of high-purity air (>99.99%) was kept constant at about 90ml/min.
throughout each test. The surface of the oxidation scales were analysed by GIXRD using Philips
X’pert MRD system with Cu Kα radiation, the incidence angle ω was 5°. The depth of X-ray
penetration was approximately 400 nm in the scanned zone.
Results
Figure1 showed the thermal response of the glass which reveals that the onset temperature for the
glass transition (Tg) and crystallization(Tx) at a scanning rate of 0.33 °C/s were 384 °C and 486 °C,
respectively. Therefore the supercooled liquid region for the BMG ∆Tx(=Tx - Tg) was approximately
102°C, indicating a good glass forming ability of Zr58Nb3Cu16Ni13Al10 .
Fig.1 DSC trace of Zr58Nb3Cu16Ni13Al10 bulk
metallic glass at heating rate 0.33°C/s.
Fig.2 Isothermal oxidation of Zr58Nb3Cu16Ni13Al10 bulk
metallic glass at 300°C, 350°C, 400°C.
The oxidation kinetics of the BMG at 300°C, 350°C following parabolic evolution and at 400°C
following linear evolution are shown in Fig. 2. The gain of mass was quite similar at 300°C and
350°C. The huge increase at 400°C indicated that the oxidation speed of the Zr58Nb3Cu16Ni13Al10
BMG in the supercooled liquid state was much faster than in the glassy state. The oxidation rate
constant (Kp) at different temperatures in dry air is tabulated in Table 1.
Zr-based BMG
300°C
350°C
400°C
5.67 x 10-8
6.75 x 10-8
linear
Table 1 The oxidation rate constant (Kp) at different temperatures in dry air (Kp:g2/cm4/s).
For better reviewing the growth process of oxide scale, the oxidation of Zr-based BMG for 1.5
hours, 25 hours and 126 hours at 400°C was plotted in Fig. 3. The structures and phases of the
oxide
Defect and Diffusion Forum Vols. 283-286
Fig.3 The oxidation kinetics of Zr58Nb3Cu16Ni13Al10 BMG
211
Fig.4 Plots of thickness growth of oxide scales vs.
time in the isotherm oxidation
scales forming at different temperatures were determined by GIXRD. Fig. 5 shows the GIXRD
patterns of the samples of Zr-based BMG which were subjected to oxidation for 1.5 hours, 25 hours
and 126 hours at 400 °C. The oxide scales consisted mainly of t-ZrO2 at 400°C for 1.5 hours. No
other metal oxides could be detected in any surface of the oxidation samples. After oxidation for 25
hours, the scale presents the phase transition including t-ZrO2, m-ZrO2, Cu10Zr7 and Cu. The change
of the scale was more complicated after 126 hours with additional Zr3O phase and ZrCu compound
compared to the phase for 25 hours.
According to GIXRD in Fig. 4 and other reports [12, 13], the characters of structure and the phase
transition in the oxidation were explained, if the density of oxide scale was chosen about 5.8g/cm3,
dominated by ZrO2 growth, the scale thickness could be estimated in the isothermal oxidation
process under different temperature conditions. In Fig. 5, it is evident that the growth of thickness at
400°C in the oxidation for 1.5 hours is faster than that at 300°C and 350°C, under both temperatures,
almost the same growth of thickness was determined.
Fig. 5 GIXRD of the oxide surface at 400°C for at 400°C for 126 hours 1.5 hours, 25hours and 126 hours
212
Diffusion in Solids and Liquids IV
Discussion
Based on the experimental results, the oxidation kinetics of Zr58Nb3Cu16Ni13Al10 amorphous alloy
was further programmed in both the glassy and supercooled liquid states in dry air. At 300°C and
350°C, the isothermal oxidation kinetics of Zr-based BMG alloy followed a parabolic law, while it
followed a linear law at 400°C in Fig. 2. This result was different from those obtained in other
research [8] in which the oxidation kinetics of BMG followed a linear behavior at 300°C. It was
evident that the mass gain was considerably larger at 400°C in the supercooled liquid state than that
at 300°C and 350°C in the glassy state that was well agreed with L. Liu [11]. The activation energy
calculated in our case was about 126KJ/mol in the temperature range from 300-400°C. In addition
to the oxidation constants in table 1, these data consisted of very fundamental parameters in the
diffusion.
As was known, the linear oxidation reflected mainly an increased rate of oxide formation, while the
dissolution at a rate determined by the oxygen gradient and diffusion coefficient of oxygen in the
metal. During linear oxidation, the oxide had a lamellar structure parallel to the metal surface. The
linear rate may reflect a reaction controlled by diffusion through a thin, inner layer of oxide that
remains approximately constant with time due to repetitive exfoliation. Alternatively, it may reflect
a rate determining of the zirconium/oxygen boundary process, e.g. nucleation and growth of the
oxide. During the linear oxidation, the metal/oxide interface moves inwards at an increased rate and
causes faster oxygen diffusion into metal. As for oxidation kinetics of Zr-based following the
parabolic law at 300°C and 350°C, according to the assumption of Wagner, the diffusion is rate
determining.
However, it was must be mentioned that the effect of crystallization in the supercooled liquid was
one of the important roles in the oxidation process. For further studying the growth of scale, the
isothermal oxidation processes for 1 hour, 25 hours, and 126 hours were analyzed by GIXRD in Fig.
5. Obviously it was found that there were two periods in the oxidation process, the first period
following a linear style, the second following a parabolic style. It was a reasonable assumption that
the crystallization in the supercooled liquid state played an important role to affect the oxidation
behavior. With increasing in the annealing time for 25 hours and even 126 hours at 400°C, the
crystallization phases increase in the outer layer of the oxide scale. The Zr3O scarcely was reported
in the oxidation kinetics study of Zr-based BMG. In Fig. 5, Cu and Cu10Zr phases were detected in
the oxide matrix at 400°C for 25 hours. It was believed that Cu and Cu10Zr demonstrated that a
massive and long distance atomic outward diffusion occurs in the supercooled liquid state. The
formation of the oxide layer was due mainly to an inward diffusion of oxygen [8].
Another worthy point was the effect of the relaxation of BMG on oxidation in the glassy state.
Metallic glasses were thermodynamically metastable with the short-range order and liquid–like
disorder which do not contain lattice defects such as dislocation and grain boundaries. The models
of defect in metallic glasses were presented in which the most common model to account for
disordering was based on the concept of free volume [14,15], even considered as extra volumes [16]
and atomic bond-deficiency[17]. The glass properties may change due to a process that was called
structural relaxation at the temperature (<Tg) including the diffusivity [18]. In this study, the
research of the effect of relaxation on oxidation was not completed, but it was one of the focuses on
oxidation in the glassy state of BMG.
Defect and Diffusion Forum Vols. 283-286
213
Conclusions
1. The oxidation kinetics of Zr58Nb3Cu16Ni13Al10 BMG in the glassy state and supercooled liquid
state followed a parabolic rate law and linear law, respectively. Different oxidation behaviors
were carried out, oxidation rate was much higher in the supercooled liquid state than in the glassy
state.
2. The oxidation process in the supercooled liquid state for 126 hours possesses two periods in
which the first period followed a linear law and the second followed a parabolic law. The surface
of the oxide scale in the glassy state consisted mainly of t-ZrO2 in the first period, however in the
second the phase transition was complicated.
3. The diffusion of Cu from the interface of oxide layer/substrate was detected in the supercooled
liquid state. It and its intermetallic alloys including Cu10Zr and CuZr could promote the oxidation
resistance.
Acknowledgements
The financial support of National Basic Research Program of China (973 Program) (Grant NO:
2007CB613900) was appreciated.
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Diffusion in Solids and Liquids IV
Diffusion in Solids and Liquids IV
doi:10.4028/3-908454-50-6
Air Oxidation Kinetics Study of
Zr58Nb3Cu16Ni13Al10</su
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doi:10.4028/3-908454-50-6.209

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