A. Peker

A. Wang

C. A. Schuh

C. L. Dai

D. C. Hofmann

D. Wang

E. Miyazaki

F. Li

G. Li

K. F. Xie

K. F. Yao

Ke-Fu Yao

L. Hu

L. Li

M. R. Reddy

Q. He

Q. Li

S. A. Raspopov

S. B. Qiu

S. G. Srinivasan

S. Packirisamy

T. Zhang

W. H. Wang

W. Zhang

W. Zhou

X. H. Zhao

X. Hui

X. Lue

Xin Wang

Y. Aoki

Y. Huang

Yang Shao

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    © The Author(s) 2012. This article is published with open access at Springerlink.com csb.scichina.com   www.springer.com/scp                       *Corresponding author (email: kfyao@tsinghua.edu.cn) Article Materials Science December 2012  Vol.57  No.36: 48014804  doi: 10.1007/s11434-012-5573-5 Chemical composition dependence of atomic oxygen erosion  resistance in Cu-rich bulk metallic glasses  WANG Xin, SHAO Yang & YAO Ke-Fu* Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Received August 16, 2012; accepted September 29, 2012  The effect of atomic oxygen (AO) on the surface oxidation of several typical Cu-based bulk metallic glasses (BMGs) was studied in the present work. The AO source using in this study is generated by discharge plasma type ground simulation equipment. The AO erosion/oxidation resistances of the amorphous alloy samples were assessed based on the analysis of mass loss, surface color and microstructure. It is found that these Cu-based BMGs possess good AO erosion/oxidation resistance and their resistance to AO erosion/oxidation strongly depends on the chemical composition. For the samples containing more Ag and/or Cu, the AO erosion/oxidation resistance is weaker. The present result is important for designing new metallic glasses using as space materials. bulk metallic glass, atomic oxygen erosion, low earth orbit, oxidation  Citation:  Wang X, Shao Y, Yao K F. Chemical composition dependence of atomic oxygen erosion resistance in Cu-rich bulk metallic glasses. Chin Sci Bull, 2012, 57: 48014804, doi: 10.1007/s11434-012-5573-5    The presence of atomic oxygen (AO) in low Earth orbit (LEO, altitude from ~200 to ~700 km) is one of important reasons for surface degradation of the aerospace materials. Atomic oxygen, which possesses high chemical activity, is the dominant chemical constituent (80%) in the neutral at-mosphere in LEO [1]. When the spacecraft locates in LEO with an orbital velocity of 7–8 km s−1, AO impacts the sur-face of the spacecraft with a kinetic energy of ~5 eV, thus can react with many elements [2]. Then, it may lead to sur-face erosion and property degradation of space materials which would further affect the safety/longevity of space-crafts. Therefore, studies on the AO effects have been widely appreciated. Through testing platforms equipped in spacecrafts and all kinds of ground effect simulation testing devices, a large number of aerospace materials such as polymer [3], inorganic material coating [4,5], silver [6], aluminum [7], titanium [8] and carbon materials [9], etc., have been studied extensively. For most metal materials, AO will react with them to form oxidized film which can prevent further oxidation to erode the material. Except AO sensitive elements such as Os, Ag and Cu, metal materials usually have relatively better AO resistance than those polymers and carbon materials. But the films generated by AO can result in a change in surface roughness and thus leading to degradation of luminous reflectance, frictional wear properties and even welding performance [7]. There-fore it is of great interest to investigate the effect of AO on metallic materials to better understand the oxidation mecha-nism. So far, the field of AO erosion study is largely fo-cused on crystalline materials. Recent decades, bulk metal-lic glasses (BMGs), new comers of metallic materials, have attracted lots of attention [10–14] due to excellent mechan-ical and functional properties [15–21], which are believed to be resulted by their unique structure [22,23]. Since high performance of BMGs make them potential candidates of space materials, their AO erosion resistance is of interest to be investigated. The main aim of the present work is to explore the influ-ence of Cu or Ag contained on the AO resistance of BMG materials. To our knowledge, those alloys containing ele-ments sensitive to AO, such as Ag and Cu, were worried about whether they are stable in LEO. In addition, as one of the major addition elements, Cu or Ag is often used to im-prove the glass forming ability or mechanical properties in 4802 Wang X, et al.   Chin Sci Bull   December (2012) Vol.57 No.36 many BMGs [24–27]. Hence, it is of great interest to know whether they adversely affect the AO erosion resistance. We choose a series of Cu-based alloys containing different amounts of Cu/Ag to study the oxidation of such glassy alloys rich in AO sensitive elements. Finally, we will pro-vide some simple empirical rules for designing metallic glasses applied in LEO environment. 1  Experimental Master alloy ingots with nominal compositions of Cu44.25- Ag14.75Zr36Ti5 [27], Cu42Zr42Ag8Al8 [26,28], Cu45Zr48Al7 [29], and Zr41Ti14Cu12.5Ni10Be22.5 (Vit1) [30], were prepared by arc melting the mixtures of raw elements with purity better than 99.4% in a Ti-gettered Ar2 atmosphere. Plates with 2 mm in thickness, 12 mm in width and 60 mm in length were prepared from the ingots by copper mold suc-tion casting method. Thin crosscut pieces of samples were cut down in the middle of the plates and then examined by X-ray diffraction (XRD) using CuKα radiation. Square plate samples with size of 2 mm × 5 mm × 5 mm and 2 mm × 10 mm × 10 mm cut from the as-cast plates, were grinded and polished using fine sandpapers. The polished samples were tested using the ground-based AO eﬀect simulation facility at Beijing University of Aero-nautics and Astronautics (BUAA). By the method of fila-ment discharge plasma, this ground-based AO eﬀect simu-lation facility can generate a larger amount of AO flux than that in LEO. Therefore, the progress of the experiment can be accelerated so as to shorten experimental period to just a few days. The configuration and characteristics of the facil-ity were detailed in reference [31]. In this case, a Kapton material, which is a commonly used polymer with a con-stant AO erosion rate of 3.0×10−24 cm3/atom, is chosen as the standard material to estimate the AO flux. In order to avoid forming the autoxidation passive film on the test samples in air and mislead the analysis of test results, all the samples have been carefully grinded with fine sandpaper prior to AO exposure. The experimental materials used in this work with the compositions of C22H10N2O5 (Kapton), Cu44.25Ag14.75Zr36Ti5, Cu42Zr42Ag8Al8, Cu45Zr48Al7, and Zr41Ti14Cu12.5Ni10Be22.5 as mentioned above, are named A0, A1, A2, A3 and A4, respectively. The mass of the samples was measured with a DT-100 balance (Beijing Optical Instrument Factory, China), with a sensitivity of 0.00005 g. The surface morphology was in-spected by a scanning electron microscope (SEM) with a field emission gun (LEO 1530).  2  Results 2.1  Changes in the colors of exposure surfaces The color change of the exposed surface is the most intui-tive evidence to determine whether a sample is sensitive to AO environment. Figure 1 shows the color changes on the surfaces of the pre/post exposure samples. Not surprise, the exposure surface colors of those samples with Cu-rich and Ag-rich have changed from a general metallic gray to a more darken color after AO exposure (Figure 1). In the three Cu-Zr-Al alloys with almost the same amount of Cu but different amounts of Ag, a clear trend can be noticed that the more Ag contained, the darken color presented after AO exposure. While, color changes of A4 samples were not that clear (Figure 1(d)). 2.2  Mass loss  Table 1 lists the mass losses of the samples exposed in AO for 40 h together with the color changes. Sample A0, which is the reference material Kapton, has an obvious mass loss of 65.78%, indicating that the experiment complied with the requirements of the AO test. Since the sensitivity of the     Figure 1  (Color online) The digit images of the pre/post exposure sam-ples for comparing their colors changes on the surfaces. (a) A1; (b) A2; (c) A3; (d) A4.  Table 1  The summarized results of the experiment including color change, microstructure change and mass loss with Cu and Ag contained Sample Composition (in at.) Cu (at %) Ag  (at %) Change  in color Change  in microstructure Mass (g) Mass loss (%) Pre-exposure Post-exposure A0 C22H10N2O5(Kapton) – – obvious obvious 0.00564 0.00193 65.78 A1 Cu44.25Ag14.75Zr36Ti5 44 15 obvious obvious 0.43460 0.43455 0.01 A2 Cu42Zr42Ag8Al8 42 8 none slight 0.49565 0.49560 0.01 A3 Cu45Zr48Al7 45 0 slight slight 0.51585 0.51585 0 A4 Zr41Ti14Cu12.5Ni10Be22.5 (Vit1) 13 0 none none 1.05880 1.05880 0   Wang X, et al.   Chin Sci Bull   December (2012) Vol.57 No.36 4803 balance is 0.00005 g, the values obtained in this case are of an instrumental error of ±0.00005 g. In other words, it is then somewhat negligible with the mass value smaller than 0.0001 g. Although the surface color changes reveal the effectiveness of the role of atomic oxygen, all the test BMG samples have no obvious mass loss more than 0.01% as listed in Table 1, indicating that glass alloy possesses very good property in AO erosion/oxidation resistance. 2.3  Surface microstructure morphology Figure 2 shows some typical surface microstructure mor-phology images of the samples. For sample A1, an obvious change between the pre-exposure and post-exposure surface can be found. It has been transformed from a smooth sur-face morphology except some scratches (Figure 2(a)) to a quite rough surface morphology with obvious AO erosion features (Figure 2(b)). On the exposed surface of A1, not only oxidation product but also some voids and cracks can be clearly observed in a relative low magnification which is a clear evidence that the interaction of AO with the sample. For sample A2, it is found that the oxidation product parti-cles with a smaller size of ~50 nm, uniformly distributed on the surface (Figure 2(c)). While for sample A3, the mor-phology is similar with A2, except particle size which rang-es from ~30 to ~200 nm (Figure 2(d)). However, for A4 (Figure 3(e), (f)), no changes could be observed before and after AO exposure, except scratches on the surface, sug-gesting a good AO erosion/oxidation resistance.      Figure 2  Typical morphologies of the pre/post exposure surface in AO for 40 h. (a) A representative morphology of the pre-exposure sample A1; (b) A1 for post-exposure; (c) A2 for post-exposure; (d) A3 for post-expo- sure; (e) A4 for pre-exposure; (f) A4 for post-exposure. 3  Discussions Based on the results reported above, Cu and Ag are key constitute elements that control the AO erosion resistance of the BMGs. Table 1 also summarizes a qualitative analysis results showing that the AO erosion resistance of the tested BMG samples depended on the Cu and/or Ag contains. Taking samples A1–A3 into account, with clearly increas-ing Ag in content, the effect of Ag on AO erosion in Cu-based BMGs can be measured. Comparing sample A2 with A3, a similar change can be found that they had similar changes in surface color and microstructure. The result demonstrates that samples A2 and A3 have similar oxida-tion behavior in AO environmental which suggested the Ag containing in sample A2 has no obvious effect on the AO erosion resistance. But when Ag content reaches 15% (sam-ple A1), the AO oxidation is quite different. For example, the color has been transformed into a pale yellow and its microstructure has become more complex which composed of particles, clusters, voids and cracks, etc. It is suggested that in Cu-based metallic glasses the surface oxidation de-pends on the Ag content. But if it is below 8%, Ag may not significantly affect the AO erosion resistance. From samples A1–A4 of Zr-Cu alloy system with a significant Cu compo-sition variation compare to other elements, the role of Cu can be roughly estimated. When the Cu content is below ~13% (such as sample A4), no change in exposed surface color or microstructure has been noticed. The result sug-gests that sample A4 possesses a better ability in anti-  erosion/oxidation of AO.  It is known that, in many BMGs, Cu or Ag is often added in the alloys for improving the GFA or turning the mechan-ical properties. If the BMGs were designed to use in AO environment, it is then necessary to avoid adding such ele-ments, or at least, to control the amount of them in the total composition. Interestingly in this case, it seems that adding these two elements with some specific amounts do not change the oxidation behavior of these alloys in AO. But the concentration of Ag and Cu must be limited to certain amount which needs lots of experiments to explore. Our work have provided some simple clues that, if Ag is added no more than about 8% or Cu is no more than about 13%, the AO effect on the BMGs is acceptable. Of course, a more detailed study is needed to be carried out in the future to confirm the maximum amount of Cu or Ag to be added without obvious change of their AO resistance. 4  Conclusion In the present work, a group of Cu-based BMGs were ex-posed in the AO environment simulated on the ground. It was found that these Cu-based BMGs possess good property of anti-erosion/oxidation of AO, despite that in the three Cu-based BMGs a clear oxidation trace could be identified, 4804 Wang X, et al.   Chin Sci Bull   December (2012) Vol.57 No.36 which was confirmed to be strongly related to the chemical composition of the BMGs. 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Chemical composition dependence of atomic oxygen erosion resistance in Cu-rich bulk metallic glasses

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Chemical composition dependence of atomic oxygen erosion resistance in Cu-rich bulk metallic glasses

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