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Hydrogenation of (Zr69.5Al7.5Cu12Ni11)100 − xTix quasicrystalline alloys and its effect on their structural and microhardness behavior
Journal of Non-Crystalline Solids 380 (2013) 11–16
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Hydrogenation of (Zr69.5Al7.5Cu12Ni11)100 − xTix quasicrystalline alloys and its effect on their structural and microhardness behavior Devinder Singh a,b,⁎, Rohit R. Shahi a, T.P. Yadav a, R.K. Mandal c, R.S. Tiwari a, O.N. Srivastava a a b c
Hydrogen Energy Centre and Unit of Nano-Science and Technology, Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Physics, Panjab University, Chandigarh 160014, India Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 13 August 2013 Available online xxxx Keywords: Hydrogen storage; Nanoquasicrystal; Melt-spinning; Microstructure; Microhardness
a b s t r a c t The present study deals with the microstructural changes with respect to the addition of Ti and their correlation with hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystalline alloys. The grain size of quasicrystals decreases with addition of Ti. It has been found that the alloy with x = 0 absorbed 1.20 wt. %, whereas the alloys with x = 4 and 12 absorbed 1.38 wt. % and 1.56 wt. % of hydrogen respectively. Hydrogenation was found to exhibit a significant effect on the structure/microstructure and microhardness behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix quasicrystalline alloys. Variation in the microhardness behavior has been discussed based on a structure–property correlation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Storage is a crucial issue for bringing in hydrogen, a renewable, inexhaustive, clean and climate friendly fuel. The storage capacity of hydrogen in metals and alloys is determined by chemical interactions between the metal and hydrogen atoms, as well as by the type, number and size of the potential interstitials sites for hydrogen. The most common materials for hydrogen storage are metals and alloys that are based on transition metals. In most of these materials, hydrogen tends to occupy tetrahedral interstitials sites. Therefore, the combination of local tetrahedral order and favorable chemical composition makes Zr– Al–Cu–Ni quasicrystalline alloys, a promising material for the hydrogen storage applications [1–4]. The quasicrystalline–glass composites as well as the glassy precursor phase are known to absorb large amounts of hydrogen during electrochemical charging, up to a hydrogen per metal atom content (H/M) of 1.6 for the quasicrystalline phase [5,6] and H/M = 1.0 for the glassy phase [7,8] respectively. However, in order to use these materials for hydrogen storage a similar hydrogen uptake from the gas phase is necessary without any irreversible phase transformations. The storage capacity was found to be higher and the absorption kinetics to be faster for the quasicrystalline phase than for the amorphous one [2,5,8]. The improved hydrogen storage capacity may result from the large number of adjacent tetrahedral sites assumed
⁎ Corresponding author at: Department of Physics, Panjab University, Chandigarh 160014, India. Tel.: +91 9452865352. E-mail address: [email protected] (D. Singh). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.08.024
for icosahedral structure. Thus, it is pertinent to study the hydrogenation characteristics of quasicrystalline phase. Zr-based as well as Ti-based quasicrystals are known to store hydrogen up to contents close to the best materials in the category of hydrogen storage alloy systems [4,9–12], but there are questions regarding their structural stability with hydrogenation. The icosahedral phase (I-phase) in Zr69.5Al7.5Cu12Ni11 alloy is metastable and it decomposes into crystalline phases when annealed for a longer time at the temperature at which it condenses out from the undercooled liquid [13,14]. These crystalline phases do not have significant polytetrahedral order, and therefore the hydrogen storage capacity will be deteriorated. It has been observed that the desorption of hydrogen was not observed to proceed at temperatures less than about 450 °C probably due to thin oxide layers formed at the surfaces of the partially quasicrystalline Zr69.5Al7.5Cu12Ni11 ribbons [3]. The I-phase decomposed during annealing at higher temperatures by a discontinuous transformation into tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2 starting with a precipitation reaction of Zr2Cu [15–17]. In view of application, the influence of hydrogenation on the structure of these quasicrystalline alloys and consequently its effect on the mechanical properties are still major concerns. Thus Zr69.5Al7.5Cu12Ni11 quasicrystalline alloy is of special interest for studying the influence of hydrogenation on the structure/microstructure and microhardness behavior. In our earlier work [18], (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with x = 0, 4, 8, 12 and 16 have been studied in terms of formation of quasicrystal–glass composites. Formation of single quasicrystalline phase by annealing the glass has been found only up to x = 12. Further increase of Ti content gives rise to the formation of a Zr2Ni type crystalline phase. The addition of Ti changes the morphology of quasicrystals. It
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D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16 Table 1 Compositions of the (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys (in at. %) based on EDX quantitative analysis. Alloy composition x (at. %)
Zr (±0.7)
Al (±0.3)
Cu (±0.5)
Ni (±0.5)
Ti (±0.5)
0 4 12
68.7 66.1 60.7
7.3 6.9 6.5
12.5 12.2 11.6
11.5 9.8 8.3
5.0 12.9
2. Experimental details
Fig. 1. (a) XRD patterns of (a) as-synthesized and (b) annealed ribbons of (Zr69.5Al7.5 Cu12 Ni11 )100 − x Ti x alloys.
is therefore imperative that a systematic examination of hydrogen storage properties in a given system is to be carried out. Keeping the aforesaid considerations in view, the hydrogenation behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with 0 ≤ x ≤ 12 has been studied. The aim of the present work is to examine the influence of hydrogenation from the gas phase on the structure/microstructure and microhardness behavior of these melt-spun quasicrystalline alloys. The investigation will be focused on the phase and microstructural changes with addition of Ti and their correlation with hydrogen storage characteristics of Zr–Al–Cu–Ni alloys.
Alloy ingots of compositions (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12 at. %) were prepared by induction melting of pure Zr (99.9%), Al (99.96%), Cu (99.99%), Ni (99.99%) and Ti (99.9%) in a silica crucible under a Ti-gettered high purity argon atmosphere. The ingots were remelted several times to attain chemical homogeneity. The alloys were then melt-spun onto a copper wheel (~14 cm diameter) rotating at a speed of 40 m/s. During melt-spinning the entire apparatus was enclosed in a steel enclosure through which argon gas was continuously flowing. This was done to prevent oxidation of the ribbons after ejection of melt from the nozzle. The length and thickness of the ribbons were ~5 cm and ~40 μm respectively. The ribbons of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys were then packed in a Ta foil which was sealed in a silica ampoule under an argon atmosphere for annealing experiments. Isothermal annealing of the ribbons was carried out in a vacuum (10−6 Torr) using a Heraeus furnace with temperature control of ±1 °C. The structural characterization was done by employing an X-ray diffractometer (X'Pert Pro PANalytical diffractometer) with CuKα radiation. The as-synthesized as well as annealed ribbons were thinned using an electrolyte (90% methanol and 10% perchloric acid) at −30 °C. The thinned samples were then studied by transmission electron microscopy (TEM) using FEI: Technai 20G2 electron microscope. An energy dispersive X-ray analysis (EDX) was employed for the compositional analysis. The hydrogen sorption characteristics of the partially crystallized ribbons were examined by using the computerized pressure–concentration– temperature (PCT) apparatus supplied by Advanced Material Corporation (USA). Temperature, pressure and the gas desorbed/absorbed through the samples were monitored by the gas reaction control based software during different experiments. The estimated error in the hydrogen storage capacity measurement for the alloys is ±0.02 wt. %. In addition to hydrogen sorption behavior, temperature programmed desorption (TPD) experiments at a heating rate of 5 °C/min were also performed. In order to understand the effect of hydrogenation on the microhardness behavior, indentation tests were conducted at room temperature using SHIMADZU HMV-2T microhardness tester under different loads. The standard diamond pyramid shape Vickers indenter with tip edge ~0.5 μm provided with the equipment by SHIMADZU was used.
Fig. 2. Bright field TEM images of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys (a) x = 0, (b) x = 4 and (c) x = 12 showing the influence of the Ti content on size of the quasicrystalline phase.
D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16
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Fig. 3. Hydrogen absorption kinetics curves of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites. The lines are drawn as guides to the eyes.
Fig. 4. Temperature programmed desorption (TPD) curves of hydrogenated (Zr69.5 Al7.5 Cu12 Ni11 )100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites.
3. Results and discussion
remaining glassy phase. These results indicate that Ti has a distinct effect on the precipitation of quasicrystals and decreases the size of the quasicrystalline particles in (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. The grain size change described here is recognized as a significant increase in nucleation rate of the I-phase with an increase of Ti content. In present case, the addition of Ti affects the nucleation and growth characteristics of quasicrystals. For the composition determination, a quantitative energy dispersive X-ray analysis (EDX) was performed. Table 1 presents EDX quantitative analysis along with deviations of the (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with x = 0, 4 and 12 respectively. The deviations in the elemental compositions have been arrived at based on semi-quantitative analysis of EDX pattern at 3–5 points. Based on the EDX quantitative analysis, the investigated compositions of the alloys are very close to stoichiometric proportions of nominal compositions. Within the native experimental error of EDX, no evidence of oxygen in the ribbons was detected.
3.1. Microstructural and structural features Fig. 1 (a) shows the XRD patterns of as-synthesized (Zr69.5Al7.5Cu12 Ni11)100 − xTix (x = 0, 4, 12 at. %) melt-spun alloys. It is seen that all the patterns of the alloys consist of only broad diffraction maxima without a detectable sharp Bragg peak, indicating that the samples are amorphous. The primary crystallization products of these melt spun alloys were examined after isothermal annealing. Fig. 1 (b) compares XRD patterns for partially crystallized ribbons with x = 0, 4 and 12. Isothermal annealing of these samples was carried out for 15 min at 698 K (for x = 0 and 4), and 693 K (for x = 12). The respective annealing temperatures for these alloys have been found through differential scanning calorimetry (DSC) investigation [18]. Formation of quasicrystalline phase has been observed in the crystallization processes of Ti bearing alloys with x = 0–12. These samples display Bragg peaks, which could be indexed on the basis of I-phase [19,20]. The XRD pattern of the sample with Ti content of 12 at. % shows significant peak broadening in comparison to the samples of lower Ti content. This is because the size of the precipitates is decreasing as we are increasing the concentration of Ti. The formation of quasicrystalline phase in these samples was further investigated by TEM. Fig. 2 (a, b and c) shows nanometer sized quasicrystalline grains of the annealed samples of the alloys with x = 0, 4 and 12 respectively. It has been found that the grain size of quasicrystals decreases with addition of Ti. The alloys with x = 0 and 4 reveals the presence of quasicrystal grains with average size of ~125 nm and ~80 nm respectively. The corresponding selected area electron diffraction (SAED) pattern shows the presence of characteristic 5-fold icosahedral symmetry. The strong reduction in the grain size has been observed for the alloy with x = 12. This reveals the formation of 5–10 nm grains. The SAED pattern shows the presence of diffraction rings (inset in Fig. 2 (c)) indexed with I-phase. The presence of diffuse ring in these diffraction patterns reveals that the nanoquasicrystals are embedded in the
Table 2 Hydrogenation characteristics quasicrystal–glass composites.
of
(Zr69.5Al7.5Cu12Ni11)100 − xTix
Alloy composition x (at. %)
Quasilattice parameter Before hydrogenation (ai0)
After hydrogenation (aiH)
0 12
5.108 Å 5.141 Å
5.201 Å 5.293 Å
(x = 0
and
3.2. Hydrogenation characteristics The hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites have been investigated by the absorption kinetic experiments performed at fixed temperature and pressure. It may be pointed out that hydrogen storage is here through dissociation of hydrogen molecule into hydrogen atoms. This
12)
Storage capacity (wt. %) (±0.02)
1.20 1.56
Fig. 5. XRD patterns of hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0 and 12) ribbons.
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D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16
Fig. 6. Bright field TEM images of hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix ribbons with (a) x = 0 and (b) x = 12.
dissociation takes place due to catalytic activity of surface transition metal atoms. The dissociation is followed by diffusion of hydrogen atoms in the interstitial sites. The hydrogenation experiments have been performed at different temperature and pressure range from 250 to 300 °C and 5–10 MPa respectively, for 2–3 h. The best results were found with temperature 270 °C and 8 MPa of hydrogen pressure. The partially crystallized ribbons were hydrogenated in a high pressure reactor. The reactor containing the ribbons was evacuated by a rotary pump (10−2 Torr) then heated up to 270 °C. This was then charged with 8 MPa of hydrogen pressure. Fig. 3 shows the hydrogen absorption characteristics for the alloys with x = 0, 4 and 12 at 270 °C temperature and 8 MPa hydrogen pressure. It is found that the hydrogen uptake capacity and the absorption kinetics of the composites increase with increasing concentration of Ti (as shown in Table 2). The hydrogen storage capacity is found to be 1.20 wt. %, 1.38 wt. % and 1.56 wt. % for the alloys with x = 0, 4 and 12 respectively. The alloy with x = 0 produces 100–150 nm grains having less storage capacity and found to be 1.20 wt. % whereas the alloy with x = 4 shows the formation of 60–100 nm grains can store 1.38 wt. % of hydrogen at same conditions of temperature and pressure. The storage capacity increases to 1.56 wt. % for the alloy with x = 12 with grain size in the range 5–10 nm. In the present case, the enhancement in storage capacity is ~23% over the normal storage capacity of Zr69.5Al7.5Cu12Ni11 quasicrystalline phase. The experimental conditions of producing melt-spun ribbons were identical for all the alloys. There are two possible reasons for the enhancement in the hydrogen storage capacity i.e. (i) it has been observed that the grain refinement occurred with increase in the value of x by XRD and TEM analysis. The increase of grain boundary density provides better interaction between hydrogen and I-phase and hence increases the hydrogen storage uptake capacity. Such an observation pertaining to the increase in the hydrogen absorption due to grain refinement has also been observed in Ti–Zr–Ni and Mg–Ni–Mm alloys [9,21]. (ii) The other possible reason for the enhancement in storage
capacity may also be due to addition of Ti. The addition of Ti has two fold effects on the hydrogen storage behavior of Zr69.5Al7.5Cu12Ni11 quasicrystalline alloy. First the addition of Ti enhances the dissociation of hydrogen molecule to hydrogen atom at the surface due to its catalytic effect [19,20,22]. Second, the increase of the combined (Zr+Ti) content, from 69.5 for x = 0 to 73.2 for x = 12, enhances the overall affinity for hydrogen and might lead to an increased number of energetically favorable sites for hydrogen. Thus, it can be said that the combined effect of above two i.e. (i) and (ii) may lead to improved hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. The hydrogen desorption behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites has been investigated by using temperature programmed desorption (TPD) experiment at heating rate 5 °C/min. Fig. 4 shows the desorption curves of (Zr69.5Al7.5 Cu12Ni11)100 − xTix (x = 0, 4 and 12) alloys. The nature of desorption curves for hydrogenated ribbons changed with increasing Ti addition. This may occur because the microstructure of the ribbon changes with addition of Ti. At higher Ti content of x = 12, a small decrease in desorption temperature has been observed. As it can be seen from Fig. 4 that full desorption was not observed in the case of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. This behavior is different from that of the stable Ti-based quasicrystals Ti45Zr38Ni17, which allows nearly full desorption [9,23]. 3.3. Influence of hydrogenation on the structural and microhardness behavior In order to investigate the structural changes with hydrogenation, the hydrogenated samples are characterized through XRD. The XRD patterns of the hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0 and 12) ribbons are shown in Fig. 5. This reveals that I-phase peaks coexist with small concentration of crystalline hydride. This hydride phase has been recognized as ZrH2 which has a tetragonal lattice with space group: I4/mmm, a = b = 3.519 Å and c = 4.450 Å. Several other
Fig. 7. Variation of hardness (VHN) with respect to load for the as-synthesized, quasicrystal(qc) –glass composites and hydrogenated qc–glass composites (a) x = 0, (b) x = 4 and (c) x = 12.
D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16 Table 3 Values of VHN (GPa) at 50 g load of as-synthesized, quasicrystal(qc)–glass composites and hydrogenated qc–glass composites of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. Alloy composition x (at. %)
As-synthesized (±0.10)
Quasicrystal(qc)–glass composite (±0.10)
Hydrogenated qc–glass composite (±0.10)
0 4 12
5.71 6.00 6.04
7.02 7.33 7.74
7.33 7.57 7.93
studies [2,3] have earlier reported the formation of such hydride phase on hydrogenation of I-phase of Zr–Al–Cu–Ni alloy. This hydride phase may form due to partial decomposition of I-phase [2]. The volume fraction of the quasicrystalline phase exhibits a significant influence on the formation of hydrides during the charging from the gas phase [2,11,24]. The decrease in the intensity along with the significant broadening of the I-phase peaks is evident for the hydrogenated samples. This is due to the decrease in the size of the grains after hydrogenation. The shift in I-phase peaks to smaller angle has been observed, thus indicating the lattice expansion upon hydrogenation. Along with identification of phase transformations with hydrogenation, X-ray diffraction patterns are also used to estimate the quasilattice parameters of hydrogenated and partially crystallized ribbons (Table 2). The formation of crystalline hydride phase along with I-phase has been observed for the hydrogenated ribbons. We have assumed that the volume fraction of crystalline hydride phase is small and the measured hydrogen absorption reflects the amount of hydrogen stored in the quasicrystal. Here, we also compare the microstructural changes observed after hydrogenation. Fig. 6 (a and b) shows the transmission electron microscopy (TEM) images of the hydrogenated ribbons with x = 0 and x = 12 respectively. In comparison to Fig. 2 (a and c), the size of the grains has decreased. The change in the morphology of the quasicrystal grains as well as weakening of the diffraction spots (marked by arrows in inset of Fig. 6 (a)) has been observed for the alloy with x = 0. The SAED pattern (inset of Fig. 6 (b)) for x = 12 shows the weak diffraction spots superimposed on the diffuse halo ring of the amorphous phase. The results obtained in the present study are similar with those obtained by Zander et al. [24]. It has been reported that even mild hydrogenation probably led to the formation of defects within the icosahedral structure. These defects probably introduce the diffuseness and the formation of domains in the microstructure and thus lead to a weakening of the contrast of the quasicrystals as well as the diffraction spots [4,24]. Further, the effect of hydrogenation on the microhardness behavior of these alloys has also been investigated. The hardness (H) was computed by the formula in GPa units [25]. H ¼ 1:854 9:8
P d2
where P is the load (g) and d is the diagonal length in μm. The mean hardness values of at least five loads will be reported here with deviations. Fig. 7 (a, b and c) show hardness (VHN) versus load (g) characteristic curves for the as-synthesized, quasicrystal(qc)–glass composite and hydrogenated qc–glass composite for the alloys with x = 0, 4 and 12 respectively. A notable difference in the microhardness behavior has been observed. The indentation tests were conducted up to the load till cracks around the indentation impression were observed. Cracks get initiated around the indent when the applied load exceeds a certain value. As evident from the curves, the indentation was conducted up to load of 100 g for the qc–glass composites while this only holds up to 50 g for the hydrogenated qc–glass composites. Thus load to fracture decreases for the hydrogenated samples. This reveals the decrease in the fracture toughness for the hydrogenated samples. The significant increase in the hardness for the qc–glass composites as compared to as-synthesized ribbons
15
is due to the precipitation of quasicrystal grains in the amorphous matrix [26–30]. The microhardness for the qc–glass composites shows only minor changes after hydrogenation. Table 3 gives hardness values at 50 g of load for all the as-synthesized, qc–glass composites and hydrogenated qc–glass composites. The hardness values of hydrogenated qc–glass composites of x = 0, 4 and 12 at 50 g load are ~ 7.33 GPa, ~ 7.57 GPa and ~ 7.93 GPa respectively. These are slightly higher than that of qc–glass composites. A slight increase in the hardness may be attributed to the microstructural variation and partial decomposition of I-phase into crystalline hydride phase during hydrogenation. During hydrogenation, the microhardness might be changed by the interstitial content and microstructural changes e.g. phase transformations as well as precipitations [31–35]. 4. Conclusions Based on the present study, it may be said that the hydrogen uptake capacity of partially quasicrystalline (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) alloys was improved by the addition of Ti. It has been found that the hydrogen storage capacity is enhanced from ∼1.20 wt. % (for x = 0) to ∼1.56 wt. % (for x = 12). The enhancement in storage capacity for the quasicrystalline alloy with x = 12 is ∼23% as compared to the storage capacity of quasicrystalline alloy with x = 0. The improvement in the hydrogen uptake capacity is assumed to be a result of grain refinement of quasicrystals due to Ti addition. The microhardness shows only minor changes after hydrogenation of quasicrystal–glass composites. It has been concluded that microstructural and morphological changes alter the hydrogen storage characteristics of quasicrystal–glass composites. Acknowledgements The authors are thankful to Dr. M. A. Shaz for many stimulating discussions. Financial assistance received from DST is gratefully acknowledged. References [1] A.D. Rud, U. Schmidt, G.M. Zclinska, A.M. Lakhnik, G. Ya Kolbasov, M.O. Danilov, J. Non-Cryst. Solids 353 (2007) 3434. [2] V.T. Huett, D. Zander, L. Jastrow, E.H. Majzoub, K.F. Kelton, U. Köster, J. Alloys Compd. 379 (2004) 16. [3] D. Zander, E. Tal Gutelmacher, L. Jastrow, U. Köster, D. Eliezer, J. Alloys Compd. 356–357 (2003) 654. [4] T. Apih, V. Khare, M. Klanjsek, P. Jeglic, J. Dolinsek, Phys. Rev. B 68 (2003) 212202. [5] N. Eliaz, D. Eliezer, E. Abramov, D. Zander, U. Köster, J. Alloys Compd. 305 (2000) 272. [6] U. Köster, D. Zander, J. Meinhardt, N. Eliaz, D. Eliezer, Hydrogen in quasicrystalline Zr-Cu-Ni-Al, in: S. Takeuchi, T. Fujiwara (Eds.), Proceedings of the Sixth International Conference on Quasicrystals, Tokyo, 1997, World Scientific, Singapore, 1998, p. 313. [7] N. Ismail, A.A. EI-Meligi, M. Uhlemann, A. Gebert, J. Eckert, L. Schultz, J. Alloys Compd. 480 (2009) 321. [8] U. Köster, D. Zander, H. Leptien, N. Eliaz, D. Eliezer, Hydrogenation and crystallization of Zr-Cu-Ni-Al metallic glasses, MRS Proceedings 554 (1998) 287, http://dx.doi.org/10.1557/PROC-554-287. [9] R.R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastava, S. Smaalen Van, Int. J. Hydrogen Energy 35 (2011) 592. [10] H. Huang, R. Li, C. Yin, S. Zheng, P. Zhang, Int. J. Hydrogen Energy 33 (2008) 4607. [11] D. Zander, U. Köster, N. Eliaz, D. Eliezer, Mater. Sci. Eng. 294–296 (2000) 112. [12] H.-R. Sinning, R. Scarfone, J.S. Golovin, Mater. Sci. Eng., A 370 (2004) 78. [13] Y.M. Wang, Structure evolution of Zr-based glass-forming alloys and composition design methodology for good glass-forming abilities. (Ph.D. thesis) City University of Hong Kong, Hong Kong, 2007. [14] U. Kühn, K. Eymann, N. Mattern, J. Eckert, A. Gebert, B. Bartusch, L. Schultz, Acta Mater. 54 (2006) 4685. [15] E. Hong, D.C. Dunand, H. Choe, Int. J. Hydrogen Energy 35 (2010) 5708. [16] D. Singh, T.P. Yadav, R.K. Mandal, R.S. Tiwari, O.N. Srivastava, Mater. Sci. Eng., A 527 (2010) 469. [17] D. Zander, H. Leptian, U. Köster, N. Eliaz, D. Eliezer, J. Non-Cryst. Solids 250–252 (1999) 893. [18] D. Singh, T.P. Yadav, R.K. Mandal, R.S. Tiwari, O.N. Srivastava, Philos. Mag. 91 (2011) 2837.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Hydrogenation of (Zr69.5Al7.5Cu12Ni11)100 − xTix quasicrystalline alloys and its effect on their structural and microhardness behavior Devinder Singh a,b,⁎, Rohit R. Shahi a, T.P. Yadav a, R.K. Mandal c, R.S. Tiwari a, O.N. Srivastava a a b c
Hydrogen Energy Centre and Unit of Nano-Science and Technology, Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Physics, Panjab University, Chandigarh 160014, India Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 13 August 2013 Available online xxxx Keywords: Hydrogen storage; Nanoquasicrystal; Melt-spinning; Microstructure; Microhardness
a b s t r a c t The present study deals with the microstructural changes with respect to the addition of Ti and their correlation with hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystalline alloys. The grain size of quasicrystals decreases with addition of Ti. It has been found that the alloy with x = 0 absorbed 1.20 wt. %, whereas the alloys with x = 4 and 12 absorbed 1.38 wt. % and 1.56 wt. % of hydrogen respectively. Hydrogenation was found to exhibit a significant effect on the structure/microstructure and microhardness behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix quasicrystalline alloys. Variation in the microhardness behavior has been discussed based on a structure–property correlation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Storage is a crucial issue for bringing in hydrogen, a renewable, inexhaustive, clean and climate friendly fuel. The storage capacity of hydrogen in metals and alloys is determined by chemical interactions between the metal and hydrogen atoms, as well as by the type, number and size of the potential interstitials sites for hydrogen. The most common materials for hydrogen storage are metals and alloys that are based on transition metals. In most of these materials, hydrogen tends to occupy tetrahedral interstitials sites. Therefore, the combination of local tetrahedral order and favorable chemical composition makes Zr– Al–Cu–Ni quasicrystalline alloys, a promising material for the hydrogen storage applications [1–4]. The quasicrystalline–glass composites as well as the glassy precursor phase are known to absorb large amounts of hydrogen during electrochemical charging, up to a hydrogen per metal atom content (H/M) of 1.6 for the quasicrystalline phase [5,6] and H/M = 1.0 for the glassy phase [7,8] respectively. However, in order to use these materials for hydrogen storage a similar hydrogen uptake from the gas phase is necessary without any irreversible phase transformations. The storage capacity was found to be higher and the absorption kinetics to be faster for the quasicrystalline phase than for the amorphous one [2,5,8]. The improved hydrogen storage capacity may result from the large number of adjacent tetrahedral sites assumed
⁎ Corresponding author at: Department of Physics, Panjab University, Chandigarh 160014, India. Tel.: +91 9452865352. E-mail address: [email protected] (D. Singh). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.08.024
for icosahedral structure. Thus, it is pertinent to study the hydrogenation characteristics of quasicrystalline phase. Zr-based as well as Ti-based quasicrystals are known to store hydrogen up to contents close to the best materials in the category of hydrogen storage alloy systems [4,9–12], but there are questions regarding their structural stability with hydrogenation. The icosahedral phase (I-phase) in Zr69.5Al7.5Cu12Ni11 alloy is metastable and it decomposes into crystalline phases when annealed for a longer time at the temperature at which it condenses out from the undercooled liquid [13,14]. These crystalline phases do not have significant polytetrahedral order, and therefore the hydrogen storage capacity will be deteriorated. It has been observed that the desorption of hydrogen was not observed to proceed at temperatures less than about 450 °C probably due to thin oxide layers formed at the surfaces of the partially quasicrystalline Zr69.5Al7.5Cu12Ni11 ribbons [3]. The I-phase decomposed during annealing at higher temperatures by a discontinuous transformation into tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2 starting with a precipitation reaction of Zr2Cu [15–17]. In view of application, the influence of hydrogenation on the structure of these quasicrystalline alloys and consequently its effect on the mechanical properties are still major concerns. Thus Zr69.5Al7.5Cu12Ni11 quasicrystalline alloy is of special interest for studying the influence of hydrogenation on the structure/microstructure and microhardness behavior. In our earlier work [18], (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with x = 0, 4, 8, 12 and 16 have been studied in terms of formation of quasicrystal–glass composites. Formation of single quasicrystalline phase by annealing the glass has been found only up to x = 12. Further increase of Ti content gives rise to the formation of a Zr2Ni type crystalline phase. The addition of Ti changes the morphology of quasicrystals. It
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D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16 Table 1 Compositions of the (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys (in at. %) based on EDX quantitative analysis. Alloy composition x (at. %)
Zr (±0.7)
Al (±0.3)
Cu (±0.5)
Ni (±0.5)
Ti (±0.5)
0 4 12
68.7 66.1 60.7
7.3 6.9 6.5
12.5 12.2 11.6
11.5 9.8 8.3
5.0 12.9
2. Experimental details
Fig. 1. (a) XRD patterns of (a) as-synthesized and (b) annealed ribbons of (Zr69.5Al7.5 Cu12 Ni11 )100 − x Ti x alloys.
is therefore imperative that a systematic examination of hydrogen storage properties in a given system is to be carried out. Keeping the aforesaid considerations in view, the hydrogenation behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with 0 ≤ x ≤ 12 has been studied. The aim of the present work is to examine the influence of hydrogenation from the gas phase on the structure/microstructure and microhardness behavior of these melt-spun quasicrystalline alloys. The investigation will be focused on the phase and microstructural changes with addition of Ti and their correlation with hydrogen storage characteristics of Zr–Al–Cu–Ni alloys.
Alloy ingots of compositions (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12 at. %) were prepared by induction melting of pure Zr (99.9%), Al (99.96%), Cu (99.99%), Ni (99.99%) and Ti (99.9%) in a silica crucible under a Ti-gettered high purity argon atmosphere. The ingots were remelted several times to attain chemical homogeneity. The alloys were then melt-spun onto a copper wheel (~14 cm diameter) rotating at a speed of 40 m/s. During melt-spinning the entire apparatus was enclosed in a steel enclosure through which argon gas was continuously flowing. This was done to prevent oxidation of the ribbons after ejection of melt from the nozzle. The length and thickness of the ribbons were ~5 cm and ~40 μm respectively. The ribbons of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys were then packed in a Ta foil which was sealed in a silica ampoule under an argon atmosphere for annealing experiments. Isothermal annealing of the ribbons was carried out in a vacuum (10−6 Torr) using a Heraeus furnace with temperature control of ±1 °C. The structural characterization was done by employing an X-ray diffractometer (X'Pert Pro PANalytical diffractometer) with CuKα radiation. The as-synthesized as well as annealed ribbons were thinned using an electrolyte (90% methanol and 10% perchloric acid) at −30 °C. The thinned samples were then studied by transmission electron microscopy (TEM) using FEI: Technai 20G2 electron microscope. An energy dispersive X-ray analysis (EDX) was employed for the compositional analysis. The hydrogen sorption characteristics of the partially crystallized ribbons were examined by using the computerized pressure–concentration– temperature (PCT) apparatus supplied by Advanced Material Corporation (USA). Temperature, pressure and the gas desorbed/absorbed through the samples were monitored by the gas reaction control based software during different experiments. The estimated error in the hydrogen storage capacity measurement for the alloys is ±0.02 wt. %. In addition to hydrogen sorption behavior, temperature programmed desorption (TPD) experiments at a heating rate of 5 °C/min were also performed. In order to understand the effect of hydrogenation on the microhardness behavior, indentation tests were conducted at room temperature using SHIMADZU HMV-2T microhardness tester under different loads. The standard diamond pyramid shape Vickers indenter with tip edge ~0.5 μm provided with the equipment by SHIMADZU was used.
Fig. 2. Bright field TEM images of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys (a) x = 0, (b) x = 4 and (c) x = 12 showing the influence of the Ti content on size of the quasicrystalline phase.
D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16
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Fig. 3. Hydrogen absorption kinetics curves of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites. The lines are drawn as guides to the eyes.
Fig. 4. Temperature programmed desorption (TPD) curves of hydrogenated (Zr69.5 Al7.5 Cu12 Ni11 )100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites.
3. Results and discussion
remaining glassy phase. These results indicate that Ti has a distinct effect on the precipitation of quasicrystals and decreases the size of the quasicrystalline particles in (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. The grain size change described here is recognized as a significant increase in nucleation rate of the I-phase with an increase of Ti content. In present case, the addition of Ti affects the nucleation and growth characteristics of quasicrystals. For the composition determination, a quantitative energy dispersive X-ray analysis (EDX) was performed. Table 1 presents EDX quantitative analysis along with deviations of the (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys with x = 0, 4 and 12 respectively. The deviations in the elemental compositions have been arrived at based on semi-quantitative analysis of EDX pattern at 3–5 points. Based on the EDX quantitative analysis, the investigated compositions of the alloys are very close to stoichiometric proportions of nominal compositions. Within the native experimental error of EDX, no evidence of oxygen in the ribbons was detected.
3.1. Microstructural and structural features Fig. 1 (a) shows the XRD patterns of as-synthesized (Zr69.5Al7.5Cu12 Ni11)100 − xTix (x = 0, 4, 12 at. %) melt-spun alloys. It is seen that all the patterns of the alloys consist of only broad diffraction maxima without a detectable sharp Bragg peak, indicating that the samples are amorphous. The primary crystallization products of these melt spun alloys were examined after isothermal annealing. Fig. 1 (b) compares XRD patterns for partially crystallized ribbons with x = 0, 4 and 12. Isothermal annealing of these samples was carried out for 15 min at 698 K (for x = 0 and 4), and 693 K (for x = 12). The respective annealing temperatures for these alloys have been found through differential scanning calorimetry (DSC) investigation [18]. Formation of quasicrystalline phase has been observed in the crystallization processes of Ti bearing alloys with x = 0–12. These samples display Bragg peaks, which could be indexed on the basis of I-phase [19,20]. The XRD pattern of the sample with Ti content of 12 at. % shows significant peak broadening in comparison to the samples of lower Ti content. This is because the size of the precipitates is decreasing as we are increasing the concentration of Ti. The formation of quasicrystalline phase in these samples was further investigated by TEM. Fig. 2 (a, b and c) shows nanometer sized quasicrystalline grains of the annealed samples of the alloys with x = 0, 4 and 12 respectively. It has been found that the grain size of quasicrystals decreases with addition of Ti. The alloys with x = 0 and 4 reveals the presence of quasicrystal grains with average size of ~125 nm and ~80 nm respectively. The corresponding selected area electron diffraction (SAED) pattern shows the presence of characteristic 5-fold icosahedral symmetry. The strong reduction in the grain size has been observed for the alloy with x = 12. This reveals the formation of 5–10 nm grains. The SAED pattern shows the presence of diffraction rings (inset in Fig. 2 (c)) indexed with I-phase. The presence of diffuse ring in these diffraction patterns reveals that the nanoquasicrystals are embedded in the
Table 2 Hydrogenation characteristics quasicrystal–glass composites.
of
(Zr69.5Al7.5Cu12Ni11)100 − xTix
Alloy composition x (at. %)
Quasilattice parameter Before hydrogenation (ai0)
After hydrogenation (aiH)
0 12
5.108 Å 5.141 Å
5.201 Å 5.293 Å
(x = 0
and
3.2. Hydrogenation characteristics The hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites have been investigated by the absorption kinetic experiments performed at fixed temperature and pressure. It may be pointed out that hydrogen storage is here through dissociation of hydrogen molecule into hydrogen atoms. This
12)
Storage capacity (wt. %) (±0.02)
1.20 1.56
Fig. 5. XRD patterns of hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0 and 12) ribbons.
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D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16
Fig. 6. Bright field TEM images of hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix ribbons with (a) x = 0 and (b) x = 12.
dissociation takes place due to catalytic activity of surface transition metal atoms. The dissociation is followed by diffusion of hydrogen atoms in the interstitial sites. The hydrogenation experiments have been performed at different temperature and pressure range from 250 to 300 °C and 5–10 MPa respectively, for 2–3 h. The best results were found with temperature 270 °C and 8 MPa of hydrogen pressure. The partially crystallized ribbons were hydrogenated in a high pressure reactor. The reactor containing the ribbons was evacuated by a rotary pump (10−2 Torr) then heated up to 270 °C. This was then charged with 8 MPa of hydrogen pressure. Fig. 3 shows the hydrogen absorption characteristics for the alloys with x = 0, 4 and 12 at 270 °C temperature and 8 MPa hydrogen pressure. It is found that the hydrogen uptake capacity and the absorption kinetics of the composites increase with increasing concentration of Ti (as shown in Table 2). The hydrogen storage capacity is found to be 1.20 wt. %, 1.38 wt. % and 1.56 wt. % for the alloys with x = 0, 4 and 12 respectively. The alloy with x = 0 produces 100–150 nm grains having less storage capacity and found to be 1.20 wt. % whereas the alloy with x = 4 shows the formation of 60–100 nm grains can store 1.38 wt. % of hydrogen at same conditions of temperature and pressure. The storage capacity increases to 1.56 wt. % for the alloy with x = 12 with grain size in the range 5–10 nm. In the present case, the enhancement in storage capacity is ~23% over the normal storage capacity of Zr69.5Al7.5Cu12Ni11 quasicrystalline phase. The experimental conditions of producing melt-spun ribbons were identical for all the alloys. There are two possible reasons for the enhancement in the hydrogen storage capacity i.e. (i) it has been observed that the grain refinement occurred with increase in the value of x by XRD and TEM analysis. The increase of grain boundary density provides better interaction between hydrogen and I-phase and hence increases the hydrogen storage uptake capacity. Such an observation pertaining to the increase in the hydrogen absorption due to grain refinement has also been observed in Ti–Zr–Ni and Mg–Ni–Mm alloys [9,21]. (ii) The other possible reason for the enhancement in storage
capacity may also be due to addition of Ti. The addition of Ti has two fold effects on the hydrogen storage behavior of Zr69.5Al7.5Cu12Ni11 quasicrystalline alloy. First the addition of Ti enhances the dissociation of hydrogen molecule to hydrogen atom at the surface due to its catalytic effect [19,20,22]. Second, the increase of the combined (Zr+Ti) content, from 69.5 for x = 0 to 73.2 for x = 12, enhances the overall affinity for hydrogen and might lead to an increased number of energetically favorable sites for hydrogen. Thus, it can be said that the combined effect of above two i.e. (i) and (ii) may lead to improved hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. The hydrogen desorption behavior of (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) quasicrystal–glass composites has been investigated by using temperature programmed desorption (TPD) experiment at heating rate 5 °C/min. Fig. 4 shows the desorption curves of (Zr69.5Al7.5 Cu12Ni11)100 − xTix (x = 0, 4 and 12) alloys. The nature of desorption curves for hydrogenated ribbons changed with increasing Ti addition. This may occur because the microstructure of the ribbon changes with addition of Ti. At higher Ti content of x = 12, a small decrease in desorption temperature has been observed. As it can be seen from Fig. 4 that full desorption was not observed in the case of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. This behavior is different from that of the stable Ti-based quasicrystals Ti45Zr38Ni17, which allows nearly full desorption [9,23]. 3.3. Influence of hydrogenation on the structural and microhardness behavior In order to investigate the structural changes with hydrogenation, the hydrogenated samples are characterized through XRD. The XRD patterns of the hydrogenated (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0 and 12) ribbons are shown in Fig. 5. This reveals that I-phase peaks coexist with small concentration of crystalline hydride. This hydride phase has been recognized as ZrH2 which has a tetragonal lattice with space group: I4/mmm, a = b = 3.519 Å and c = 4.450 Å. Several other
Fig. 7. Variation of hardness (VHN) with respect to load for the as-synthesized, quasicrystal(qc) –glass composites and hydrogenated qc–glass composites (a) x = 0, (b) x = 4 and (c) x = 12.
D. Singh et al. / Journal of Non-Crystalline Solids 380 (2013) 11–16 Table 3 Values of VHN (GPa) at 50 g load of as-synthesized, quasicrystal(qc)–glass composites and hydrogenated qc–glass composites of (Zr69.5Al7.5Cu12Ni11)100 − xTix alloys. Alloy composition x (at. %)
As-synthesized (±0.10)
Quasicrystal(qc)–glass composite (±0.10)
Hydrogenated qc–glass composite (±0.10)
0 4 12
5.71 6.00 6.04
7.02 7.33 7.74
7.33 7.57 7.93
studies [2,3] have earlier reported the formation of such hydride phase on hydrogenation of I-phase of Zr–Al–Cu–Ni alloy. This hydride phase may form due to partial decomposition of I-phase [2]. The volume fraction of the quasicrystalline phase exhibits a significant influence on the formation of hydrides during the charging from the gas phase [2,11,24]. The decrease in the intensity along with the significant broadening of the I-phase peaks is evident for the hydrogenated samples. This is due to the decrease in the size of the grains after hydrogenation. The shift in I-phase peaks to smaller angle has been observed, thus indicating the lattice expansion upon hydrogenation. Along with identification of phase transformations with hydrogenation, X-ray diffraction patterns are also used to estimate the quasilattice parameters of hydrogenated and partially crystallized ribbons (Table 2). The formation of crystalline hydride phase along with I-phase has been observed for the hydrogenated ribbons. We have assumed that the volume fraction of crystalline hydride phase is small and the measured hydrogen absorption reflects the amount of hydrogen stored in the quasicrystal. Here, we also compare the microstructural changes observed after hydrogenation. Fig. 6 (a and b) shows the transmission electron microscopy (TEM) images of the hydrogenated ribbons with x = 0 and x = 12 respectively. In comparison to Fig. 2 (a and c), the size of the grains has decreased. The change in the morphology of the quasicrystal grains as well as weakening of the diffraction spots (marked by arrows in inset of Fig. 6 (a)) has been observed for the alloy with x = 0. The SAED pattern (inset of Fig. 6 (b)) for x = 12 shows the weak diffraction spots superimposed on the diffuse halo ring of the amorphous phase. The results obtained in the present study are similar with those obtained by Zander et al. [24]. It has been reported that even mild hydrogenation probably led to the formation of defects within the icosahedral structure. These defects probably introduce the diffuseness and the formation of domains in the microstructure and thus lead to a weakening of the contrast of the quasicrystals as well as the diffraction spots [4,24]. Further, the effect of hydrogenation on the microhardness behavior of these alloys has also been investigated. The hardness (H) was computed by the formula in GPa units [25]. H ¼ 1:854 9:8
P d2
where P is the load (g) and d is the diagonal length in μm. The mean hardness values of at least five loads will be reported here with deviations. Fig. 7 (a, b and c) show hardness (VHN) versus load (g) characteristic curves for the as-synthesized, quasicrystal(qc)–glass composite and hydrogenated qc–glass composite for the alloys with x = 0, 4 and 12 respectively. A notable difference in the microhardness behavior has been observed. The indentation tests were conducted up to the load till cracks around the indentation impression were observed. Cracks get initiated around the indent when the applied load exceeds a certain value. As evident from the curves, the indentation was conducted up to load of 100 g for the qc–glass composites while this only holds up to 50 g for the hydrogenated qc–glass composites. Thus load to fracture decreases for the hydrogenated samples. This reveals the decrease in the fracture toughness for the hydrogenated samples. The significant increase in the hardness for the qc–glass composites as compared to as-synthesized ribbons
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is due to the precipitation of quasicrystal grains in the amorphous matrix [26–30]. The microhardness for the qc–glass composites shows only minor changes after hydrogenation. Table 3 gives hardness values at 50 g of load for all the as-synthesized, qc–glass composites and hydrogenated qc–glass composites. The hardness values of hydrogenated qc–glass composites of x = 0, 4 and 12 at 50 g load are ~ 7.33 GPa, ~ 7.57 GPa and ~ 7.93 GPa respectively. These are slightly higher than that of qc–glass composites. A slight increase in the hardness may be attributed to the microstructural variation and partial decomposition of I-phase into crystalline hydride phase during hydrogenation. During hydrogenation, the microhardness might be changed by the interstitial content and microstructural changes e.g. phase transformations as well as precipitations [31–35]. 4. Conclusions Based on the present study, it may be said that the hydrogen uptake capacity of partially quasicrystalline (Zr69.5Al7.5Cu12Ni11)100 − xTix (x = 0, 4 and 12) alloys was improved by the addition of Ti. It has been found that the hydrogen storage capacity is enhanced from ∼1.20 wt. % (for x = 0) to ∼1.56 wt. % (for x = 12). The enhancement in storage capacity for the quasicrystalline alloy with x = 12 is ∼23% as compared to the storage capacity of quasicrystalline alloy with x = 0. The improvement in the hydrogen uptake capacity is assumed to be a result of grain refinement of quasicrystals due to Ti addition. The microhardness shows only minor changes after hydrogenation of quasicrystal–glass composites. It has been concluded that microstructural and morphological changes alter the hydrogen storage characteristics of quasicrystal–glass composites. Acknowledgements The authors are thankful to Dr. M. A. Shaz for many stimulating discussions. Financial assistance received from DST is gratefully acknowledged. References [1] A.D. Rud, U. Schmidt, G.M. Zclinska, A.M. Lakhnik, G. Ya Kolbasov, M.O. Danilov, J. Non-Cryst. Solids 353 (2007) 3434. [2] V.T. Huett, D. Zander, L. Jastrow, E.H. Majzoub, K.F. Kelton, U. Köster, J. Alloys Compd. 379 (2004) 16. [3] D. Zander, E. Tal Gutelmacher, L. Jastrow, U. Köster, D. Eliezer, J. Alloys Compd. 356–357 (2003) 654. [4] T. Apih, V. Khare, M. Klanjsek, P. Jeglic, J. Dolinsek, Phys. Rev. B 68 (2003) 212202. [5] N. Eliaz, D. Eliezer, E. Abramov, D. Zander, U. Köster, J. Alloys Compd. 305 (2000) 272. [6] U. Köster, D. Zander, J. Meinhardt, N. Eliaz, D. Eliezer, Hydrogen in quasicrystalline Zr-Cu-Ni-Al, in: S. Takeuchi, T. Fujiwara (Eds.), Proceedings of the Sixth International Conference on Quasicrystals, Tokyo, 1997, World Scientific, Singapore, 1998, p. 313. [7] N. Ismail, A.A. EI-Meligi, M. Uhlemann, A. Gebert, J. Eckert, L. Schultz, J. Alloys Compd. 480 (2009) 321. [8] U. Köster, D. Zander, H. Leptien, N. Eliaz, D. Eliezer, Hydrogenation and crystallization of Zr-Cu-Ni-Al metallic glasses, MRS Proceedings 554 (1998) 287, http://dx.doi.org/10.1557/PROC-554-287. [9] R.R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastava, S. Smaalen Van, Int. J. Hydrogen Energy 35 (2011) 592. [10] H. Huang, R. Li, C. Yin, S. Zheng, P. Zhang, Int. J. Hydrogen Energy 33 (2008) 4607. [11] D. Zander, U. Köster, N. Eliaz, D. Eliezer, Mater. Sci. Eng. 294–296 (2000) 112. [12] H.-R. Sinning, R. Scarfone, J.S. Golovin, Mater. Sci. Eng., A 370 (2004) 78. [13] Y.M. Wang, Structure evolution of Zr-based glass-forming alloys and composition design methodology for good glass-forming abilities. (Ph.D. thesis) City University of Hong Kong, Hong Kong, 2007. [14] U. Kühn, K. Eymann, N. Mattern, J. Eckert, A. Gebert, B. Bartusch, L. Schultz, Acta Mater. 54 (2006) 4685. [15] E. Hong, D.C. Dunand, H. Choe, Int. J. Hydrogen Energy 35 (2010) 5708. [16] D. Singh, T.P. Yadav, R.K. Mandal, R.S. Tiwari, O.N. Srivastava, Mater. Sci. Eng., A 527 (2010) 469. [17] D. Zander, H. Leptian, U. Köster, N. Eliaz, D. Eliezer, J. Non-Cryst. Solids 250–252 (1999) 893. [18] D. Singh, T.P. Yadav, R.K. Mandal, R.S. Tiwari, O.N. Srivastava, Philos. Mag. 91 (2011) 2837.
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