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Oxidation of Cu60Zr30Ti10 metallic glasses
Materials Science and Engineering A 449–451 (2007) 165–168
Oxidation of Cu60Zr30Ti10 metallic glasses Uwe K¨oster ∗ , Lioba Jastrow, Monika Meuris Department of Biochemical & Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany Received 25 August 2005; received in revised form 20 February 2006; accepted 26 February 2006
Abstract Cu–Zr-based bulk metallic glasses are of increasing interest due to their excellent properties. Cu-rich Cu60 Zr30 Ti10 and Zr-rich Zr69.5 Cu12 Ni11 Al7.5 glasses (numbers indicate at.%) are examples which combine good glass forming ability with excellent mechanical properties making them a material of choice for a variety of applications. Any application, however, requires adequate thermal stability, i.e. resistance against crystallization and oxidation. The aim of this paper is a detailed investigation on the oxidation of Cu60 Zr30 Ti10 metallic glasses. Whereas oxidation in Zr-rich Zr69.5 Cu12 Ni11 Al7.5 is assumed to be controlled by oxygen diffusion through a homogeneous scale towards the ZrO2 /glass interface, in Cu-rich Cu60 Zr30 Ti10 metallic glasses multilayered scales were found to develop with an assembly of Cu-oxide needles at the outer surface. Due to developing stresses and the formation of voids the outer oxide layer loses contact at the interface and starts to peel off during ongoing oxidation. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Cu60 Zr30 Ti10 ; Oxidation; Multilayered scale; Cross-sectional microscopy
1. Introduction
2. Experimental
Cu–Zr-based alloys are among the most promising systems for bulk glass formation. Zr-rich bulk metallic glasses are already used due to their excellent mechanical properties, i.e. high elastic limit and strength, e.g. for golf clubs or penetrators [1,2]. The only recently developed Cu-rich Zr–Cu-based bulk metallic glasses have some potential advantages such as an even higher elastic limit and strength in comparison to those based on early transition metals: Cu60 Zr30 Ti10 (numbers indicate at.%) [3] exhibits an elastic modulus of about 120 GPa and a strength of more than 2000 N/mm2 . For any application, however, thermal stability, i.e. resistance against crystallization as well as oxidation has to be adequate. Since the glass transition temperature increases with the Cu-content, crystallization is not expected to limit the use of Cu-rich glasses. Oxidation of the Zr-rich glasses, e.g. Zr69.5 Cu12 Ni11 Al7.5 has been studied in some detail only recently [4,5], but only very little is known on the oxidation of Cu-rich metallic glasses [6]. The aim of this paper is to study the micromechanism during oxidation of Cu60 Zr30 Ti10 metallic glasses in detail.
Cu60 Zr30 Ti10 , Cu50 Zr50 and Cu30 Zr70 pre-alloys were prepared by arc-melting in an Ar atmosphere from the pure elements. Amorphous ribbons with a thickness of ∼30 m and a width of 2–3 mm were prepared by melt-spinning in a dry Heatmosphere of 300 mbar. Oxidation of the melt-spun ribbons was studied by thermogravimetric analysis (Netzsch TG 400) in synthetic air. For these experiments the glassy ribbons were always freshly ground. The microstructure of the oxide scales was studied by Xray diffraction as well as by transmission electron microscopy (TEM; Philips CM 200), optical microscopy and scanning electron microscopy (SEM; Hitachi S-4500), in particular by crosssectional microscopy in order to reveal the rather complicated microstructure of the scale in detail.
∗
Corresponding author. Tel.: +49 231 755 2678; fax: +49 231 755 5978. E-mail address: [email protected] (U. K¨oster).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.291
3. Results and discussion Cu–Zr-based metallic glasses undergo significant oxidation in the temperature range above about 300 ◦ C. Fig. 1 shows the increase in weight for glasses with different Cu content during oxidation in dry air at 320 ◦ C: the oxidation kinetics is accelerated significantly with increasing Cu content. X-ray diffraction (see Fig. 2) reveals the formation of cubic ZrO2 and crystalline Cu as well as CuO and Cu2 O on the Cu60 Zr30 Ti10 metallic glass.
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Fig. 1. Influence of the Zr-content on the oxidation kinestics of Cu–Zr glasses as measured by thermogravimetry.
The microstructure of the formed oxide scale was analyzed by SEM and TEM either by flat-on view onto the oxidized surface or cross-sectional after suitable specimen preparation. Fig. 3 shows the specimen surface after oxidation for 2 h at 320 ◦ C. There are large Cu2 O globules decorated with thin needles. During further oxidation the number of these globules were observed to increase as well as their diameter. After 16 h oxidation these globules form a continuous layer, but start to delaminate and to peel off. The formation of such needles or whisker-like crystals during oxidation is already well-known from crystalline Cu [7,8]: The number of these needles increases with the oxidation temperature. Their composition and crystal structure changes from cubic Cu2 O at 200 ◦ C, CuO and tetragonal 6Cu2 O·CuO between 300 and 500 ◦ C towards monoclinic CuO at 500 ◦ C [7]. The needles formed during the oxidation of Cu60 Zr30 Ti10 metallic glasses can be scratch off and deposited onto a carbon
Fig. 3. Surface (SEM image) of oxidized Cu60 Zr30 Ti10 metallic glass (2 h at 320 ◦ C) exhibiting Cu2 O globules and CuO needles.
Fig. 2. X-ray diffraction pattern of Cu60 Zr30 Ti10 metallic glasses as oxidized for different times at 320 ◦ C in dry air.
Fig. 4. Cross-section (SEM image) of an oxidized Cu60 Zr30 Ti10 metallic glass (16 h at 320 ◦ C).
film coated TEM grid; they are about 20 nm thick and up to a few m long. The electron diffraction pattern can be indexed assuming crystalline CuO, but with a slightly changed lattice parameter. Fig. 4 shows the cross-section of a Cu60 Zr30 Ti10 metallic glass oxidized for 16 h at 320 ◦ C; there are two oxide layers very different in their composition and separated by a broad dark line. The outer layer formed by the globules contains only Cu and a high concentration of oxygen. Electron microscopy (shown in Fig. 5) reveals nanocrystals in the range of 50 nm; nanobeam electron diffraction can be indexed very well assuming Cu2 O. The dark line which separates the outer Cu2 O layer from the inner layer is probably an area with less density containing already some holes and cracks. After longer oxidation treatments the outer oxide layer was observed to start peelingoff along this line. It is not clear whether the delamination and peeling off proceeds already during the oxidation process due to the formation of Kirkendall voids or as a consequence of different thermal expansion coefficients during the cooling down to room temperature which may lead to significant stresses. A similar behavior is known from the oxidation of crystalline Cu
U. K¨oster et al. / Materials Science and Engineering A 449–451 (2007) 165–168
167
Fig. 5. Microstructure (TEM) of the outer oxide layer consisting of Cu2 O nanocrystals (nanobeam electron diffraction inserted).
Fig. 6. Cross-section of the inner oxide layer consisting of (Zr, Ti)O2 lamellae embedding Cu nanocrystals. The inserted electron diffraction pattern reveals continuous rings for the Cu and discrete spots for the oxide.
when the oxide layer delaminates fully after a critical thickness has been reached. The inner oxide layer (see Fig. 6) consists of cubic ZrO2 lamellae with interlamellar spacings in the range of 5 nm embedding Cu nanocrystals. TEM darkfield images as well as first HRTEM-photos [9] not shown here indicate that the ZrO2 lamellae are connected over larger areas or exhibit at least parallel orientation. A schematic sketch for the formation of the multilayered scale during oxidation of Cu60 Zr30 Ti10 metallic glasses is shown in Fig. 7. Nucleation of the oxidation process seems to start with the formation of Cu2 O crystals along grooves of the freshly ground surface followed by the formation of (Zr, Ti)O2 . There is some evidence that this nucleation step and oxidation kinetics are influenced by the surface preparation prior to oxidation. Only very recently, Chang et al. [10] have demonstrated that fresh prepared surfaces of Cu-rich Cu–Zr–Ti glasses are very prone to oxidation reactions during exposure to air and stated a strong influence of the sample preparation. This influence may explain also the differences between our results and those of Tam and Shek [5] who polished their specimen prior to the oxidation treatment.
Treating multilayered scale growth, one has to consider more than one parabolic rate constant, since the growth rate of one layer depends also on ion mobility in the adjacent oxide phase. The growth of the inner layer was observed to follow a squareroot-over-time law assuming an oxygen diffusivity of about 3 × 10−16 m2 /s. Similar diffusivities are observed for the oxygen diffusion in ZrO2 scales on Zr-rich metallic glasses [4]. Growth of the outer oxide layer and the whisker-like CuO crystals can proceed only, if the Cu supply by diffusion through the inner layer of the scale is sufficient. Since the inner layer exhibits a lamellar structure with spacings in the range of 5 nm or even less, enough easy diffusion paths for the necessary fast outward diffusion of Cu should be available. During an increase of the oxidation time from 2 to 16 h the diameter of the Cu2 O globules is doubled; this means that their volume increases linear in time, thus indicating a Cu supply through the inner oxide layer independent on its thickness. Probably this is possible due to the very fine lamellar structure of the inner oxide layer. The Cu-rich Cu60 Zr30 Ti10 metallic glass was observed to oxidize significantly faster than the Zr-rich ones, thus compromising its technical applications even despite their better mechanical properties. Therefore, it is of utmost importance to
Fig. 7. Schematic sketch of the oxidation of Cu60 Zr30 Ti10 metallic glasses at 320 ◦ C in dry air.
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develop strategies for improving the oxidation resistance. In Zrrich metallic glasses the addition of small amounts of Sn, Si or in particular Be is known to improve the oxidation resistance by reducing the rate controlling oxygen diffusivity. In future investigations the influence of such additions on the oxidation of Cu-rich metallic glasses will be studied in detail.
glassy matrix, but also very fast Cu diffusion towards the outer side. Cu-rich metallic glasses possess better mechanical properties than Zr-rich ones, but are prone to faster oxidation. This has to be taken into account for designing of improved Cu-rich bulk metallic glasses.
4. Conclusion
References
Whereas in slow oxidizing Zr-rich metallic glasses oxidation proceeds by the formation of ZrO2 nodules or lamellae embedding nanocrystals of the alloying metals, the fast oxidation is correlated with the formation of a fine lamellar structure consisting mainly of the two ZrO2 phases (tetragonal and monoclinic ZrO2 ), thus allowing the extreme fast oxygen diffusion. Cu-rich metallic glasses, however, form multilayered oxide scales with an outer layer consisting mainly of Cu2 O and CuO whiskers (in particular at higher oxidation temperatures); the inner layer possesses a very fine lamellar structure (ZrO2 + Cu) which allows not only very fast oxygen diffusion towards the
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
W.L. Johnson, JOM 54 (2002) 40. D.V. Louzguine, A. Inoue, J. Mater. Res. 17 (2002) 2112. D. Xu, G. Duan, W.L. Johnson, Phys. Rev. Lett. 92 (2004) 245504. U. K¨oster, L. Jastrow, Arch. Mater. Sci. 25 (2004) 283. Triwikantoro, Oxidation metallener Gl¨aser auf Zr-Basis, Cuvillier-Verlag, G¨ottingen, 2001. C.Y. Tam, C.H. Shek, J. Mater. Res. 20 (2005) 1396. C. Kaito, Y. Nakata, Y. Saito, T. Naika, K. Fujita, J. Cryst. Growth 74 (1986) 469. R. Haugsrud, J. Electrochem. Soc. 149 (2002) B14. U. K¨oster, X.L. Ma, D. Zander, M. Meuris, Private Communication, 2005. H.J. Chang, E.S. Park, Y.C. Kim, D.H. Kim, Mater. Sci. Eng. A 406 (2005) 4999.
Oxidation of Cu60Zr30Ti10 metallic glasses Uwe K¨oster ∗ , Lioba Jastrow, Monika Meuris Department of Biochemical & Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany Received 25 August 2005; received in revised form 20 February 2006; accepted 26 February 2006
Abstract Cu–Zr-based bulk metallic glasses are of increasing interest due to their excellent properties. Cu-rich Cu60 Zr30 Ti10 and Zr-rich Zr69.5 Cu12 Ni11 Al7.5 glasses (numbers indicate at.%) are examples which combine good glass forming ability with excellent mechanical properties making them a material of choice for a variety of applications. Any application, however, requires adequate thermal stability, i.e. resistance against crystallization and oxidation. The aim of this paper is a detailed investigation on the oxidation of Cu60 Zr30 Ti10 metallic glasses. Whereas oxidation in Zr-rich Zr69.5 Cu12 Ni11 Al7.5 is assumed to be controlled by oxygen diffusion through a homogeneous scale towards the ZrO2 /glass interface, in Cu-rich Cu60 Zr30 Ti10 metallic glasses multilayered scales were found to develop with an assembly of Cu-oxide needles at the outer surface. Due to developing stresses and the formation of voids the outer oxide layer loses contact at the interface and starts to peel off during ongoing oxidation. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Cu60 Zr30 Ti10 ; Oxidation; Multilayered scale; Cross-sectional microscopy
1. Introduction
2. Experimental
Cu–Zr-based alloys are among the most promising systems for bulk glass formation. Zr-rich bulk metallic glasses are already used due to their excellent mechanical properties, i.e. high elastic limit and strength, e.g. for golf clubs or penetrators [1,2]. The only recently developed Cu-rich Zr–Cu-based bulk metallic glasses have some potential advantages such as an even higher elastic limit and strength in comparison to those based on early transition metals: Cu60 Zr30 Ti10 (numbers indicate at.%) [3] exhibits an elastic modulus of about 120 GPa and a strength of more than 2000 N/mm2 . For any application, however, thermal stability, i.e. resistance against crystallization as well as oxidation has to be adequate. Since the glass transition temperature increases with the Cu-content, crystallization is not expected to limit the use of Cu-rich glasses. Oxidation of the Zr-rich glasses, e.g. Zr69.5 Cu12 Ni11 Al7.5 has been studied in some detail only recently [4,5], but only very little is known on the oxidation of Cu-rich metallic glasses [6]. The aim of this paper is to study the micromechanism during oxidation of Cu60 Zr30 Ti10 metallic glasses in detail.
Cu60 Zr30 Ti10 , Cu50 Zr50 and Cu30 Zr70 pre-alloys were prepared by arc-melting in an Ar atmosphere from the pure elements. Amorphous ribbons with a thickness of ∼30 m and a width of 2–3 mm were prepared by melt-spinning in a dry Heatmosphere of 300 mbar. Oxidation of the melt-spun ribbons was studied by thermogravimetric analysis (Netzsch TG 400) in synthetic air. For these experiments the glassy ribbons were always freshly ground. The microstructure of the oxide scales was studied by Xray diffraction as well as by transmission electron microscopy (TEM; Philips CM 200), optical microscopy and scanning electron microscopy (SEM; Hitachi S-4500), in particular by crosssectional microscopy in order to reveal the rather complicated microstructure of the scale in detail.
∗
Corresponding author. Tel.: +49 231 755 2678; fax: +49 231 755 5978. E-mail address: [email protected] (U. K¨oster).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.291
3. Results and discussion Cu–Zr-based metallic glasses undergo significant oxidation in the temperature range above about 300 ◦ C. Fig. 1 shows the increase in weight for glasses with different Cu content during oxidation in dry air at 320 ◦ C: the oxidation kinetics is accelerated significantly with increasing Cu content. X-ray diffraction (see Fig. 2) reveals the formation of cubic ZrO2 and crystalline Cu as well as CuO and Cu2 O on the Cu60 Zr30 Ti10 metallic glass.
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Fig. 1. Influence of the Zr-content on the oxidation kinestics of Cu–Zr glasses as measured by thermogravimetry.
The microstructure of the formed oxide scale was analyzed by SEM and TEM either by flat-on view onto the oxidized surface or cross-sectional after suitable specimen preparation. Fig. 3 shows the specimen surface after oxidation for 2 h at 320 ◦ C. There are large Cu2 O globules decorated with thin needles. During further oxidation the number of these globules were observed to increase as well as their diameter. After 16 h oxidation these globules form a continuous layer, but start to delaminate and to peel off. The formation of such needles or whisker-like crystals during oxidation is already well-known from crystalline Cu [7,8]: The number of these needles increases with the oxidation temperature. Their composition and crystal structure changes from cubic Cu2 O at 200 ◦ C, CuO and tetragonal 6Cu2 O·CuO between 300 and 500 ◦ C towards monoclinic CuO at 500 ◦ C [7]. The needles formed during the oxidation of Cu60 Zr30 Ti10 metallic glasses can be scratch off and deposited onto a carbon
Fig. 3. Surface (SEM image) of oxidized Cu60 Zr30 Ti10 metallic glass (2 h at 320 ◦ C) exhibiting Cu2 O globules and CuO needles.
Fig. 2. X-ray diffraction pattern of Cu60 Zr30 Ti10 metallic glasses as oxidized for different times at 320 ◦ C in dry air.
Fig. 4. Cross-section (SEM image) of an oxidized Cu60 Zr30 Ti10 metallic glass (16 h at 320 ◦ C).
film coated TEM grid; they are about 20 nm thick and up to a few m long. The electron diffraction pattern can be indexed assuming crystalline CuO, but with a slightly changed lattice parameter. Fig. 4 shows the cross-section of a Cu60 Zr30 Ti10 metallic glass oxidized for 16 h at 320 ◦ C; there are two oxide layers very different in their composition and separated by a broad dark line. The outer layer formed by the globules contains only Cu and a high concentration of oxygen. Electron microscopy (shown in Fig. 5) reveals nanocrystals in the range of 50 nm; nanobeam electron diffraction can be indexed very well assuming Cu2 O. The dark line which separates the outer Cu2 O layer from the inner layer is probably an area with less density containing already some holes and cracks. After longer oxidation treatments the outer oxide layer was observed to start peelingoff along this line. It is not clear whether the delamination and peeling off proceeds already during the oxidation process due to the formation of Kirkendall voids or as a consequence of different thermal expansion coefficients during the cooling down to room temperature which may lead to significant stresses. A similar behavior is known from the oxidation of crystalline Cu
U. K¨oster et al. / Materials Science and Engineering A 449–451 (2007) 165–168
167
Fig. 5. Microstructure (TEM) of the outer oxide layer consisting of Cu2 O nanocrystals (nanobeam electron diffraction inserted).
Fig. 6. Cross-section of the inner oxide layer consisting of (Zr, Ti)O2 lamellae embedding Cu nanocrystals. The inserted electron diffraction pattern reveals continuous rings for the Cu and discrete spots for the oxide.
when the oxide layer delaminates fully after a critical thickness has been reached. The inner oxide layer (see Fig. 6) consists of cubic ZrO2 lamellae with interlamellar spacings in the range of 5 nm embedding Cu nanocrystals. TEM darkfield images as well as first HRTEM-photos [9] not shown here indicate that the ZrO2 lamellae are connected over larger areas or exhibit at least parallel orientation. A schematic sketch for the formation of the multilayered scale during oxidation of Cu60 Zr30 Ti10 metallic glasses is shown in Fig. 7. Nucleation of the oxidation process seems to start with the formation of Cu2 O crystals along grooves of the freshly ground surface followed by the formation of (Zr, Ti)O2 . There is some evidence that this nucleation step and oxidation kinetics are influenced by the surface preparation prior to oxidation. Only very recently, Chang et al. [10] have demonstrated that fresh prepared surfaces of Cu-rich Cu–Zr–Ti glasses are very prone to oxidation reactions during exposure to air and stated a strong influence of the sample preparation. This influence may explain also the differences between our results and those of Tam and Shek [5] who polished their specimen prior to the oxidation treatment.
Treating multilayered scale growth, one has to consider more than one parabolic rate constant, since the growth rate of one layer depends also on ion mobility in the adjacent oxide phase. The growth of the inner layer was observed to follow a squareroot-over-time law assuming an oxygen diffusivity of about 3 × 10−16 m2 /s. Similar diffusivities are observed for the oxygen diffusion in ZrO2 scales on Zr-rich metallic glasses [4]. Growth of the outer oxide layer and the whisker-like CuO crystals can proceed only, if the Cu supply by diffusion through the inner layer of the scale is sufficient. Since the inner layer exhibits a lamellar structure with spacings in the range of 5 nm or even less, enough easy diffusion paths for the necessary fast outward diffusion of Cu should be available. During an increase of the oxidation time from 2 to 16 h the diameter of the Cu2 O globules is doubled; this means that their volume increases linear in time, thus indicating a Cu supply through the inner oxide layer independent on its thickness. Probably this is possible due to the very fine lamellar structure of the inner oxide layer. The Cu-rich Cu60 Zr30 Ti10 metallic glass was observed to oxidize significantly faster than the Zr-rich ones, thus compromising its technical applications even despite their better mechanical properties. Therefore, it is of utmost importance to
Fig. 7. Schematic sketch of the oxidation of Cu60 Zr30 Ti10 metallic glasses at 320 ◦ C in dry air.
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U. K¨oster et al. / Materials Science and Engineering A 449–451 (2007) 165–168
develop strategies for improving the oxidation resistance. In Zrrich metallic glasses the addition of small amounts of Sn, Si or in particular Be is known to improve the oxidation resistance by reducing the rate controlling oxygen diffusivity. In future investigations the influence of such additions on the oxidation of Cu-rich metallic glasses will be studied in detail.
glassy matrix, but also very fast Cu diffusion towards the outer side. Cu-rich metallic glasses possess better mechanical properties than Zr-rich ones, but are prone to faster oxidation. This has to be taken into account for designing of improved Cu-rich bulk metallic glasses.
4. Conclusion
References
Whereas in slow oxidizing Zr-rich metallic glasses oxidation proceeds by the formation of ZrO2 nodules or lamellae embedding nanocrystals of the alloying metals, the fast oxidation is correlated with the formation of a fine lamellar structure consisting mainly of the two ZrO2 phases (tetragonal and monoclinic ZrO2 ), thus allowing the extreme fast oxygen diffusion. Cu-rich metallic glasses, however, form multilayered oxide scales with an outer layer consisting mainly of Cu2 O and CuO whiskers (in particular at higher oxidation temperatures); the inner layer possesses a very fine lamellar structure (ZrO2 + Cu) which allows not only very fast oxygen diffusion towards the
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
W.L. Johnson, JOM 54 (2002) 40. D.V. Louzguine, A. Inoue, J. Mater. Res. 17 (2002) 2112. D. Xu, G. Duan, W.L. Johnson, Phys. Rev. Lett. 92 (2004) 245504. U. K¨oster, L. Jastrow, Arch. Mater. Sci. 25 (2004) 283. Triwikantoro, Oxidation metallener Gl¨aser auf Zr-Basis, Cuvillier-Verlag, G¨ottingen, 2001. C.Y. Tam, C.H. Shek, J. Mater. Res. 20 (2005) 1396. C. Kaito, Y. Nakata, Y. Saito, T. Naika, K. Fujita, J. Cryst. Growth 74 (1986) 469. R. Haugsrud, J. Electrochem. Soc. 149 (2002) B14. U. K¨oster, X.L. Ma, D. Zander, M. Meuris, Private Communication, 2005. H.J. Chang, E.S. Park, Y.C. Kim, D.H. Kim, Mater. Sci. Eng. A 406 (2005) 4999.