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Nanocrystallization and hydriding properties of amorphous melt-spun Mg65Cu25Nd10 alloy
Journal of Alloys and Compounds 432 (2007) 172–176
Nanocrystallization and hydriding properties of amorphous melt-spun Mg65Cu25Nd10 alloy L.J. Huang ∗ , G.Y. Liang, Z.B. Sun, Y.F. Zhou Department of Materials Physics, Science School, State Key Laboratory of Mechanical Behavior for Materials, Xi’an Jiaotong University, Xi’an 710049, China Received 11 May 2006; received in revised form 5 June 2006; accepted 6 June 2006 Available online 10 July 2006
Abstract Amorphous Mg-based alloy Mg65 Cu25 Nd10 was prepared by melt spinning. Thermal stability and phase transition in the as-quenched alloy were studied by TEM, DSC, X-ray and electron diffraction. It was found that the crystallization process of melt-spun Mg65 Cu25 Nd10 ribbons consists of three steps. The first crystallization reaction at about 180 ◦ C is connected with the formation of Mg2 Cu nanocrystalline phase with average grain size in the range 5–10 nm, followed by formation of a coarser grained ␣-Mg crystalline phase (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected and the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present with the biggest grain size about 100 nm. The hydrogenation characteristics of the as-quenched and the annealed samples (partially and fully crystallized) were compared as well. The amorphous Mg65 Cu25 Nd10 alloy shows the best initial hydrogenation rate and the maximum hydrogen capacity (3.2 wt.%) in comparison with those of partially crystalline and completely crystallized microstructural states. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Melt spinning; Amorphous alloy; Hydrogen storage
1. Introduction The search for new magnesium-based alloys is of great economic interest due to their advantages such as light weight, great abundance and high hydrogen capacity of the hydride and their promising application potential in the automotive industry. The hydrogen storage capacities in the common Mg-based hydrides are 3.6 wt.% for Mg2 NiH4 , 4.5 wt.% for Mg2 CoH5 , 5.4 wt.% for Mg2 FeH6 and 2.6 wt.% for Mg–Cu–H system (2Mg2 Cu + 3H2 = 3MgH2 + MgCu2 ). Among these alloys, Mg2 NiH4 has been widely studied and there are thousands of detailed papers on its hydrogen storage properties since Reilly reported the hydrogen storage study of Mg–Ni–H system in 1968 [1]. However, there is little work on the hydrogen storage study of Mg–Cu–H system even while they reported the Mg–Cu–H system earlier in 1967 [2]. This is mainly attributed to the facts that the hydrogen storage capacity of Mg2 Cu is only 2.6 wt.% compared to 3.6 wt.% for Mg2 Ni and
∗
Corresponding author. E-mail address: [email protected] (L.J. Huang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.026
the difficultly in preparation of Mg2 Cu compound. In the last decades, high energy ball milling [3–6] and rapid solidification [7–9] technologies were applied to produce nanocrystalline or amorphous structures, to improve their hydrogenation characteristics and meet the requirement of using at low temperature, which is a major step forward towards commercialization. It was found that magnesium and its binary compound with refined microstructures, especially amorphous structure, show an enhanced hydrogen sorption rate and the high diffusivity and solubility of hydrogen in amorphous Mg-based alloys at room temperature can be attributed to the existence of a lot of interstitial sites in the short range-order of the amorphous phase. Those interstitial sites associated with a wide energy distribution for hydrogen and make hydrogen atoms easy to be filled in. In the last years new bulk glass-forming Mg-based alloys have been developed, which are for example Mg–Cu–Y [10–12] and Mg–Ni–Y [13] systems. The Mg65 Cu25 Y10 alloy was found to show the highest glass-forming ability among these systems. In 2001, Gebert et al. [14] produced the amorphous Mg65 Cu25 Y10 ribbons by single roller melt spinning and found its good stability in aqueous electrolytes. Later in 2004, Savyak et al. [15] studied the electrochemical hydrogenation of Mg65 Cu25 Y10 metallic
L.J. Huang et al. / Journal of Alloys and Compounds 432 (2007) 172–176
glass by hot extraction and reported the best hydrogen content (3.7 wt.%) in Mg–Cu–H system so far. Our recent investigations [16,17] demonstrated the effect of microstructure, composition and phase composition on the hydrogen storage capacity as well as on the kinetics of hydrogen absorption. The best hydriding properties were found in the amorphous (Mg60 Ni25 )90 Nd10 alloy with maximum hydrogen capacity of 4.2 wt.%. Furthermore, a more beneficial effect on the glass-forming ability for Mg-based alloys of Nd than Y was detected. In this way, the present paper is devoted to an investigation of the hydrogenation characteristics of rapidly quenched Mg65 Cu25 Nd10 alloy with the same composition to that possessing the best H-storage characteristics found by Savyak et al. [15] in Mg–Cu–H system so far, but replacing Y by Nd. The thermal stability and crystallization of the as-cast alloy were also studied in order to obtain stable microstructures favorable for hydrogen storage. The hydrogenation characteristics of the as-quenched and annealed alloys (partially and fully crystallized) were compared as well. At the same time, we also prepared Mg65 Cu25 Y10 ribbons and studied its H-storage characteristics at the same conditions with Mg65 Cu25 Nd10 ribbons in order to compare their H-storage characteristics. 2. Experimental procedure Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 alloy ingots were prepared by induction melting a mixture of pure Nd(Y) metal and Mg–Cu alloy in a vacuum furnace under the protection of argon gas. Based on the low melting point and the high vapor pressure of Mg, a special melting technique, that is the repeated melting at positive pressure protection, has to be taken to prevent massive Mg evaporation and ensure composition homogeneity during master alloy ingot preparation. The amorphous ribbons were produced by a single roller melt-spinning technique (copper quenching disc with a diameter of 250 mm and surface velocity of about 39 m s−1 ) with a argon atmosphere of 400 mbar. The microstructure of the as-quenched ribbons was characterized by transmission electron microscopy (Philips CM200 operated at 200 kV) and by X-ray (using Cu K␣ radiation) and electron diffraction. The glass transition and crystallization of as-quenched ribbons were investigated by means of a differential scanning calorimeter (DSC, TA-Instruments, DSC2910). Hydrogen charging of ribbon samples with a length of 20 mm was carried out electrolytically under galvanostatic conditions in 6 mol/l KOH electrolyte containing 20 g/l LiOH at room temperature and a cathodic current densities i = 1 mA/cm2 . The hydrogen content was measured from the increase in mass after hydrogenation by a microbalance (Perkin-Elmer TGS2) with an accuracy of 0.1 g.
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Fig. 2. TEM image and electron diffraction pattern of as-quenched Mg65 Cu25 Nd10 .
only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. It is also seen that the typical amorphous peaks of Mg65 Cu25 Nd10 is more smooth and more low than that of Mg65 Cu25 Y10 , which implies that Nd has a more beneficial effect on the glass-forming ability than Y for Mg–Cu–M (M = Nd, Y) alloys. The TEM image and electron diffraction pattern of as-quenched Mg65 Cu25 Nd10 alloy are showed in Fig. 2. It is found that a uniform amorphous structure was obtained. Fig. 3 shows sections (50–350 ◦ C) of DSC scans of Mg65 Cu25 Nd10 samples recorded at 20 ◦ C/min. On one hand the DSC scans of the as-quenched glassy ribbon exhibits the characteristic wide undercooled liquid region, on the other hand during heating the sample crystallize completely, as the crystallization process consists of several steps. The first crystallization reaction at about 180 ◦ C is connected with a sharp exothermic DSC peak, followed by a smaller and wider peak (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected. In order to understand the sequence of the crystalline phase formed during continuous heating as well as the microstructure development, the crystallization process was stopped at different temperatures and then X-ray diffractograms were taken for Mg65 Cu25 Nd10 alloy, Fig. 4.
3. Experimental results Fig. 1 presents the X-ray diffraction patterns of the as-quenched Mg65 Cu25 Nd10 , and Mg65 Cu25 Y10 . It is seen that both melt-spun alloys show
Fig. 1. X-ray diffraction patterns (Cu K␣) of as-quenched alloys.
Fig. 3. DSC curves (at 20 ◦ C/min heating rate) of as-quenched Mg65 Cu25 Nd10 alloy.
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Fig. 4. X-ray diffraction of the annealed Mg65 Cu25 Nd10 melt-spun samples with 20 ◦ C/min at 125 ◦ C (a), 170 ◦ C (b), 210 ◦ C (c), 250 ◦ C (d), and 335 ◦ C (e).
Fig. 6. Kinetics of hydrogenation for Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 alloy.
Fig. 5 shows the rate of hydrogen absorption for the Mg65 Cu25 Nd10 alloy at three different microstructural states: amorphous, partially crystalline (annealed at 210 ◦ C) and completely crystallized (annealed at 335 ◦ C). As already shown in our recent studies [16] the amorphous alloy exhibits very fast initial hydrogenation kinetics due to the rapid diffusion of hydrogen and has the maximum hydrogen capacity of 3.2 wt.%. The fully crystallized Mg65 Cu25 Nd10 alloy shows the lowest initial hydrogenation rate as compared to the other two microstructural states. The partially crystalline alloy shows a lower initial hydrogenation rate at first as compared to the amorphous alloy, but at longer hydriding times the amount of hydrogen absorbed in the amorphous and partially crystalline alloys is practically the same. This result also support the Spassov’s [18] reports in his investigations of Mg-based alloys. Fig. 6 shows the rate of hydrogen absorption for the Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 melt-spun alloys at the same conditions. It exhibits that the Mg65 Cu25 Nd10 alloy shows a higher initial hydrogenation rate and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy. Fig. 7. TEM image and corresponding selected area electron diffraction pattern for Mg65 Cu25 Nd10 alloy heat treated to 210 ◦ C.
Fig. 5. Kinetics of hydrogenation for amorphous, partially and fully crystallized Mg65 Cu25 Nd10 alloy.
Fig. 8. TEM image and electron diffraction pattern of Mg65 Cu25 Nd10 alloy heat treated to 335 ◦ C.
L.J. Huang et al. / Journal of Alloys and Compounds 432 (2007) 172–176
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Tx = Tx − Tg = 88 ◦ C, resulting from a glass-transition temperature Tg = 85 ◦ C and a crystallization temperature Tx = 173 ◦ C. According to the results of Savyak et al. [15] reported, the undercooled liquid region (Tx ) of as-cast Mg65 Cu25 Y10 alloy is only 55 K. Therefore, the GFA of the amorphous Mg65 Cu25 Nd10 alloy is stronger than that of Mg65 Cu25 Y10 alloy. Considering the above discussed, the Mg65 Cu25 Nd10 alloy shows a higher initial hydrogenation rate and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy (Fig. 6). 5. Conclusions Fig. 9. XRD patterns of amorphous Mg65 Cu25 Nd10 alloy charged with 3.2 wt.% H.
4. Discussion Here we discuss the reasons for the different rates of hydrogen absorption for the Mg65 Cu25 Nd10 alloy at three different microstructural states. Heating until the end of the first sharp DSC peak for the Mg65 Cu25 Nd10 alloy leads to formation of Mg2 Cu nanocrystalline phase with average grain size in the range 5–10 nm (Figs. 3, 4 and 7). Heating to temperatures above the second exothermic peak causes formation of a coarser grained ␣-Mg crystalline phase (Figs. 3 and 4). At higher temperatures of annealing (above the third small DSC peak at about 320 ◦ C) the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present with the biggest grain size about 100 nm (Figs. 3, 4 and 8). According to the results of Orimo and Fujii [19] reported and our previous studied [16], ␣-Mg and Cu5 Nd cannot reacted with H at room temperature, but Mg2 Cu can do. Therefore, the formation of ␣-Mg and Cu5 Nd phases leads to the decrease amount of hydrogen absorbed in the partially crystalline and completely crystallized alloys. Fig. 9 shows the XRD patterns of amorphous Mg65 Cu25 Nd10 alloy charged with 3.2 wt.% H. It exhibits that amorphous alloy sample that charged hydrogen content to 3.2 wt.% results in the formation of a mainly nanocrystals structures besides a few residual amorphous phase. The main nanocrystals products during hydrogenation processes are MgH2 , Cu2 Mg and Nd2 H5 , which formed by direct metal hydride reaction. Like the amorphous Mg65 Cu25 Y10 alloy investigated by Savyak et al. [15], amorphous Mg65 Cu25 Nd10 alloy also transforms during room temperature hydrogen charging into a very fine nanocrystalline state. Our recent investigations [16,17] demonstrated the relationship between the glass-forming ability (GFA) of Mg-based alloys and the rate of hydrogen absorption for Mg–Ni–Nd alloys. It is that the stronger of the GFA means the better kinetics of hydrogenation. This fact was also observed for the recently studied Mg65 Cu25 Y10 alloy [15]. Inoue [20] found that the GFA of the amorphous alloys can be denoted with their super-cooled liquid region Tx (Tx = Tx − Tg , Tx is the crystallization-onset temperature, Tg is the glass-transition temperature). The DSC scans (Fig. 3) of the as-quenched Mg65 Cu25 Nd10 glassy ribbon exhibits the characteristic wide undercooled liquid region,
The nanocrystallization and hydriding properties of the meltspun Mg65 Cu25 Nd10 amorphous alloys were examined. The results obtained are summarized as follows: 1. The crystallization process of melt-spun Mg65 Cu25 Nd10 amorphous ribbons consists of three steps. The first crystallization reaction at about 180 ◦ C is connected with the formation of Mg2 Cu nanocrystalline phase, followed by formation of a coarser grained ␣-Mg crystalline phase (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected and the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present. 2. The amorphous Mg65 Cu25 Nd10 alloy shows the best initial hydrogenation rate and the maximum hydrogen capacity (3.2 wt.%) in comparison with those of partially crystalline and completely crystallized microstructural states. 3. Amorphous Mg65 Cu25 Nd10 alloy sample that charged hydrogen content to 3.2 wt.% results in the formation of a mainly nanocrystals structure besides a few residual amorphous phases. The products of hydrogenation processes, MgH2 , Cu2 Mg and Nd2 H5 were detected. 4. The Mg65 Cu25 Nd10 alloy shows a stronger GFA and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy. Acknowledgements This study was supported by the Doctoral Foundation of Xi’an Jiaotong University (Grant no. DFXJTU-200516) and National Natural Science Foundation of China (Grant no. 50371066). References [1] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 7 (1968) 2254. [2] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 6 (1967) 2220. [3] H.Y. Lee, N.H. Goo, W.T. Jeong, K.S. Lee, J. Alloys Compd. 313 (2000) 258. [4] T.-W. Hong, Y.-J. Kim, J. Alloys Compd. 330–332 (2002) 584. [5] A.A. Mohamad, N.S. Mohamed, Y. Alias, A.K. Arof, J. Power Sources 115 (2003) 161. [6] M. Abdellaoui, S. Mokbli, F. Cuevas, M. Latroche, A. Percheron-Gu´egan, H. Zarrouk, J. Alloys Compd. 356/357 (2003) 557. [7] T. Spassov, U. K¨oster, J. Alloys Compd. 279 (1998) 279. [8] L.-B. Wang, Y.-H. Tang, Y.-J. Wang, Q.-D. Li, H.-N. Song, H.-B. Yang, J. Alloys Compd. 336 (2002) 297.
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[9] S. Yamaura, H.Y. Kim, H.M. Kimura, A. Inoue, Y. Arata, J. Alloys Compd. 347 (2002) 239. [10] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mater. Trans. JIM 33 (1992) 937. [11] S.S. Wu, T.S. Chin, K.C. Su, Int. J. Rapid Solid. 8 (1993) 65. [12] A. Inoue, T. Masumoto, Mater. Sci. Eng. A 173 (1993) 1. [13] C.H. Kam, Y. Li, S.C. Ng, A. Wee, J.S. Pan, H. Jones, J. Mater. Res. 14 (1999) 1638. [14] A. Gebert, U. Wolff, A. John, J. Eckert, L. Schultz, Mater. Sci. Eng. A 299 (2001) 125.
[15] M. Savyak, S. Hirnyj, H.-D. Bauer, M. Uhlemann, J. Eckert, L. Schultz, A. Gebert, J. Alloys Compd. 364 (2004) 229. [16] L.J. Huang, G.Y. Liang, Z.B. Sun, J. Alloys Compd. 421 (2006) 279– 282. [17] L.J. Huang, G.Y. Liang, Z.B. Sun, D.C. Wu, J. Power Sources. 160 (2006) 684–687. [18] T. Spassov, V. Rangelova, N. Neykov, J. Alloys Compd. 334 (2002) 219. [19] S. Orimo, H. Fujii, Appl. Phys. A 72 (2001) 167. [20] A. Inoue, Acta Mater. 48 (2000) 278.
Nanocrystallization and hydriding properties of amorphous melt-spun Mg65Cu25Nd10 alloy L.J. Huang ∗ , G.Y. Liang, Z.B. Sun, Y.F. Zhou Department of Materials Physics, Science School, State Key Laboratory of Mechanical Behavior for Materials, Xi’an Jiaotong University, Xi’an 710049, China Received 11 May 2006; received in revised form 5 June 2006; accepted 6 June 2006 Available online 10 July 2006
Abstract Amorphous Mg-based alloy Mg65 Cu25 Nd10 was prepared by melt spinning. Thermal stability and phase transition in the as-quenched alloy were studied by TEM, DSC, X-ray and electron diffraction. It was found that the crystallization process of melt-spun Mg65 Cu25 Nd10 ribbons consists of three steps. The first crystallization reaction at about 180 ◦ C is connected with the formation of Mg2 Cu nanocrystalline phase with average grain size in the range 5–10 nm, followed by formation of a coarser grained ␣-Mg crystalline phase (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected and the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present with the biggest grain size about 100 nm. The hydrogenation characteristics of the as-quenched and the annealed samples (partially and fully crystallized) were compared as well. The amorphous Mg65 Cu25 Nd10 alloy shows the best initial hydrogenation rate and the maximum hydrogen capacity (3.2 wt.%) in comparison with those of partially crystalline and completely crystallized microstructural states. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Melt spinning; Amorphous alloy; Hydrogen storage
1. Introduction The search for new magnesium-based alloys is of great economic interest due to their advantages such as light weight, great abundance and high hydrogen capacity of the hydride and their promising application potential in the automotive industry. The hydrogen storage capacities in the common Mg-based hydrides are 3.6 wt.% for Mg2 NiH4 , 4.5 wt.% for Mg2 CoH5 , 5.4 wt.% for Mg2 FeH6 and 2.6 wt.% for Mg–Cu–H system (2Mg2 Cu + 3H2 = 3MgH2 + MgCu2 ). Among these alloys, Mg2 NiH4 has been widely studied and there are thousands of detailed papers on its hydrogen storage properties since Reilly reported the hydrogen storage study of Mg–Ni–H system in 1968 [1]. However, there is little work on the hydrogen storage study of Mg–Cu–H system even while they reported the Mg–Cu–H system earlier in 1967 [2]. This is mainly attributed to the facts that the hydrogen storage capacity of Mg2 Cu is only 2.6 wt.% compared to 3.6 wt.% for Mg2 Ni and
∗
Corresponding author. E-mail address: [email protected] (L.J. Huang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.026
the difficultly in preparation of Mg2 Cu compound. In the last decades, high energy ball milling [3–6] and rapid solidification [7–9] technologies were applied to produce nanocrystalline or amorphous structures, to improve their hydrogenation characteristics and meet the requirement of using at low temperature, which is a major step forward towards commercialization. It was found that magnesium and its binary compound with refined microstructures, especially amorphous structure, show an enhanced hydrogen sorption rate and the high diffusivity and solubility of hydrogen in amorphous Mg-based alloys at room temperature can be attributed to the existence of a lot of interstitial sites in the short range-order of the amorphous phase. Those interstitial sites associated with a wide energy distribution for hydrogen and make hydrogen atoms easy to be filled in. In the last years new bulk glass-forming Mg-based alloys have been developed, which are for example Mg–Cu–Y [10–12] and Mg–Ni–Y [13] systems. The Mg65 Cu25 Y10 alloy was found to show the highest glass-forming ability among these systems. In 2001, Gebert et al. [14] produced the amorphous Mg65 Cu25 Y10 ribbons by single roller melt spinning and found its good stability in aqueous electrolytes. Later in 2004, Savyak et al. [15] studied the electrochemical hydrogenation of Mg65 Cu25 Y10 metallic
L.J. Huang et al. / Journal of Alloys and Compounds 432 (2007) 172–176
glass by hot extraction and reported the best hydrogen content (3.7 wt.%) in Mg–Cu–H system so far. Our recent investigations [16,17] demonstrated the effect of microstructure, composition and phase composition on the hydrogen storage capacity as well as on the kinetics of hydrogen absorption. The best hydriding properties were found in the amorphous (Mg60 Ni25 )90 Nd10 alloy with maximum hydrogen capacity of 4.2 wt.%. Furthermore, a more beneficial effect on the glass-forming ability for Mg-based alloys of Nd than Y was detected. In this way, the present paper is devoted to an investigation of the hydrogenation characteristics of rapidly quenched Mg65 Cu25 Nd10 alloy with the same composition to that possessing the best H-storage characteristics found by Savyak et al. [15] in Mg–Cu–H system so far, but replacing Y by Nd. The thermal stability and crystallization of the as-cast alloy were also studied in order to obtain stable microstructures favorable for hydrogen storage. The hydrogenation characteristics of the as-quenched and annealed alloys (partially and fully crystallized) were compared as well. At the same time, we also prepared Mg65 Cu25 Y10 ribbons and studied its H-storage characteristics at the same conditions with Mg65 Cu25 Nd10 ribbons in order to compare their H-storage characteristics. 2. Experimental procedure Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 alloy ingots were prepared by induction melting a mixture of pure Nd(Y) metal and Mg–Cu alloy in a vacuum furnace under the protection of argon gas. Based on the low melting point and the high vapor pressure of Mg, a special melting technique, that is the repeated melting at positive pressure protection, has to be taken to prevent massive Mg evaporation and ensure composition homogeneity during master alloy ingot preparation. The amorphous ribbons were produced by a single roller melt-spinning technique (copper quenching disc with a diameter of 250 mm and surface velocity of about 39 m s−1 ) with a argon atmosphere of 400 mbar. The microstructure of the as-quenched ribbons was characterized by transmission electron microscopy (Philips CM200 operated at 200 kV) and by X-ray (using Cu K␣ radiation) and electron diffraction. The glass transition and crystallization of as-quenched ribbons were investigated by means of a differential scanning calorimeter (DSC, TA-Instruments, DSC2910). Hydrogen charging of ribbon samples with a length of 20 mm was carried out electrolytically under galvanostatic conditions in 6 mol/l KOH electrolyte containing 20 g/l LiOH at room temperature and a cathodic current densities i = 1 mA/cm2 . The hydrogen content was measured from the increase in mass after hydrogenation by a microbalance (Perkin-Elmer TGS2) with an accuracy of 0.1 g.
173
Fig. 2. TEM image and electron diffraction pattern of as-quenched Mg65 Cu25 Nd10 .
only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. It is also seen that the typical amorphous peaks of Mg65 Cu25 Nd10 is more smooth and more low than that of Mg65 Cu25 Y10 , which implies that Nd has a more beneficial effect on the glass-forming ability than Y for Mg–Cu–M (M = Nd, Y) alloys. The TEM image and electron diffraction pattern of as-quenched Mg65 Cu25 Nd10 alloy are showed in Fig. 2. It is found that a uniform amorphous structure was obtained. Fig. 3 shows sections (50–350 ◦ C) of DSC scans of Mg65 Cu25 Nd10 samples recorded at 20 ◦ C/min. On one hand the DSC scans of the as-quenched glassy ribbon exhibits the characteristic wide undercooled liquid region, on the other hand during heating the sample crystallize completely, as the crystallization process consists of several steps. The first crystallization reaction at about 180 ◦ C is connected with a sharp exothermic DSC peak, followed by a smaller and wider peak (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected. In order to understand the sequence of the crystalline phase formed during continuous heating as well as the microstructure development, the crystallization process was stopped at different temperatures and then X-ray diffractograms were taken for Mg65 Cu25 Nd10 alloy, Fig. 4.
3. Experimental results Fig. 1 presents the X-ray diffraction patterns of the as-quenched Mg65 Cu25 Nd10 , and Mg65 Cu25 Y10 . It is seen that both melt-spun alloys show
Fig. 1. X-ray diffraction patterns (Cu K␣) of as-quenched alloys.
Fig. 3. DSC curves (at 20 ◦ C/min heating rate) of as-quenched Mg65 Cu25 Nd10 alloy.
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Fig. 4. X-ray diffraction of the annealed Mg65 Cu25 Nd10 melt-spun samples with 20 ◦ C/min at 125 ◦ C (a), 170 ◦ C (b), 210 ◦ C (c), 250 ◦ C (d), and 335 ◦ C (e).
Fig. 6. Kinetics of hydrogenation for Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 alloy.
Fig. 5 shows the rate of hydrogen absorption for the Mg65 Cu25 Nd10 alloy at three different microstructural states: amorphous, partially crystalline (annealed at 210 ◦ C) and completely crystallized (annealed at 335 ◦ C). As already shown in our recent studies [16] the amorphous alloy exhibits very fast initial hydrogenation kinetics due to the rapid diffusion of hydrogen and has the maximum hydrogen capacity of 3.2 wt.%. The fully crystallized Mg65 Cu25 Nd10 alloy shows the lowest initial hydrogenation rate as compared to the other two microstructural states. The partially crystalline alloy shows a lower initial hydrogenation rate at first as compared to the amorphous alloy, but at longer hydriding times the amount of hydrogen absorbed in the amorphous and partially crystalline alloys is practically the same. This result also support the Spassov’s [18] reports in his investigations of Mg-based alloys. Fig. 6 shows the rate of hydrogen absorption for the Mg65 Cu25 Nd10 and Mg65 Cu25 Y10 melt-spun alloys at the same conditions. It exhibits that the Mg65 Cu25 Nd10 alloy shows a higher initial hydrogenation rate and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy. Fig. 7. TEM image and corresponding selected area electron diffraction pattern for Mg65 Cu25 Nd10 alloy heat treated to 210 ◦ C.
Fig. 5. Kinetics of hydrogenation for amorphous, partially and fully crystallized Mg65 Cu25 Nd10 alloy.
Fig. 8. TEM image and electron diffraction pattern of Mg65 Cu25 Nd10 alloy heat treated to 335 ◦ C.
L.J. Huang et al. / Journal of Alloys and Compounds 432 (2007) 172–176
175
Tx = Tx − Tg = 88 ◦ C, resulting from a glass-transition temperature Tg = 85 ◦ C and a crystallization temperature Tx = 173 ◦ C. According to the results of Savyak et al. [15] reported, the undercooled liquid region (Tx ) of as-cast Mg65 Cu25 Y10 alloy is only 55 K. Therefore, the GFA of the amorphous Mg65 Cu25 Nd10 alloy is stronger than that of Mg65 Cu25 Y10 alloy. Considering the above discussed, the Mg65 Cu25 Nd10 alloy shows a higher initial hydrogenation rate and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy (Fig. 6). 5. Conclusions Fig. 9. XRD patterns of amorphous Mg65 Cu25 Nd10 alloy charged with 3.2 wt.% H.
4. Discussion Here we discuss the reasons for the different rates of hydrogen absorption for the Mg65 Cu25 Nd10 alloy at three different microstructural states. Heating until the end of the first sharp DSC peak for the Mg65 Cu25 Nd10 alloy leads to formation of Mg2 Cu nanocrystalline phase with average grain size in the range 5–10 nm (Figs. 3, 4 and 7). Heating to temperatures above the second exothermic peak causes formation of a coarser grained ␣-Mg crystalline phase (Figs. 3 and 4). At higher temperatures of annealing (above the third small DSC peak at about 320 ◦ C) the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present with the biggest grain size about 100 nm (Figs. 3, 4 and 8). According to the results of Orimo and Fujii [19] reported and our previous studied [16], ␣-Mg and Cu5 Nd cannot reacted with H at room temperature, but Mg2 Cu can do. Therefore, the formation of ␣-Mg and Cu5 Nd phases leads to the decrease amount of hydrogen absorbed in the partially crystalline and completely crystallized alloys. Fig. 9 shows the XRD patterns of amorphous Mg65 Cu25 Nd10 alloy charged with 3.2 wt.% H. It exhibits that amorphous alloy sample that charged hydrogen content to 3.2 wt.% results in the formation of a mainly nanocrystals structures besides a few residual amorphous phase. The main nanocrystals products during hydrogenation processes are MgH2 , Cu2 Mg and Nd2 H5 , which formed by direct metal hydride reaction. Like the amorphous Mg65 Cu25 Y10 alloy investigated by Savyak et al. [15], amorphous Mg65 Cu25 Nd10 alloy also transforms during room temperature hydrogen charging into a very fine nanocrystalline state. Our recent investigations [16,17] demonstrated the relationship between the glass-forming ability (GFA) of Mg-based alloys and the rate of hydrogen absorption for Mg–Ni–Nd alloys. It is that the stronger of the GFA means the better kinetics of hydrogenation. This fact was also observed for the recently studied Mg65 Cu25 Y10 alloy [15]. Inoue [20] found that the GFA of the amorphous alloys can be denoted with their super-cooled liquid region Tx (Tx = Tx − Tg , Tx is the crystallization-onset temperature, Tg is the glass-transition temperature). The DSC scans (Fig. 3) of the as-quenched Mg65 Cu25 Nd10 glassy ribbon exhibits the characteristic wide undercooled liquid region,
The nanocrystallization and hydriding properties of the meltspun Mg65 Cu25 Nd10 amorphous alloys were examined. The results obtained are summarized as follows: 1. The crystallization process of melt-spun Mg65 Cu25 Nd10 amorphous ribbons consists of three steps. The first crystallization reaction at about 180 ◦ C is connected with the formation of Mg2 Cu nanocrystalline phase, followed by formation of a coarser grained ␣-Mg crystalline phase (210–225 ◦ C) corresponding to a second crystallization reaction. At higher temperatures a third exothermic effect (at about 320 ◦ C) can be detected and the stable Mg2 Cu, ␣-Mg and Cu5 Nd phases are present. 2. The amorphous Mg65 Cu25 Nd10 alloy shows the best initial hydrogenation rate and the maximum hydrogen capacity (3.2 wt.%) in comparison with those of partially crystalline and completely crystallized microstructural states. 3. Amorphous Mg65 Cu25 Nd10 alloy sample that charged hydrogen content to 3.2 wt.% results in the formation of a mainly nanocrystals structure besides a few residual amorphous phases. The products of hydrogenation processes, MgH2 , Cu2 Mg and Nd2 H5 were detected. 4. The Mg65 Cu25 Nd10 alloy shows a stronger GFA and a better hydrogen capacity than that of Mg65 Cu25 Y10 alloy. Acknowledgements This study was supported by the Doctoral Foundation of Xi’an Jiaotong University (Grant no. DFXJTU-200516) and National Natural Science Foundation of China (Grant no. 50371066). References [1] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 7 (1968) 2254. [2] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 6 (1967) 2220. [3] H.Y. Lee, N.H. Goo, W.T. Jeong, K.S. Lee, J. Alloys Compd. 313 (2000) 258. [4] T.-W. Hong, Y.-J. Kim, J. Alloys Compd. 330–332 (2002) 584. [5] A.A. Mohamad, N.S. Mohamed, Y. Alias, A.K. Arof, J. Power Sources 115 (2003) 161. [6] M. Abdellaoui, S. Mokbli, F. Cuevas, M. Latroche, A. Percheron-Gu´egan, H. Zarrouk, J. Alloys Compd. 356/357 (2003) 557. [7] T. Spassov, U. K¨oster, J. Alloys Compd. 279 (1998) 279. [8] L.-B. Wang, Y.-H. Tang, Y.-J. Wang, Q.-D. Li, H.-N. Song, H.-B. Yang, J. Alloys Compd. 336 (2002) 297.
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