**2. Investigation of nominal composition: Zr69.5Al7.5Cu12Ni11**

The Zr69.5Al7.5Cu12Ni11 alloy with a thickness of ∼40–50 μm and lengths of ∼1–2 m has been synthesized using melt-spinning technique [23, 35]. 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. For annealing

experiments, the ribbons were packed in a Ta foil and then sealed in a silica ampoule under an argon atmosphere. The ribbons were isothermally annealed in vacuum (10−6 Torr) using a Heraeus furnace with temperature control of ±1°C.

Quasicrystals have been found to exhibit certain important characteristic properties like high hardness, high oxidation resistance, low surface energy, and low thermal conductivity which make these materials attractive for technological applications [1–7]. These materials have received considerable attention as hydrogen storage materials after the discovery of thermodynamically stable icosahedral quasicrystalline phase in Ti-based alloys [8, 9]. Due to the presence of high density of interstitial voids in quasicrystalline alloys, these materials have shown to reversibly store a large amount of hydrogen. The storage capacity can reach up to 3 wt.% of hydrogen [10]. This may be better than that of the crystalline La-based, V-based, and Fe-Ti hydrogen storage alloys [11–13]. Ti- and Zr-based quasicrystalline alloys have a large number of tetrahedral coordinated sites. The Zr-Al-Cu-Ni quasicrystalline alloy is a promising material for the hydrogen storage applications due to the presence of local tetrahedral order and favorable chemical composition [14–17]. The metallic glasses and their composites are known to absorb hydrogen during electrochemical charging, up to a hydrogen per metal atom content (H/M) of 1.6 for the quasicrystalline phase [18, 19] and H/M = 1.0 for the glassy phase [20, 21], respectively. A similar hydrogen uptake from the gas phase is needed with no irreversible phase transformation in order to use these materials for hydrogen storage. It has been found that the hydrogen storage capacity is higher, and the absorption kinetics is faster for the quasicrystalline phase than for the glassy phase [15, 18, 21]. The presence of a large number of tetrahedral sites assumed for icosahedral structure has led to enhanced hydrogen storage capacity. Thus, it is relevant to study the hydrogenation characteristics of quasicrystalline phase. Though Zr-based and Ti-based quasicrystals are among the best in the group of hydrogen storage alloy systems [17, 22–26], still there are issues in regard to their structural stability with hydrogenation. The icosahedral phase (I-phase) in Zr69.5Al7.5Cu12Ni11 alloy is metastable. When the Zr69.5Al7.5Cu12Ni11 alloy is annealed for a longer time, the metastable Iphase decomposes into crystalline phases [27–31]. Hence, the hydrogen storage capacity will be deteriorated due to the lack of significant polytetrahedral order in these crystalline phases. The desorption of hydrogen was not observed to ensue at temperatures less than about 450°C, most likely due to thin oxide layers formed at the surfaces of the partially quasicrystalline Zr69.5Al7.5Cu12Ni11 ribbons [16]. During annealing at higher temperatures, the I-phase decomposed by a discontinuous transformation into tetragonal Zr2Cu, tetragonal Zr2Ni, and hexagonal Zr6NiAl2 starting with a precipitation reaction of Zr2Cu [32–34]. In view of application, there are still major concerns regarding the effect of hydrogenation on the structure of these quasicrystalline alloys and consequently its influence on the mechanical properties. Thus, the study of hydrogenation effect on the structure/microstructure and mechanical behavior of

112 New Advances in Hydrogenation Processes - Fundamentals and Applications

Zr69.5Al7.5Cu12Ni11 quasicrystalline alloy is of special interest.

**2. Investigation of nominal composition: Zr69.5Al7.5Cu12Ni11**

The Zr69.5Al7.5Cu12Ni11 alloy with a thickness of ∼40–50 μm and lengths of ∼1–2 m has been synthesized using melt-spinning technique [23, 35]. 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. For annealing For exploring the gross structural feature, X-ray diffraction (XRD) studies of the as-formed ribbons were done. **Figure 1a** shows the representative XRD pattern of the Zr69.5Al7.5Cu12Ni11 as-synthesized ribbon. As can be seen, the XRD pattern shows two broad halos. This shows that the as-formed material is amorphous (glassy). However, for the as-synthesized ribbon (c.f. **Figure 1a**), broad halo within the angular range of 30–45° followed by another weaker second broad halo within the angular range of 60–70° is seen. The material was then annealed in the anticipation that the synthesized amorphous phase will transform to the quasicrystalline/ crystalline version. In order to estimate the transformation temperature, differential scanning calorimetry (DSC) technique was employed. **Figure 1b** shows the constant-rate heating DSC scan (20 K/min) of the melt-spun Zr69.5Al7.5Cu12Ni11 alloy. Heating the melt-spun ribbon from room temperature up to elevated temperatures reveals a distinct endothermic event characteristic of the glass transition (Tg) at 624 K. With increasing temperature the DSC curve exhibits two peaks (Tp1 and Tp2) with onset temperatures Tx1 and Tx2 (702 K and 727 K, respectively). The supercooled liquid region, defined as ∆Tx = Tx1−Tg corresponds to the stability of the supercooled liquid against crystallization and is found to be 78 K. The melting temperature (Tm) of the melt-spun Zr69.5Al7.5Cu12Ni11 is found to be 1064 K. The first peak (Tp1) results from the quasicrystal (qc) formation. The second peak (Tp2) corresponds to the decomposition of quasicrystals as well as the transformation of the remaining amorphous matrix into a crystalline phase.

**Figure 1.** (a) XRD patterns of as-synthesized and annealed ribbons of Zr69.5Al7.5Cu12Ni11 alloy at 693 K and 733 K for 15 min and (b) DSC curve of the melt-spun Zr69.5Al7.5Cu12Ni11 alloy at the heating rate of 20 K/min.

The XRD patterns of the melt-spun ribbons (**Figure 1a**) exhibit the typical broad diffuse maxima characteristic of amorphous materials and no traces of crystalline phases. When the sample is isothermally heated at 693 K (i.e., near Tx1) for 15 min, the XRD pattern displays diffraction peaks with positions and intensity ratios that are typical for powder diffraction patterns of quasicrystals. The reflections can be indexed to I-phase. One can notice the broad overlapping diffraction peak due to amorphous phase in addition to the quasicrystalline diffraction peaks in the annealed XRD patterns. This indicates that during the first devitrification step, the phase transformation from glass to quasicrystalline is not complete and a residual glassy phase retained in the sample. This suggests that quasicrystal formation is a primary transformation. As evident from **Figure 1a**, heating the ribbon to 733 K (i.e., near Tx2) leads to the formation of a tetragonal Zr2Cu-type phase (space group I4/mmm (139), a = 3.218 Ǻ, c = 11.180 Ǻ). No trace of quasicrystals is detected at this stage of crystallization, indicating that the quasicrystalline phase is metastable and transforms into more stable crystalline phase.

The Zr69.5Al7.5Cu12Ni11 melt-spun ribbon was observed to be fully amorphous by means of transmission electron microscopy (TEM). **Figure 2a** and the inset show the TEM micrograph and corresponding selected area electron diffraction (SAED) pattern displaying diffuse halos for Zr69.5Al7.5Cu12Ni11 alloy. The presence of two halos is suggestive of the presence of shortrange order with two different correlation lengths. The TEM bright-field micrograph displaying no discernible contrast indicates the formation of glassy phase in the system. **Figure 2b** shows the typical microstructure of a partially transformed Zr69.5Al7.5Cu12Ni11 ribbon with spherical quasicrystals. The inset of **Figure 2b** shows the fivefold diffraction pattern of the quasicrystalline phase. Since no diffraction patterns corresponding to the other crystalline structure are seen, the initial precipitation phase in Zr69.5Al7.5Cu12Ni11 alloy is only quasicrystalline phase.

**Figure 2.** (a) TEM image and the corresponding diffraction pattern of as-synthesized Zr69.5Al7.5Cu12Ni11 alloy. (b) TEM microstructure of Zr69.5Al7.5Cu12Ni11 alloy showing the presence of spherical quasicrystals. Inset of (b) shows the SAED pattern of the quasicrystalline (qc) phase along the fivefold direction.
