This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Journal of Alloys and Compounds 483 (2009) Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: Amorphous and nanocrystalline sputtered Mg Cu thin films H.S. Chou a, J.C. Huang a,, Y.H. Lai a, L.W. Chang a, X.H. Du a,b, J.P. Chu c, T.G. Nieh d a Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC b Department of Materials Engineering, Shenyang Institute of Aeronautical Engineering, Shenyang , PR China c Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC d Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN , USA article info abstract Article history: Received 30 August 2007 Received in revised form 17 July 2008 Accepted 27 July 2008 Available online 12 December 2008 Keywords: Thin films Amorphous materials Scanning and transmission electron microscopy Intermetallics Binary Mg Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg 1 x Cu x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg 2 Cu or MgCu 2 nanocrystalline phases are induced in the Mg-rich or Curich thin films, respectively. Due to the presence of fine nanocrystalline Mg 2 Cu or MgCu 2 particles in the Mg Cu amorphous matrix, the as-sputtered thin films show satisfactory Young s modulus 100 GPa and hardness 4GPa Elsevier B.V. All rights reserved. 1. Introduction Since 1960s, amorphous alloys have attracted more and more attentions duo to their specific characteristics, such as high elastic energy, high hardness, good hydrogen storage, good wear and corrosion resistance [1]. For example, the Fe and Co-based amorphous alloys show special magnetic properties and the Zr and Pd bases metallic glasses exhibit attractive mechanical properties [1]. Recently, the Mg or Cu-based metallic glasses are also widely studied due to their lighter weight or special applications in electronic and bio-medical areas. The major Mg amorphous system is the ternary Mg Cu Y alloy, where Cu and Y can be fully or partially replaced by Ni and Gd, respectively [2 4]. In contrast, the binary Mg Cu system has rarely been reported until now. In 1990, the glass forming range of the Mg-rich binary Mg Cu amorphous alloy was determined to be within Mg 92 Cu 8 to Mg 72 Cu 28 and Mg 82 Cu 18 to Mg 89 Cu 11 via the theoretical thermodynamic calculation and melt spinning experiments, respectively [5 7]. As for the Cu-rich Mg Cu amorphous alloy, there has been hardly any report till now due to the need of a very high cooling rate via liquid quenching. In this study, the co-sputtering process provides another alternate to overcome the difficulty in liquid quenching, arisen from the large difference of the melting points in Mg (923 K) and Cu (1338 K). Corresponding author. address: jacobc@mail.nsysu.edu.tw (J.C. Huang). The vapor solid quenching during sputtering enables the amorphous or nanocrystalline phases to form. The thin film amorphous alloys are generally called as the thin film metallic glasses (TFMGs), which can be prepared by many physical vapor deposition methods such as sputtering or evaporation. TFMGs might have tremendous application potentials in the areas of surface coating and microelectro-mechanical systems (MEMS) [8 11]. Co-sputtering by two or three guns appears to be a promising process in studying the TFMGs with a wide variation of composition and microstructure. 2. Experimental The Mg Cu metallic thin films of different compositions are deposited on 8mm 8 mm cleaned P-type (1 0 0) Si wafers via co-sputtering. The purities of the Mg and Cu targets are both 99.99%. The compositions of the as-deposited Mg Cu films can be adjusted utilizing different powers for the Mg and Cu targets, as shown in Table 1. The resulting Mg Cu metallic thin films are characterized by the Siemens D5000 X-ray diffractometry (XRD) with the Cu K radiation, operated at 40 kv and 30 ma and equipped with a 0.02 mm graphite monochromator. The thermal properties are traced by the PerkinElmer Pyris Diamond differential scanning calorimetry (DSC). The thin films specimens were first heated to 343 K in DSC and then hold for 10 min to remove possible water vapor in the chamber and then heated to 523 K at a heating rate of 5 K/min. Ar atmosphere was filled in the chamber to prevent from side reactions during the entire process. In order to examine the structural transition, the Mg Cu films are isothermally annealed at 423 K at an atmosphere below 10 3 Torr. The plane-view microstructures of the Mg Cu metallic thin films, prepared by ion milling, are characterized via the JEOL 3010 transmission electron microscopy (TEM). The nano-mechanical properties of the Mg Cu thin films of 4 m in thickness are tested by the MTS nanoindenter using the continuous stiffness measurement (CSM) mode. The films are indented to a depth of 400 nm at a strain rate of s /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.jallcom

3 342 H.S. Chou et al. / Journal of Alloys and Compounds 483 (2009) Table 1 The sputtering conditions of the Mg Cu thin films; RF and DC are referred to the radio frequency and direct current. Sample Deposition condition Mg 17.7Cu 82.3 Mg RF 100 W Cu DC 150 W Mg 23.5Cu 76.5 Mg RF 100 W Cu DC 100 W Mg 40.4Cu 59.6 Mg RF 100 W Cu DC 50 W Mg 61.9Cu 38.1 Mg RF 100 W Cu DC 25 W Fig. 1. XRD patterns of the Mg Cu co-sputtered thin films. 3. Results and discussion As seen in Fig. 1, the XRD patterns for some of the representative Mg Cu co-sputtered thin films, around 500 nm in thickness, basically exhibit one major diffuse hump, suggesting the mainly amorphous nature with possible some nanocrystalline particles. The hump central position appears to shift to the left as Cu content decreases. By closer examination of the XRD pattern in Fig. 1 for the Mg 61.9 Cu 38.1 specimen, there appears a sharper peak at the center of the diffuse hump, indexed as the (0 8 0) peak of the Mg 2 Cu nanocrystalline phase, suggesting that the Mg 2 Cu nanoparticles are present in the Mg Cu amorphous matrix. Considering the packing of Mg and Cu, the Mg atoms (with an atomic radius of nm) are much larger than the Cu atoms (with an atomic radius of nm), resulting in a large difference in atomic size of 26%. Hence, the Cu atoms would play a major role to fill the free volume among Mg atoms. Also, in this study, it is often found that the Mg-rich thin films tend to be oxidized slightly upon exposure in air (judging from the change in film color), whereas the Cu-rich counterparts do not have such a problem. The DSC heating scan of the Mg 17.7 Cu 82.3 thin films with a heating rate of 5 K/min reveals the glass transition temperature (T g ) and the crystallization temperature (T x ) of 428 K and 460 K, respectively. The same DSC scan for the Mg 23.5 Cu 76.5 thin films yields similar T g and T x at 425 K and 460 K. Note that the Mg-rich thin films are difficult to be split from the Si substrate, thereby no DSC information was obtained. Compared with the thermal properties of the ternary Mg 65 Cu 25 Y 10 and Mg 65 Cu 25 Gd 10 bulk metallic glasses, the supercooled temperature range, T x (=T x T g ), of the Mg 17.7 Cu 82.3 and Mg 23.5 Cu 76.5 binary thin films (about 30 K) is apparently narrower than those of the ternary Mg-based alloys, as compared in Table 2. Generally speaking, the addition of the third element usually increases the thermal properties and crystallization resistance [12]. In the Mg Cu binary thin film system, due to the lack of the third element with a large atomic size, such as Y, Gd or Nd, the Mg and Cu atoms can easily diffuse with each other and form the crystalline Mg 2 Cu or MgCu 2 nanoparticles. This will lower the crystallization temperature T x and the supercooled temperature range T x. It is of concern to discern which kind of the nanocrystalline compound would form in the binary Mg Cu thin films after thermal annealing. It turns out to be a function of the composition location in the Mg Cu binary diagram. After the isothermal annealing at 423 K for 1 h and above, the major compounds in the Mg-rich and Cu-rich specimens, which belong to the Mg 2 Cu MgCu 2 eutectic region and the Cu MgCu 2 eutectic region, are Mg 2 Cu and MgCu 2, respectively. For example, Mg 2 Cu is induced in Mg 61.9 Cu 38.1 after annealing at 423 K for 1 h and above, as shown in Fig. 2(a), and the MgCu 2 is gradually and slowly formed in Mg 17.7 Cu 82.3 after 3 h annealing, as evident from the gradual transformation of the smooth XRD hump into a shaper peak in Fig. 2(b). It has been reported that the glass forming ability of the Mg-rich thin films is superior to that of the Cu-rich films [6]; but from the current study the thermal stability or the resistance against crystallization of the Mg-rich semi-amorphous thin films is inferior to that of the Cu-rich thin films. The TEM bright-field images of the as-deposited Mg 17.7 Cu 82.3 and Mg 23.5 Cu 76.5 thin films, shown in Fig. 3(a) and (c), both reveal some MgCu 2 nanoparticles (about nm) in the amorphous matrix, as evident from the selected area diffraction patterns, shown in Fig. 3(b) and (d). The nanocrystalline MgCu 2 (about nm) particles can be more readily seen from the higher magnification bright-field image of Mg 23.5 Cu 76.5, as shown in Fig. 4(a). The high-resolution TEM images of the as-deposited Mg 23.5 Cu 76.5 and Mg 17.7 Cu 82.3 thin films also reveals finer MgCu 2 nanoparticles (Fig. 4(b) and (c)), around only 5 nm in size, embedded in the Mg Cu amorphous matrix. The TEM and XRD evidences demonstrate the formation of the Mg Cu TFMGs with the nanocrystalline particles. In order to extract the intrinsic mechanical properties of the films reducing the substrate effect, thinker co-sputtered Mg Cu films to a film thickness of 4 m are prepared. The indenta- Table 2 Thermal properties of the binary Mg Cu thin films and ternary Mg-based amorphous alloys. Alloys Sample type Structure T g (K) T x (K) T x (K) Mg 23.5Cu 76.5 Thin film Partially amorphous Mg 17.7Cu 82.3 Thin film Partially amorphous Mg 65Cu 25Y 10 [16] Bulk Fully amorphous Mg 65Cu 25Gd 10 [16] Bulk Fully amorphous

4 H.S. Chou et al. / Journal of Alloys and Compounds 483 (2009) Fig. 2. Phase transformation during post-annealing in (a) Mg-rich metallic thin films, and (b) Cu-rich metallic thin films. Fig. 3. Bright-field image and the selected area diffraction pattern of the Mg 17.7Cu 82.3 specimen in (a) and (b), and of the Mg 23.5Cu 76.5 specimen in (c) and (d). tion depth is set to be 400 nm, only 1/10 of the film thickness. Fig. 5 shows the load displacement curves of the Mg 17.7 Cu 82.3, Mg 23.5 Cu 76.5 and Mg 40.4 Cu 59.6 films, all revealing the pop-in effect. The pop-in phenomenon in the nano-indentation response curve has been observed and discussed before [13 15], and is a typical inhomogeneous deformation at a specific temperature range due to shear bands nucleation and propagation in the amorphous alloys. The pop-in frequency can be a function of test temperature and strain rate. The Young s modulus and hardness of the Mg 17.7 Cu 82.3, Mg 23.5 Cu 76.5 and Mg 40.4 Cu 59.6 films are all similar, scattering within GPa and GPa, respectively, as listed in Table 3. With the higher occurrence of the nano-crystalline MgCu 2 particles in the Mg 23.5 Cu 76.5 thin film, both the modulus and hardness are higher than the other two films. Table 3 Comparison of the mechanical properties of the current Mg Cu amorphous thin film and the typical bulk metallic glasses. Materials Sample type Young s modulus (GPa) Hardness (GPa) Mg 17.7Cu 82.3 Thin film ± ± 0.2 Mg 23.5Cu 76.5 Thin film 118 ± ± 0.2 Mg 40.4Cu 59.6 Thin film 96.8 ± ± 0.1 Zr 41.25Ti 13.75Cu 12.5Ni 10Be 22.5 [17] Bulk 96 ± ± 0.1 Zr 60Cu 20Pd 10Al 10 [18] Bulk 87 a 5.7 a Pd 40Ni 40P 20 [18] Bulk 108 a 5.5 a Mg 65Ni 20Nd 15 [18] Bulk 57 a 3.4 a Mg 60Cu 30Y 10 [19] Bulk 51.5 ± ± 0.1 Cu 60Zr 22Ti 18 [20] Bulk ± ± 0.4 a The error bars of the reference data are not available.

5 344 H.S. Chou et al. / Journal of Alloys and Compounds 483 (2009) Fig. 4. (a) High-magnification bright-field image of Mg 23.5Cu 76.5, containing the MgCu 2 nanoparticles in the Mg Cu amorphous matrix, (b) high-resolution lattice image of Mg 23.5Cu 76.5, and (c) high-resolution lattice image of Mg 17.7Cu It is found that the occurrence of the nanoparticles in the Mg 23.5Cu 76.5 film is higher than that in the Mg 17.7Cu 82.3 one. The hardness of the MgCu 2 intermetallic crystalline compound has been measured by Chen and Sun [21] using nano-indentation, revealing a hardness reading of in the Vicker s scale (or 5.3 GPa). The latter value is higher than those of the Mg 17.7 Cu 82.3, Mg 23.5 Cu 76.5 and Mg 40.4 Cu 59.6 thin film specimens listed in Table 3. It is likely a result of the softer Mg Cu amorphous matrix. Compared with the fully amorphous Zr-, Mg-, and Pd-based bulk metallic glasses, the current Cu-rich Mg Cu metallic thin films exhibit a sufficiently high Young s modulus and hardness, due to the presence of abundant MgCu 2 nanoparticles in the amorphous matrix, as compared in Table Conclusion The binary Mg 1 x Cu x (x varying from 38 at.% to 82 at.%) thin films consisting of the Mg Cu amorphous matrix and the MgCu 2 or Mg 2 Cu nanocrystals are fabricated by co-sputtering. In addition to liquid quenching, sputtering provides another alternate in preparing amorphous alloys in the form of thin film. Due to the lack of the third element with a larger atomic size such as Y or Gd, the thermal stability or the crystalline resistance of the current TFMGs is inferior to that of the ternary Mg Cu Y(Gd) amorphous alloys. The Mg Cu co-sputtered amorphous thin films with the nanocrystalline MgCu 2 or Mg 2 Cu phases exhibit satisfactory mechanical properties. The measured Young s modulus and hardness of the Mg Cu thin films is about 100 GPa and 4 GPa, respectively. Acknowledgement The authors gratefully acknowledge the sponsorship by National Science Council of Taiwan, ROC, under the Project No. NSC E References Fig. 5. Load displacement curves of the Mg Cu co-sputtered thin films. [1] A. Inoue, Mater. Sci. Eng. A (2001) 1. [2] Y.T. Cheng, T.H. Hung, J.C. Huang, J.S.C. Jang, Chi Y.A. Tsao, P.Y. Lee, Intermetallics 14 (2006) 866. [3] H.M. Chen, Y.C. Chang, T.H. Hung, X.H. Du, J.C. Huang, J.S.C. Jang, P.K. Liaw, Mater. Trans. 48 (2007) [4] L.J. Chang, G.R. Fang, J.S.C. Jang, I.S. Lee, J.C. Huang, Chi Y.A. Tsao, J.L. Jou, Mater. Trans. 48 (2007) [5] F. Sommer, G. Bucher, B. Predel, J. Phys. Colloque C-8 (1980) 41.

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