Hydrogenation of Zr-Based Quasicrystals

In this chapter, results of our recent investigations on the hydrogenation behavior of Zrbased quasicrystalline alloys and its effect on their structural and microhardness behavior have been discussed. The microstructural changes with respect to the addition of Ti and their correlation with hydrogen storage characteristics of (Zr69.5Al7.5Cu12 Ni11)100−xTix (x = 0, 4 and 12) quasicrystalline alloys have been studied. The substitution of Ti affects the nucleation and growth characteristics of nano-quasicrystals. The grain size of quasicrystals decreases with addition of Ti. The hydrogen uptake capacity of partially quasicrystalline alloys has been improved by the addition of Ti. The alloys with x = 0, 4, and 12 absorbed 1.20 wt.%, 1.38 wt.%, and 1.56 wt.% of hydrogen, respectively. A significant effect on the structure/microstructure and mechanical behavior of (Zr69.5Al7.5Cu12Ni11)100−xTix quasicrystalline alloys due to hydrogenation has been observed. The change in the microhardness behavior has been discussed based on microstructural variation resulting to Ti addition. The study is focused on investigations of these materials to understand the structure (microstructure)-property correlations.


Introduction
Storage is an important issue with hydrogen to become a renewable, clean, and climate friendly fuel. The chemical interactions between the metal and hydrogen atoms, number, type, and size of the interstitial sites for hydrogen mainly determine the storage capacity of hydrogen in metals and alloys. Metals and alloys based on transition metals are the most common materials for hydrogen storage. Hydrogen tends to occupy tetrahedral interstitial sites in these materials.
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][2][3][4][5][6][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][12][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][15][16][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][23][24][25][26], still there are issues in regard to their structural stability with hydrogenation. The icosahedral phase (I-phase) in Zr 69.5 Al 7.5 Cu 12 Ni 11 alloy is metastable. When the Zr 69.5 Al 7.5 Cu 12 Ni 11 alloy is annealed for a longer time, the metastable Iphase decomposes into crystalline phases [27][28][29][30][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 Zr 69.5 Al 7.5 Cu 12 Ni 11 ribbons [16]. During annealing at higher temperatures, the I-phase decomposed by a discontinuous transformation into tetragonal Zr 2 Cu, tetragonal Zr 2 Ni, and hexagonal Zr 6 NiAl 2 starting with a precipitation reaction of Zr 2 Cu [32][33][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 Zr 69.5 Al 7.5 Cu 12 Ni 11 quasicrystalline alloy is of special interest.

Investigation of nominal composition: Zr 69.5 Al 7.5 Cu 12 Ni 11
The Zr 69.5 Al 7.5 Cu 12 Ni 11 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.
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 Zr 69.5 Al 7.5 Cu 12 Ni 11 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 Zr 69.5 Al 7.5 Cu 12 Ni 11 alloy. Heating the melt-spun ribbon from room temperature up to elevated temperatures reveals a distinct endothermic event characteristic of the glass transition (T g ) at 624 K. With increasing temperature the DSC curve exhibits two peaks (T p1 and T p2 ) with onset temperatures T x1 and T x2 (702 K and 727 K, respectively). The supercooled liquid region, defined as ∆T x = T x1 −T g corresponds to the stability of the supercooled liquid against crystallization and is found to be 78 K. The melting temperature (T m ) of the melt-spun Zr 69.5 Al 7.5 Cu 12 Ni 11 is found to be 1064 K. The first peak (T p1 ) results from the quasicrystal (qc) formation. The second peak (T p2 ) corresponds to the decomposition of quasicrystals as well as the transformation of the remaining amorphous matrix into a crystalline phase. 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 T x1 ) 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 T x2 ) leads to the formation of a tetragonal Zr 2 Cu-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 Zr 69.5 Al 7.5 Cu 12 Ni 11 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 Zr 69.5 Al 7.5 Cu 12 Ni 11 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 Zr 69.5 Al 7.5 Cu 12 Ni 11 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 Zr 69.5 Al 7.5 Cu 12 Ni 11 alloy is only quasicrystalline phase.

Effect of material tailoring on the hydrogen storage properties
The structure and phase formation of Zr-Al-Cu-Ni alloy system is strongly affected by material tailoring. Several studies reveal that the compositional changes may influence the hydrogen storage capacity of Zr-based alloys [19][20][21]. In the present chapter, the effect of Ti addition on the phase formation and the hydrogen storage characteristics of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys with 0 ≤ x ≤ 12 have been discussed. The influence of hydrogenation from the gas phase on the structure/microstructure and microhardness behavior of these melt-spun quasicrystal-line alloys have been studied. The present study deals with the synthesis, phase formation, change in the microstructure with respect to Ti addition, and their correlation with hydrogen storage characteristics of Zr-Al-Cu-Ni alloys. Figure 3a shows the XRD patterns of as-synthesized (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4, 12 at. %) melt-spun alloys. It can be 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 in nature. The TEM image and its corresponding SAED pattern displaying diffuse halos for the alloy with x = 12 is shown in Figure 3b. The TEM microstructure for the samples with x = 12 or less displays a featureless contrast, which is the typical appearance of an amorphous phase. The DSC scans for the samples recorded at a heating rate of 20 K/min are shown in Figure 4. Table 1 gives the thermal stability data for all the investigated samples. With increasing Ti content, the onset crystallization temperature (T x ) decreases gradually, whereas the change of glass transition temperature (T g ) is very subtle. The crystallization for the alloys with x = 0-12 follows a two-step process.

Microstructural and structural features
The primary crystallization products of these melt-spun alloys were examined after isothermal annealing. Figure 5 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 425°C (for x = 0 and 4) and 420°C (for x = 12). The respective annealing temperatures for these alloys have been found through DSC investigation (c.f. Table 1). The crystallization processes of alloys with x = 0-12 show the formation of quasicrystalline phase. The indexing of the Bragg peaks in the XRD patterns was done on the basis of I-phase [36,37]. For the alloy with x = 12, the XRD pattern shows considerable peak broadening as compared to the alloys with x = 0 and 4. This is for the reason that the size of the precipitates is decreasing with increasing Ti content. The quasicrystalline-phase formation in these samples was further investigated by TEM. The nanometer-sized quasicrystalline grains of the annealed samples for the alloys with x = 0, 4, and 12 are shown in Figure 6a-c. The grain size of quasicrystals decreases with addition of Ti. The quasicrystal grains with average size of ~125 nm and ~80 nm have been observed for the alloys with x = 0 and 4, respectively. The presence of characteristic fivefold icosahedral symmetry can be seen in the corresponding SAED pattern. The alloy with x = 12 reveals the formation of 5-10 nm grains. Thus, the strong reduction in the grain size has been observed for this alloy. The corresponding SAED pattern for the alloy with x = 12 shows the presence of diffraction rings (inset in Figure 6c) indexed with I-phase. The diffraction patterns with diffuse ring confirm the presence of a residual glassy phase after the first devitrification step. These results show that the size of the quasicrystalline grains decreases with addition of Ti in (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys. The nucleation rate of the Iphase increases with increase in the concentration of Ti. Thus, in the present case, it can be said that the addition of Ti affects the nucleation and growth characteristics of quasicrystals significantly.   New Advances in Hydrogenation Processes -Fundamentals and Applications  A quantitative energy-dispersive X-ray (EDX) analysis was employed for the composition determination. Figure 7a-c shows EDX spectra of the (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys with x = 0, 4, and 12, respectively. Table 2 presents EDX quantitative analysis along with deviations of the (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0-12) alloys. The semiquantitative analysis of EDX pattern at 3-5 points gives the deviations in the elemental compositions. Based on the EDX quantitative analysis, it has been found that the investigated compositions of the alloys are very close to stoichiometric proportions of nominal compositions. The presence of oxygen within the detectable limit of EDX was not found.

Hydrogenation characteristics
In this section, we discuss the results of the hydrogenation characteristics of three glassy composites. The computerized pressure-concentration-temperature (PCT) apparatus supplied by Advance Materials Corporation (USA) has been used to study the hydrogen sorption characteristics of the partially crystallized ribbons. The gas reaction control-based software has been used to monitor the temperature, pressure, and gas desorbed/absorbed through the samples. The estimated error in the hydrogen storage capacity measurement for the alloys is ±0.02 wt.%. Temperature-programmed desorption (TPD) experiments at a heating rate of 5°C/ min were also performed in addition to hydrogen sorption behavior. The absorption kinetic experiments were performed at fixed temperature and pressure to study the hydrogen storage characteristics of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4, and 12) quasicrystal-glass composites. It may be noted that hydrogen storage here is through dissociation of hydrogen molecule into hydrogen atoms. The catalytic activity of surface transition metal atoms is responsible for the dissociation of hydrogen molecule into hydrogen atoms. The dissociation is followed by diffusion of hydrogen atoms in the interstitial sites. Different temperatures and pressures range from 250 to 300°C and 5-10 MPa for 2-3 h have been performed for the hydrogenation experiments. The best results were found with temperature 270°C and 8 MPa of hydrogen pressure. The hydrogenation for the partially crystallized ribbons was done in a high-pressure reactor. The reactor containing the ribbons was evacuated by a rotary pump (10 −2 Torr), heated up to 270°C, and then charged with 8 MPa of hydrogen pressure.
The hydrogen absorption characteristics for the alloys with x = 0, 4, and 12 at 270°C temperature and 8 MPa hydrogen pressure are shown in Figure 8. The absorption kinetics and hydrogen uptake capacity of the composites increase with increasing concentration of Ti (as shown in Table 3). The hydrogen storage capacity for the alloys with x = 0, 4, and 12 is found to be 1.20 wt. %, 1.38 wt.%, and 1.56 wt.%, respectively. The alloy with x = 0 having less hydrogen storage capacity of 1.20 wt.% with 100-150 nm grains whereas the alloy with x = 4 can store 1.38 wt.% of hydrogen with 60-100 nm grains at same conditions of temperature and pressure. The hydrogen storage capacity further increases to 1.56 wt.% for the alloy with x = 12 for grain size in the range 5-10 nm. It may be noted here that the experimental conditions for the synthesis of melt-spun ribbons were identical for all the alloys. In the present case, the enhancement in storage capacity is ~23% over the normal storage capacity of Zr 69.5 Al 7.5 Cu 12 Ni 11 quasicrystalline phase.   Table 3. Hydrogenation characteristics of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4, and 12) quasicrystal-glass composites (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).
The probable reasons for the enhancement in the hydrogen storage capacity of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys are: (i) it has been observed by XRD and TEM analysis that the grain refinement occurred with increase in the concentration of Ti. The increase of grain boundary density provides better interaction between hydrogen and I-phase and consequently increases the hydrogen storage uptake capacity. The enhancement in hydrogen storage capacity due to grain refinement has also been reported in Ti-Zr-Ni and Mg-Ni-Mm alloys [22,38]. (ii) The other probable reason for the increase in hydrogen absorption may also be due to Ti addition. The addition of Ti in Zr 69.5 Al 7.5 Cu 12 Ni 11 quasicrystalline alloy may have twofold effects on the hydrogen storage behavior. First, the catalytic effect of Ti plays an important role in the improvement of hydrogen storage behavior. The Ti addition enhances the dissociation of hydrogen molecule to hydrogen atom at the surface due to its catalytic effect [36,37,39]. Second, the overall affinity for hydrogen increases due to the increase in the (Zr + Ti) combined content, from 69.5 for x = 0 to 73.2 for x = 12 and might lead to an increased number of energetically favorable sites for hydrogen. Thus, it can be said that the combined effect of the above two probable reasons may lead to improved hydrogen storage characteristics of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys.
Temperature-programmed desorption (TPD) experiment at heating rate 5°C/min has been done to study the hydrogen desorption behavior of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4, and 12) quasicrystal-glass composites. The desorption curves of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4, and 12) alloys are shown in Figure 9. The nature of desorption curves for hydrogenated ribbons changed with increasing Ti addition, and this may be attributed to the change in the microstructure of the ribbon with addition of Ti. A small decrease in desorption temperature has been observed at higher Ti content of x = 12. It can be seen from Figure 9 that full desorption was not observed in the case of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys. This behavior is in contrast to stable Ti-based quasicrystal Ti 45 Zr 38 Ni 17 , which allows nearly full desorption [22,40].

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 (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0 and 12) ribbons are shown in Figure 10. This reveals that Iphase peaks coexist with small concentration of crystalline hydride. This hydride phase has been recognized as ZrH 2 which has a tetragonal lattice with space group: I4/mmm, a = b = 3.519 Å, and c = 4.450 Å. Several earlier studies [15,16] have reported the formation of such hydride phase on hydrogenation of I-phase in Zr-Al-Cu-Ni alloy. The partial decomposition of I-phase may lead to the formation of hydride phase [15]. During charging from the gas phase, the volume fraction of the quasicrystalline phase exhibits a significant effect on the formation of hydrides [15,25,41]. The decrease in the intensity as well as the significant broadening of the I-phase peaks is evident in the XRD patterns for the hydrogenated samples. This is due to the decrease in the size of the grains after hydrogenation. It has been observed that the I-phase peaks are shifted to smaller angle, thus indicating the lattice expansion upon hydrogenation. In the present study, the X-ray diffraction patterns are not only used for the identification of phase transformations with hydrogenation but also used to estimate the quasilattice parameters of hydrogenated and partially crystallized ribbons ( Table 3). The XRD patterns for the hydrogenated ribbons show the formation of crystalline hydride phase along with I-phase. We believe 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. Figure 11a and b shows the TEM micrographs of the hydrogenated ribbons with x = 0 and x = 12, respectively. In comparison with Figure 6a 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 the inset of Figure 11a) has been observed for the alloy with x = 0. The weak diffraction spots superimposed on the diffuse halo ring of the amorphous phase can be seen in the SAED pattern for the alloy with x = 12 (inset of Figure 11b). These results are in agreement with the results obtained by Zander et al. [41]. It has been observed earlier that even weak hydrogenation may lead to the generation of defects in the icosahedral structure. These defects may be responsible for the diffuseness and domain formation in the microstructure. This would lead to a weakening of the contrast of quasicrystal grains as well as the diffraction spots [17,41].
where P is the load (g) and d is the diagonal length in μm. The mean hardness values of at least five loads are reported here with deviations. A notable difference in the indentation behavior of as-synthesized, quasicrystal (qc)-glass composite and hydrogenated qc-glass composite has been observed for the alloys with x = 0, 4, and 12 (c.f. Figure 12a-c). Cracks around the indentation impression start to appear when the applied load goes above a certain value. The microhardness tests in the present study were carried out up to this critical load. It can be seen from the hardness versus load characteristic curves that the microhardness tests were carried out up to load of 100 g for the qc-glass composites and 50 g for the hydrogenated qc-glass composites. Hence, load to fracture decreases, and this reveals the decrease in the fracture toughness for the hydrogenated samples. The significant increase in the hardness for the qcglass composites as compared to as-synthesized ribbons has been observed, and this is due to the precipitation of nano-quasicrystal grains in the amorphous matrix [45][46][47][48][49][50][51][52][53]. Minor changes in the microhardness behavior after hydrogenation have been observed for the qc-glass composites. Table 4 gives hardness values at 50 g of load for the as-synthesized, qc-glass composites, and hydrogenated qc-glass composites. The hardness values of hydrogenated qcglass composites of x = 0, 4, and 12 at 50 g load are found to be ~7.33 GPa, ~7.57 GPa, and 7.93 GPa, respectively. These are slightly higher than that of qc-glass composites. The microstructural variation and the partial decomposition of I-phase into crystalline hydride phase during hydrogenation may lead to slight increase in the hardness values. The microhardness might change during hydrogenation by the interstitial content and microstructural changes, e.g., phase transformations as well as precipitations [54][55][56][57][58][59][60].  Table 4. Values of hardness (VHN) (GPa) at 50 g load of as-synthesized, quasicrystal (qc)-glass composites, and hydrogenated qc-glass composites of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

Conclusion
In this chapter, the hydrogenation behavior of Zr-based quasicrystalline alloy system has been discussed. Based on this study, it may be said that the hydrogen storage capacity of partially quasicrystalline (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x (x = 0, 4 and 12) alloys has been improved by the addition of Ti. 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 alloy with x = 12 is ∼23% as compared to the storage capacity of alloy with x = 0. The grain refinement of quasicrystals resulting due to Ti addition has led to the improvement in the hydrogen uptake capacity of (Zr 69.5 Al 7.5 Cu 12 Ni 11 ) 100−x Ti x alloys. The microhardness shows only minor changes after hydrogenation of quasicrystal-glass composites. The microstructural and morphological changes affect the hydrogen storage characteristics of quasicrystal-glass composites.