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Effects of Si addition on the oxidation behavior of a Cu–Zr-based bulk metallic alloy
Intermetallics 18 (2010) 1994e1999
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effects of Si addition on the oxidation behavior of a CueZr–based bulk metallic alloy W. Kai a, *, P.C. Kao a, P.C. Lin a, I.F. Ren a, J.S.C. Jang b a b
Institute of Materials Engineering, National Taiwan Ocean University, Keelung, 202, Taiwan, ROC Department of Mechanical Engineering, National Central University, Taoyuan, 320, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2009 Received in revised form 3 March 2010 Accepted 3 March 2010
The effect of Si addition on the oxidation of (Cu43Zr43Al7Ag7)99.5Si0.5 bulk metallic glass (CZ43S-BMG) was investigated over the temperature range of 375e500 C in dry air. The results generally showed that the oxidation rates of the CZ43S-BMG followed a multi-stage parabolic-rate law, and the oxidation rate constants (kp values) fluctuated with increasing temperature. It was found that the kp values of the CZ43S-BMG were slightly slower than those of the Si-free glassy alloy (Cu43Zr43Al7Ag7, named as CZ43BMG). The scales formed on the CZ43S-BMG strongly depend on temperature, consisting of 3 different modifications of ZrO2 and CuO, and minor amounts of Cu2O (formed at T 425 C) and Al2O3 (only detected at 500 C). In addition, the glassy substrate remained amorphous nature at T 400 C, while it transferred to the crystalline phases of Cu10Zr7, ZrAl, and ZrSi2 after the oxidation at higher temperatures. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: B. Oxidation B. Glasses, metallic
1. Introduction CueZr-based bulk metallic glasses (BMGs), consisting of ternary or multi-component systems have been extensively studied since the past decades [1e6]. Several attractive potentials for the development of CueZr-based BMGs are noted, consisting of high glassforming ability (GFA) with critical cooling rates as low as 1 K/s, high fracture strength of w1600 MPa, and relatively low materialcost [2,3]. Most recently, a series of (Cu50Zr50)100xAlx and (Cu50Zr50)100xyAlxAgy amorphous systems were developed, and a few results revealed that those systems exhibited unique thermal and mechanical properties. For example, the glass transition temperature (Tg) and crystallization temperature (TX) of the CZ43BMG were reported to be about 449 and 521 C, respectively, having a larger supercooled temperature of 72 C [6]. This BMG also exhibited a higher compressive strength near 2000 MPa and a plastic deformation around 8% [7]. In addition, another study reported that small amounts of Si addition (such as the CZ43SBMG) could further increase the GFA of the glassy alloy, which offered a better thermal property to resist the devitrification for industrial applications [8]. To use those BMGs at elevated temperatures for practical purposes, it is essential to understand their airoxidation behavior. Thus, the goal of this study is to investigate the
* Corresponding author. E-mail address: [email protected] (W. Kai).
oxidation of CZ43- and CZ43S-BMGs, and in particular, the effect of Si on the oxidation kinetics is explored. 2. Experimental Both CZ43- and CZ43S-BMG rods with 3 mm in diameter and 15 mm in length were prepared by a drop cast technique, as described elsewhere [9]. The detail composition of the alloy ingots analyzed by X-ray wavelength dispersive spectroscopy (WDS) gave Cue43.12Zre7.28Ale7.25Ag and Cue43.08Zre7.09Ale7.07Age0.51Si (in at.%), respectively. BMG samples were directly sliced into about 1 mm in thickness, ground and polished with a 1-mm diamond paste, cleaned with acetone and methanol, immediately dried before the tests. Oxidation tests and characterization of the substrate and scales were similar to those described in the previous study [10]. 3. Results and discussion A typical X-ray diffraction (XRD) analysis of the CZ43- and CZ43S-BMGs, as shown in Fig. 1, reveals a wide-broadening peak near 2q ¼ 39.8 and 39.6 , respectively, indicative of the amorphous nature of both substrates. The DSC curves of the two alloys obtained at a heating rate of 20 C/min are shown in Fig. 2, revealing that the Tg and TX temperatures of the CZ43-BMG are around 431.0 and 506.1 C, respectively, while those of the CZ43S-BMG are around 428.2 and 519.0 C, respectively. Thus, the supercooled liquid region (DTX ¼ TX Tg) are about 75.1 (CZ43-BMG) and 90.8 C
0966-9795/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.03.005
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
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Fig. 1. XRD spectra of the CZ43- and CZ43S-BMGs.
Fig. 3. Parabolic plots of the mass-gain data from (a) CZ43-BMG and (b) CZ43S-BMG.
Fig. 2. DSC curves of the CZ43- and CZ43S-BMGs obtained at a heating rate of 20 C/min.
(CZ43S-BMG), and the oxidation tests were set to encompass the temperature range of 375e500 C. Parabolic plots of the mass-gain data of the CZ43- and CZ43SBMGs over the temperature range of 375e500 C are shown in Fig. 3. The oxidation kinetics of the two alloys generally followed a multi-stage parabolic-rate law, consisting at least of an initial incubation period of time (w0.03 h), followed by a fast-growth transient oxidation (0.03 w 2 h), by a decreased oxidation stage (from 2 to 4.5 h), and by a final steady-state stage (after 4.5 h). The oxidation rate constants (kp values) at the steady-state stage of both alloys are summarized in Table 1. The kp values of CZ43-BMG progressively increase with increasing temperature when it is below 450 C, while a significant lower kp value is obtained at 500 C. For comparative purposes, the kp values of Cu45Zr45Al5Ag5
BMG (CZ45-BMG) in the previous study are also tabulated in the same table [10]. In general, the CZ43-BMG exhibited slightly lower kp values at T 425 C, while the reverse condition was observed at higher temperatures. In addition, the temperature effect on the kp values of the CZ43S-BMG followed a similar trend as that of the Sifree alloy, and the kinetics results showed that an additional small amount of Si could further reduce the oxidation reaction to a certain extent at T 450 C, as discussed later. For comparative purposes, it is useful to review the scale constitution and phases and the crystallization sequence of CZ45BMG after air oxidation at the same temperature range [10]. The scales formed on CZ45-BMG were strongly temperature-dependent, consisting mostly of CuO and tetragonal-ZrO2 (t-ZrO2), minor amounts of monoclinic- and orthorhombic-ZrO2 (m- and o-ZrO2), and uncorroded Ag particles at T 450 C, while an additional aAl2O3 phase was also detected at 500 C. In addition, the crystallization sequence for this BMG at 400 C underwent an initial selective oxidation of Zr to form t- and o-ZrO2 (governed by inward diffusion of O), followed by the growth of CuO and m-ZrO2, while the
Table 1 Parabolic-rate constants of the CZ-based BMGs in dry air (kp unit: g2 cm4 s1). 375 C CZ43-BMG CZ43S-BMG CZ45-BMG
400 C 12
1.17 10 1.08 1012 1.49 1012
425 C 12
2.22 10 1.85 1012 2.56 1012
450 C 12
2.50 10 2.16 1012 3.65 1012
500 C 12
5.38 10 3.13 1012 3.82 1012
4.00 1012 2.04 1012 1.12 1012
Fig. 4. (a) Cross-sectional BEI micrograph and XRD spectra of the CZ43-BMG oxidized for 36 h at 375 C, and cross-sectional BEI micrographs of the same alloy for 36 h at (b) 400 C and (c) 425 C.
Fig. 5. Cross-sectional BEI micrographs and XRD spectra of the CZ43-BMG oxidized for 36 h at (a) 450 C and (b) 500 C.
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
substrate retained amorphous nature. However, the substrate started to form Cu10Zr7 at 450 C with a 260 min exposure, and then, followed by the development of ZrAl after a 360 min exposure. The following section presents and discusses microstructure results for CZ43- and CZ43S-BMGs. Typical backscattered electron image (BEI) micrographs of the cross-sections formed on the CZ43BMG and XRD analyses at 375e425 C for 36 h are shown in Fig. 4. The scales remained good adherent to the substrate and their constitutions were nearly identical to the CZ45-BMG, except for having an unknown peak near 2q ¼ 38.21 in the outer-portion of scales. In addition, the Cu10Zr7 phase was detected in the substrate beneath the scales after the oxidation at 375 C, indicating that the crystallization of CZ43-BMG occurred at a lower temperature with respect to that of CZ45-BMG (at 425e450 C). Furthermore, the scales formed at 450e500 C, as shown in Fig. 5, were nearly identical to those at T 425 C, except that the scale/substrate interfaces became more non-planar, and minor amounts of Cu2O and Al2O3 were detected at 450 and 500 C, respectively. An interesting aspect of the results is to form another ZrAl phase which was absent at lower temperatures. Very likely, the growth of ZrAl under a thermal-activated reaction between Zr and Al in the CZ43BMG substrate was not occurred at T 425 C but prevailed at higher temperatures. In addition, the scale constitution and phases of the CZ43S-BMG were further discussed after SEM/EPMA/XRD/TEM analyses (Figs. 6e8). Cross-sectional BEI micrographs of the CZ43S-BMG oxidized for 36 h at 375e400 C and XRD spectra at 400 C are shown in Fig. 6. The scale constitution and phases were similar to that of CZ43-BMG, except that the amount of o-ZrO2 was much abundant but that of t-ZrO2 was nearly invisible in the inner-
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portion scales. In addition, the substrate remained amorphous nature after the oxidation at 400 C although a few tiny peaks were noted near the wide-broadening peak. Those exceptional observations could be due to the effect of Si addition, which could cause the atomic-structure changes in the BMGs, and in turn provided a contribution to not only enhance the growth of o-ZrO2 but stabilize the amorphous substrate to a higher temperature, in comparison to that of CZ43-BMG. Another BEI micrograph and Xray maps of alloying elements and O of the cross-sections of the CZ43S-BMG oxidized at 425 C for 36 h are shown in Fig. 7. Unlike the results at 375e400 C, some cracks near the Ag particles inside the scales and a few prolong rod-shaped precipitates in the scale/ substrate interface were noted. Based on X-ray maps, those precipitates enriched Zr and Si, however, EPMA failed to identify their exact composition. Thus, further TEM analyses were given, with the bright-field images and inserted selected-area diffraction (SAD) patterns in various regions of the cross-sectional scales (Fig. 8). Fine nano-grains of mostly CuO intermixed with minor Cu2O were observed in the outer-portion scales, which were confirmed by SAD ring patterns (Fig. 8a). The Cu2O phase was not detected by XRD but confirmed by TEM, which could be due to its low amounts beyond XRD-detection limits. Another TEM micrograph taken from the scale/substrate interface (Fig. 8b) revealed a larger dark precipitate of intermetallic ZrSi2 (w750 nm in diameter), which was also confirmed by the SAD spot patterns. The observed ZrSi2 precipitate was unexpected, which deserved further discussion. Based on the thermodynamic data of pure substances [11], the Gibbs free energy of formation (DGof in unit of kJ/mol O2) of SiO2 at 425 C is about 779.2, which is much more negative than those of CuO (184.5) and Cu2O (234.5), but less negative than
Fig. 6. Cross-sectional BEI micrograph and XRD spectra of the CZ43S-BMG oxidized for 36 h at (a) 375 C and (b) 400 C.
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W. Kai et al. / Intermetallics 18 (2010) 1994e1999
Fig. 7. (a) Cross-sectional BEI micrograph and X-ray maps of (b) Cu, (c) Zr, (d) Al, (e) Ag, (f) Si, and (g) O of the CZ43S-BMG oxidized at 425 C for 36 h.
those of Al2O3 (971.1) and ZrO2 (963.7). It is possible that SiO2 is favorable to form in the scales intermixed with other oxides (CuO/ Cu2O, Al2O3 or ZrO2) for thermodynamic reasons. However, it was never detected by XRD or TEM. As similar to the previous result [10], the oxidation mechanism of CZ43S-BMG was also predominant by inward oxygen diffusion, so that the first growth of ZrO2
(three different modifications) and CuO/Cu2O was expected because the relative activities of Zr and Cu were much higher than those of other elements (Al, Ag, and Si). After the development of intermixed scales of ZrO2 and CuO/Cu2O, the relative activities of Al, Ag, and Si could be slightly increased, which possibly leaded to a further oxidation of Al and Si (Ag was considered as an inert
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
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Fig. 8. TEM bright-field images and inserted SAD patterns of the CZ43S-BMG oxidized at 425 C for 36 h, showing (a) the copper oxides in the outer-portion scales and (b) the intermetallic ZrSi2 in the scale/substrate interface.
element). However, this prediction disagreed to the results mentioned above because Al2O3 was only detected at 500 C and SiO2 was absent. Very likely, Si did not react with oxygen but diffused back into the glassy substrate, which in turn resulted in forming the crystalline ZreSi phases. Besides, the heat of mixing (in unit of kJ/mol) for the ZreSi intermetallics was about 84, which is much more negative than that of ZreAl (44), ZreCu (23), ZreAg (20) CueSi (19), and CueAl (1) [8,12]. Thus, the formation of ZrSi2 precipitates in the scale/substrate interface became thermodynamically and kinetically favorable. Another interesting aspect to be discussed is to form abundant amounts of o-ZrO2 in the inner-portion of the scales of the CZ43SBMG, which resulted in the slow oxidation rates, as compared to those of CZ43-BMGs. As reported previously, the higher the amount of t-ZrO2 presented in the scales, the faster the oxidation rates obtained for Zr-based BMGs [13,14]. The same trend could be expected for the oxidation of CZ43-based BMGs. The detail mechanism for the formation of o-ZrO2 was still unclear, however, its abundant amounts did progressively reduce the formation of t-ZrO2 in the scales when the available amounts of Zr were fixed in the substrate. Perhaps, the oxygen inward diffusion in the sublattice of t-ZrO2 is much faster than that in both m- and o-ZrO2, so that the more the o-ZrO2 phase and the less the t-ZrO2 phase presented in the scales after the oxidation at T < 450 C, the lower the kp values obtained. However, this deduction became invalid when Al2O3 formed in the scales of the glassy alloys at 500 C. Although not shown here, the scales formed on the CZ43- and CZ43S-BMG at 450e500 C are similar to those of the CZ45-BMG exception that the Cu2O presented in the scales of CZ43-BMG was nearly invisible in the CZ43S-BMG, while an additional amount of Al2O3 was always presented in the scales of the three BMGs at 500 C. As also mentioned before [10], the presence of Al2O3 could play a partly blocking effect to reduce inward diffusion of oxygen, which in turn imparted the oxidation rates, leading to a lower kp with respect to those at lower temperatures. Most likely, the growth of Al2O3 makes a much more protective barrier to reduce the oxidation rate of the alloys than that of o-ZrO2. As a result, the kp values at 500 C for the three BMGs are lower than those at 450 C.
1. The oxidation kinetics of the two BMGs obeyed a multi-stage parabolic-rate law with their kp values fluctuating with temperature. 2. A small amount of the Si addition provided a better oxidation resistance for the CZ43S-BMG at T 450 C, while no improvement was obtained at 500 C. 3. The scales formed on the CZ43S-BMG strongly depended on temperature, consisting mostly of CuO and t-ZrO2, minor amounts of o- and m-ZrO2, Cu2O, and uncorroded Ag particles at T 450 C, while Al2O3 were also detected at 500 C. 4. The additional Si plays an important role in forming ZrSi2 precipitates near the scale/substrate interface, which enhances the oxidation resistance of CZ43S-BMG. Acknowledgments The authors are thankful for financial support by the National Science Council of Republic of China under the Grant No. of NSC-982221-E-019-007 and TGA equipment support by the National Taiwan Ocean University under the Grant No. of NTOU-RD972-0403-01-01. Special thanks are indebted to Dr. R.T. Huang in our institute and Mr. C.T. Wu in the Department of System Engineering and Science, National Tsing Hua University (Hsin-Chu, Taiwan) for their technical supports of TEM. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions [12]
The effects of Si addition on the oxidation behavior of the CZ43and CZ43S-BMGs over the temperature range of 375e500 C was characterized. Several conclusions can be reached.
[13] [14]
Inoue A, Koshiba M, Zhang T, Masumoto T. J Appl Phys 1998;83:1967. Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1995;36:391. Kündig AA, Ohnuma M, Ohkubo T, Hono K. Acta Mater 2005;53:2091. Inoue A, Zhang T, Chen MW, Sakurai T. J Mater Res 2000;15:2195. Sung DS, Kwon OJ, Fleury E, Kim KB, Lee JC, Kim DH, et al. Met Mater Int 2004;10:575. Park SO, Lee JC, Kim YC, Fleury E, Sung DS, Kim DH. Mater Sci Eng 2007;A449eA451:561. Oh JC, Ohkubo T, Kim YC, Fleury E, Hono K. Scripta Mater 2005;53:165. Zhang B, Jia Y, Wang S, Li G, Shan S, Zhan Z, et al. J Alloys Compd 2009;468:187. Kai W, Ho TH, Hsieh HH, Chen YR, Qiao DC, Jiang F, et al. Metall Mater Trans A 2008;39:1838. Kai W, Ren IF, Kao PC, Wang RF, Chuang CP, Freels MW, et al. Adv Eng Mater 2009;11:380. Barin I. Thermochemical data for pure substance. 3rd ed. Weinheim, New York: American Chemical Society and American Institute of Physics for National Bureau of Standards; 1995. Boer F, Boom R, Matterns W, Miedema A, Niessen A. Cohesion in metals transition metal alloy. North-Holland; 1988. Hsieh HH, Kai W, Huang RT, Pan MX, Nieh TG. Intermetallics 2004;12:1089. Kai W, Chen YR, Ho TH, Hsieh HH, Qiao DC, Jiang F, et al. J Alloys Compd 2009;483:519.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effects of Si addition on the oxidation behavior of a CueZr–based bulk metallic alloy W. Kai a, *, P.C. Kao a, P.C. Lin a, I.F. Ren a, J.S.C. Jang b a b
Institute of Materials Engineering, National Taiwan Ocean University, Keelung, 202, Taiwan, ROC Department of Mechanical Engineering, National Central University, Taoyuan, 320, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2009 Received in revised form 3 March 2010 Accepted 3 March 2010
The effect of Si addition on the oxidation of (Cu43Zr43Al7Ag7)99.5Si0.5 bulk metallic glass (CZ43S-BMG) was investigated over the temperature range of 375e500 C in dry air. The results generally showed that the oxidation rates of the CZ43S-BMG followed a multi-stage parabolic-rate law, and the oxidation rate constants (kp values) fluctuated with increasing temperature. It was found that the kp values of the CZ43S-BMG were slightly slower than those of the Si-free glassy alloy (Cu43Zr43Al7Ag7, named as CZ43BMG). The scales formed on the CZ43S-BMG strongly depend on temperature, consisting of 3 different modifications of ZrO2 and CuO, and minor amounts of Cu2O (formed at T 425 C) and Al2O3 (only detected at 500 C). In addition, the glassy substrate remained amorphous nature at T 400 C, while it transferred to the crystalline phases of Cu10Zr7, ZrAl, and ZrSi2 after the oxidation at higher temperatures. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: B. Oxidation B. Glasses, metallic
1. Introduction CueZr-based bulk metallic glasses (BMGs), consisting of ternary or multi-component systems have been extensively studied since the past decades [1e6]. Several attractive potentials for the development of CueZr-based BMGs are noted, consisting of high glassforming ability (GFA) with critical cooling rates as low as 1 K/s, high fracture strength of w1600 MPa, and relatively low materialcost [2,3]. Most recently, a series of (Cu50Zr50)100xAlx and (Cu50Zr50)100xyAlxAgy amorphous systems were developed, and a few results revealed that those systems exhibited unique thermal and mechanical properties. For example, the glass transition temperature (Tg) and crystallization temperature (TX) of the CZ43BMG were reported to be about 449 and 521 C, respectively, having a larger supercooled temperature of 72 C [6]. This BMG also exhibited a higher compressive strength near 2000 MPa and a plastic deformation around 8% [7]. In addition, another study reported that small amounts of Si addition (such as the CZ43SBMG) could further increase the GFA of the glassy alloy, which offered a better thermal property to resist the devitrification for industrial applications [8]. To use those BMGs at elevated temperatures for practical purposes, it is essential to understand their airoxidation behavior. Thus, the goal of this study is to investigate the
* Corresponding author. E-mail address: [email protected] (W. Kai).
oxidation of CZ43- and CZ43S-BMGs, and in particular, the effect of Si on the oxidation kinetics is explored. 2. Experimental Both CZ43- and CZ43S-BMG rods with 3 mm in diameter and 15 mm in length were prepared by a drop cast technique, as described elsewhere [9]. The detail composition of the alloy ingots analyzed by X-ray wavelength dispersive spectroscopy (WDS) gave Cue43.12Zre7.28Ale7.25Ag and Cue43.08Zre7.09Ale7.07Age0.51Si (in at.%), respectively. BMG samples were directly sliced into about 1 mm in thickness, ground and polished with a 1-mm diamond paste, cleaned with acetone and methanol, immediately dried before the tests. Oxidation tests and characterization of the substrate and scales were similar to those described in the previous study [10]. 3. Results and discussion A typical X-ray diffraction (XRD) analysis of the CZ43- and CZ43S-BMGs, as shown in Fig. 1, reveals a wide-broadening peak near 2q ¼ 39.8 and 39.6 , respectively, indicative of the amorphous nature of both substrates. The DSC curves of the two alloys obtained at a heating rate of 20 C/min are shown in Fig. 2, revealing that the Tg and TX temperatures of the CZ43-BMG are around 431.0 and 506.1 C, respectively, while those of the CZ43S-BMG are around 428.2 and 519.0 C, respectively. Thus, the supercooled liquid region (DTX ¼ TX Tg) are about 75.1 (CZ43-BMG) and 90.8 C
0966-9795/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.03.005
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
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Fig. 1. XRD spectra of the CZ43- and CZ43S-BMGs.
Fig. 3. Parabolic plots of the mass-gain data from (a) CZ43-BMG and (b) CZ43S-BMG.
Fig. 2. DSC curves of the CZ43- and CZ43S-BMGs obtained at a heating rate of 20 C/min.
(CZ43S-BMG), and the oxidation tests were set to encompass the temperature range of 375e500 C. Parabolic plots of the mass-gain data of the CZ43- and CZ43SBMGs over the temperature range of 375e500 C are shown in Fig. 3. The oxidation kinetics of the two alloys generally followed a multi-stage parabolic-rate law, consisting at least of an initial incubation period of time (w0.03 h), followed by a fast-growth transient oxidation (0.03 w 2 h), by a decreased oxidation stage (from 2 to 4.5 h), and by a final steady-state stage (after 4.5 h). The oxidation rate constants (kp values) at the steady-state stage of both alloys are summarized in Table 1. The kp values of CZ43-BMG progressively increase with increasing temperature when it is below 450 C, while a significant lower kp value is obtained at 500 C. For comparative purposes, the kp values of Cu45Zr45Al5Ag5
BMG (CZ45-BMG) in the previous study are also tabulated in the same table [10]. In general, the CZ43-BMG exhibited slightly lower kp values at T 425 C, while the reverse condition was observed at higher temperatures. In addition, the temperature effect on the kp values of the CZ43S-BMG followed a similar trend as that of the Sifree alloy, and the kinetics results showed that an additional small amount of Si could further reduce the oxidation reaction to a certain extent at T 450 C, as discussed later. For comparative purposes, it is useful to review the scale constitution and phases and the crystallization sequence of CZ45BMG after air oxidation at the same temperature range [10]. The scales formed on CZ45-BMG were strongly temperature-dependent, consisting mostly of CuO and tetragonal-ZrO2 (t-ZrO2), minor amounts of monoclinic- and orthorhombic-ZrO2 (m- and o-ZrO2), and uncorroded Ag particles at T 450 C, while an additional aAl2O3 phase was also detected at 500 C. In addition, the crystallization sequence for this BMG at 400 C underwent an initial selective oxidation of Zr to form t- and o-ZrO2 (governed by inward diffusion of O), followed by the growth of CuO and m-ZrO2, while the
Table 1 Parabolic-rate constants of the CZ-based BMGs in dry air (kp unit: g2 cm4 s1). 375 C CZ43-BMG CZ43S-BMG CZ45-BMG
400 C 12
1.17 10 1.08 1012 1.49 1012
425 C 12
2.22 10 1.85 1012 2.56 1012
450 C 12
2.50 10 2.16 1012 3.65 1012
500 C 12
5.38 10 3.13 1012 3.82 1012
4.00 1012 2.04 1012 1.12 1012
Fig. 4. (a) Cross-sectional BEI micrograph and XRD spectra of the CZ43-BMG oxidized for 36 h at 375 C, and cross-sectional BEI micrographs of the same alloy for 36 h at (b) 400 C and (c) 425 C.
Fig. 5. Cross-sectional BEI micrographs and XRD spectra of the CZ43-BMG oxidized for 36 h at (a) 450 C and (b) 500 C.
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
substrate retained amorphous nature. However, the substrate started to form Cu10Zr7 at 450 C with a 260 min exposure, and then, followed by the development of ZrAl after a 360 min exposure. The following section presents and discusses microstructure results for CZ43- and CZ43S-BMGs. Typical backscattered electron image (BEI) micrographs of the cross-sections formed on the CZ43BMG and XRD analyses at 375e425 C for 36 h are shown in Fig. 4. The scales remained good adherent to the substrate and their constitutions were nearly identical to the CZ45-BMG, except for having an unknown peak near 2q ¼ 38.21 in the outer-portion of scales. In addition, the Cu10Zr7 phase was detected in the substrate beneath the scales after the oxidation at 375 C, indicating that the crystallization of CZ43-BMG occurred at a lower temperature with respect to that of CZ45-BMG (at 425e450 C). Furthermore, the scales formed at 450e500 C, as shown in Fig. 5, were nearly identical to those at T 425 C, except that the scale/substrate interfaces became more non-planar, and minor amounts of Cu2O and Al2O3 were detected at 450 and 500 C, respectively. An interesting aspect of the results is to form another ZrAl phase which was absent at lower temperatures. Very likely, the growth of ZrAl under a thermal-activated reaction between Zr and Al in the CZ43BMG substrate was not occurred at T 425 C but prevailed at higher temperatures. In addition, the scale constitution and phases of the CZ43S-BMG were further discussed after SEM/EPMA/XRD/TEM analyses (Figs. 6e8). Cross-sectional BEI micrographs of the CZ43S-BMG oxidized for 36 h at 375e400 C and XRD spectra at 400 C are shown in Fig. 6. The scale constitution and phases were similar to that of CZ43-BMG, except that the amount of o-ZrO2 was much abundant but that of t-ZrO2 was nearly invisible in the inner-
1997
portion scales. In addition, the substrate remained amorphous nature after the oxidation at 400 C although a few tiny peaks were noted near the wide-broadening peak. Those exceptional observations could be due to the effect of Si addition, which could cause the atomic-structure changes in the BMGs, and in turn provided a contribution to not only enhance the growth of o-ZrO2 but stabilize the amorphous substrate to a higher temperature, in comparison to that of CZ43-BMG. Another BEI micrograph and Xray maps of alloying elements and O of the cross-sections of the CZ43S-BMG oxidized at 425 C for 36 h are shown in Fig. 7. Unlike the results at 375e400 C, some cracks near the Ag particles inside the scales and a few prolong rod-shaped precipitates in the scale/ substrate interface were noted. Based on X-ray maps, those precipitates enriched Zr and Si, however, EPMA failed to identify their exact composition. Thus, further TEM analyses were given, with the bright-field images and inserted selected-area diffraction (SAD) patterns in various regions of the cross-sectional scales (Fig. 8). Fine nano-grains of mostly CuO intermixed with minor Cu2O were observed in the outer-portion scales, which were confirmed by SAD ring patterns (Fig. 8a). The Cu2O phase was not detected by XRD but confirmed by TEM, which could be due to its low amounts beyond XRD-detection limits. Another TEM micrograph taken from the scale/substrate interface (Fig. 8b) revealed a larger dark precipitate of intermetallic ZrSi2 (w750 nm in diameter), which was also confirmed by the SAD spot patterns. The observed ZrSi2 precipitate was unexpected, which deserved further discussion. Based on the thermodynamic data of pure substances [11], the Gibbs free energy of formation (DGof in unit of kJ/mol O2) of SiO2 at 425 C is about 779.2, which is much more negative than those of CuO (184.5) and Cu2O (234.5), but less negative than
Fig. 6. Cross-sectional BEI micrograph and XRD spectra of the CZ43S-BMG oxidized for 36 h at (a) 375 C and (b) 400 C.
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W. Kai et al. / Intermetallics 18 (2010) 1994e1999
Fig. 7. (a) Cross-sectional BEI micrograph and X-ray maps of (b) Cu, (c) Zr, (d) Al, (e) Ag, (f) Si, and (g) O of the CZ43S-BMG oxidized at 425 C for 36 h.
those of Al2O3 (971.1) and ZrO2 (963.7). It is possible that SiO2 is favorable to form in the scales intermixed with other oxides (CuO/ Cu2O, Al2O3 or ZrO2) for thermodynamic reasons. However, it was never detected by XRD or TEM. As similar to the previous result [10], the oxidation mechanism of CZ43S-BMG was also predominant by inward oxygen diffusion, so that the first growth of ZrO2
(three different modifications) and CuO/Cu2O was expected because the relative activities of Zr and Cu were much higher than those of other elements (Al, Ag, and Si). After the development of intermixed scales of ZrO2 and CuO/Cu2O, the relative activities of Al, Ag, and Si could be slightly increased, which possibly leaded to a further oxidation of Al and Si (Ag was considered as an inert
W. Kai et al. / Intermetallics 18 (2010) 1994e1999
1999
Fig. 8. TEM bright-field images and inserted SAD patterns of the CZ43S-BMG oxidized at 425 C for 36 h, showing (a) the copper oxides in the outer-portion scales and (b) the intermetallic ZrSi2 in the scale/substrate interface.
element). However, this prediction disagreed to the results mentioned above because Al2O3 was only detected at 500 C and SiO2 was absent. Very likely, Si did not react with oxygen but diffused back into the glassy substrate, which in turn resulted in forming the crystalline ZreSi phases. Besides, the heat of mixing (in unit of kJ/mol) for the ZreSi intermetallics was about 84, which is much more negative than that of ZreAl (44), ZreCu (23), ZreAg (20) CueSi (19), and CueAl (1) [8,12]. Thus, the formation of ZrSi2 precipitates in the scale/substrate interface became thermodynamically and kinetically favorable. Another interesting aspect to be discussed is to form abundant amounts of o-ZrO2 in the inner-portion of the scales of the CZ43SBMG, which resulted in the slow oxidation rates, as compared to those of CZ43-BMGs. As reported previously, the higher the amount of t-ZrO2 presented in the scales, the faster the oxidation rates obtained for Zr-based BMGs [13,14]. The same trend could be expected for the oxidation of CZ43-based BMGs. The detail mechanism for the formation of o-ZrO2 was still unclear, however, its abundant amounts did progressively reduce the formation of t-ZrO2 in the scales when the available amounts of Zr were fixed in the substrate. Perhaps, the oxygen inward diffusion in the sublattice of t-ZrO2 is much faster than that in both m- and o-ZrO2, so that the more the o-ZrO2 phase and the less the t-ZrO2 phase presented in the scales after the oxidation at T < 450 C, the lower the kp values obtained. However, this deduction became invalid when Al2O3 formed in the scales of the glassy alloys at 500 C. Although not shown here, the scales formed on the CZ43- and CZ43S-BMG at 450e500 C are similar to those of the CZ45-BMG exception that the Cu2O presented in the scales of CZ43-BMG was nearly invisible in the CZ43S-BMG, while an additional amount of Al2O3 was always presented in the scales of the three BMGs at 500 C. As also mentioned before [10], the presence of Al2O3 could play a partly blocking effect to reduce inward diffusion of oxygen, which in turn imparted the oxidation rates, leading to a lower kp with respect to those at lower temperatures. Most likely, the growth of Al2O3 makes a much more protective barrier to reduce the oxidation rate of the alloys than that of o-ZrO2. As a result, the kp values at 500 C for the three BMGs are lower than those at 450 C.
1. The oxidation kinetics of the two BMGs obeyed a multi-stage parabolic-rate law with their kp values fluctuating with temperature. 2. A small amount of the Si addition provided a better oxidation resistance for the CZ43S-BMG at T 450 C, while no improvement was obtained at 500 C. 3. The scales formed on the CZ43S-BMG strongly depended on temperature, consisting mostly of CuO and t-ZrO2, minor amounts of o- and m-ZrO2, Cu2O, and uncorroded Ag particles at T 450 C, while Al2O3 were also detected at 500 C. 4. The additional Si plays an important role in forming ZrSi2 precipitates near the scale/substrate interface, which enhances the oxidation resistance of CZ43S-BMG. Acknowledgments The authors are thankful for financial support by the National Science Council of Republic of China under the Grant No. of NSC-982221-E-019-007 and TGA equipment support by the National Taiwan Ocean University under the Grant No. of NTOU-RD972-0403-01-01. Special thanks are indebted to Dr. R.T. Huang in our institute and Mr. C.T. Wu in the Department of System Engineering and Science, National Tsing Hua University (Hsin-Chu, Taiwan) for their technical supports of TEM. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions [12]
The effects of Si addition on the oxidation behavior of the CZ43and CZ43S-BMGs over the temperature range of 375e500 C was characterized. Several conclusions can be reached.
[13] [14]
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