**3.1. Microstructural and structural features**

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

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 quasicrys-

**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

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 [19–21]. In the present chapter, the effect of Ti addition on the phase formation and the hydrogen storage characteristics of (Zr69.5Al7.5Cu12Ni11)100−xTix 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-

**3. Effect of material tailoring on the hydrogen storage properties**

pattern of the quasicrystalline (qc) phase along the fivefold direction.

phase is metastable and transforms into more stable crystalline phase.

114 New Advances in Hydrogenation Processes - Fundamentals and Applications

talline phase.

**Figure 3a** shows the XRD patterns of as-synthesized (Zr69.5Al7.5Cu12Ni11)100−xTix (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 (Tx) decreases gradually, whereas the change of glass transition temperature (Tg) is very subtle. The crystallization for the alloys with x = 0–12 follows a two-step process.

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 (Zr69.5Al7.5Cu12Ni11)100−xTix 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.

**Figure 3.** (a) XRD patterns of as-synthesized ribbons of (Zr69.5Al7.5Cu12Ni11)100−xTix alloys. (b) TEM image and the corresponding diffraction pattern of as-synthesized (Zr69.5Al7.5Cu12Ni11)88Ti12 alloy (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

**Figure 4.** DSC curves of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix alloys [35].


Tg: glass transition temperature; Tx1: first-onset crystallization temperature; Tx2: second-onset crystallization temperature; ΔTx: supercooled liquid region

**Table 1.** Thermal analysis of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix ribbons [35].

**Figure 3.** (a) XRD patterns of as-synthesized ribbons of (Zr69.5Al7.5Cu12Ni11)100−xTix alloys. (b) TEM image and the corresponding diffraction pattern of as-synthesized (Zr69.5Al7.5Cu12Ni11)88Ti12 alloy (reprinted with kind permission from Ref.

[23], Copyright 2013, Elsevier).

**Figure 4.** DSC curves of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix alloys [35].

116 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Table 1.** Thermal analysis of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix ribbons [35].

temperature; ΔTx: supercooled liquid region

**x (at.%) Tg (°C) Tx1 (°C) Tx2 (°C) ΔTx (°C)** 351 429 453 78 355 429 461 74 355 420 474 65 355 413 480 58

Tg: glass transition temperature; Tx1: first-onset crystallization temperature; Tx2: second-onset crystallization

**Figure 5.** XRD patterns of annealed ribbons of (Zr69.5Al7.5Cu12Ni11)100−xTix alloys (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

**Figure 6.** 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 (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

A quantitative energy-dispersive X-ray (EDX) analysis was employed for the composition determination. **Figure 7a**–**c** shows EDX spectra of the (Zr69.5Al7.5Cu12Ni11)100−xTix alloys with x = 0, 4, and 12, respectively. **Table 2** presents EDX quantitative analysis along with deviations of the (Zr69.5Al7.5Cu12Ni11)100−xTix (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.

**Figure 7.** Energy-dispersive spectra of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4 and 12) alloys.


**Table 2.** Compositions of the (Zr69.5Al7.5Cu12Ni11)100−xTix alloys (in at.%) based on EDX quantitative analysis (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

#### **3.2. 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 (Zr69.5Al7.5Cu12Ni11)100−xTix (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 Zr69.5Al7.5Cu12Ni11 quasicrystalline phase.

**Figure 7.** Energy-dispersive spectra of the melt-spun (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4 and 12) alloys.

with kind permission from Ref. [23], Copyright 2013, Elsevier).

118 New Advances in Hydrogenation Processes - Fundamentals and Applications

**3.2. Hydrogenation characteristics**

**Alloy composition x (at.%) Zr (±0.7) Al (±0.3) Cu (±0.5) Ni (±0.5) Ti (±0.5)** 68.7 7.3 12.5 11.5 – 66.1 6.9 12.2 9.8 5.0 60.7 6.5 11.6 8.3 12.9

**Table 2.** Compositions of the (Zr69.5Al7.5Cu12Ni11)100−xTix alloys (in at.%) based on EDX quantitative analysis (reprinted

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 (Zr69.5Al7.5Cu12Ni11)100−xTix (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

**Figure 8.** Hydrogen absorption kinetic curves of (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4 and 12) quasicrystal-glass composites (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).


**Table 3.** Hydrogenation characteristics of (Zr69.5Al7.5Cu12Ni11)100−xTix (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 (Zr69.5Al7.5Cu12Ni11)100−xTix 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 Zr69.5Al7.5Cu12Ni11 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 (Zr69.5Al7.5Cu12Ni11)100−xTix alloys.

Temperature-programmed desorption (TPD) experiment at heating rate 5°C/min has been done to study the hydrogen desorption behavior of (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4, and 12) quasicrystal-glass composites. The desorption curves of (Zr69.5Al7.5Cu12Ni11)100−xTix (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 (Zr69.5Al7.5Cu12Ni11)100−xTix alloys. This behavior is in contrast to stable Ti-based quasicrystal Ti45Zr38Ni17, which allows nearly full desorption [22, 40].

**Figure 9.** Temperature-programmed desorption (TPD) curves of hydrogenated (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4, and 12) quasicrystal-glass composites (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

#### **3.3. Influence of hydrogenation on the structural and microhardness behavior**

The probable reasons for the enhancement in the hydrogen storage capacity of (Zr69.5Al7.5Cu12Ni11)100−xTix 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 Zr69.5Al7.5Cu12Ni11 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

120 New Advances in Hydrogenation Processes - Fundamentals and Applications

Temperature-programmed desorption (TPD) experiment at heating rate 5°C/min has been done to study the hydrogen desorption behavior of (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4, and 12) quasicrystal-glass composites. The desorption curves of (Zr69.5Al7.5Cu12Ni11)100−xTix (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 (Zr69.5Al7.5Cu12Ni11)100−xTix alloys. This behavior is in contrast to

stable Ti-based quasicrystal Ti45Zr38Ni17, which allows nearly full desorption [22, 40].

**Figure 9.** Temperature-programmed desorption (TPD) curves of hydrogenated (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0, 4, and

12) quasicrystal-glass composites (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

(Zr69.5Al7.5Cu12Ni11)100−xTix alloys.

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 **Figure 10**. This reveals that Iphase 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 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.

**Figure 10.** XRD patterns of hydrogenated (Zr69.5Al7.5Cu12Ni11)100−xTix (x = 0 and 12) ribbons (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

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].

**Figure 11.** Bright-field TEM micrographs of hydrogenated (Zr69.5Al7.5Cu12Ni11)100−xTix ribbons with (a) x = 0 and (b) x = 12 (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

Indentation tests were conducted at room temperature in order to understand the effect of hydrogenation on the microhardness behavior. The SHIMADZU HMV-2T microhardness tester was used under different loads. The standard diamond-pyramid-shaped Vickers indenter with a tip size of ~0.5 μm was used. The hardness (H) was computed in GPa units by employing the following relationship [42–44]:

$$H = 1.854 \times 9.8 \times \frac{P}{d^2} \tag{1}$$

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 [45–53]. 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 [54–60].

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

**Figure 11.** Bright-field TEM micrographs of hydrogenated (Zr69.5Al7.5Cu12Ni11)100−xTix ribbons with (a) x = 0 and (b) x = 12

Indentation tests were conducted at room temperature in order to understand the effect of hydrogenation on the microhardness behavior. The SHIMADZU HMV-2T microhardness tester was used under different loads. The standard diamond-pyramid-shaped Vickers indenter with a tip size of ~0.5 μm was used. The hardness (H) was computed in GPa units by

<sup>2</sup> 1.854 9.8 *<sup>P</sup> H= × ×*

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 [45–53]. Minor changes in the microhardness behavior after hydrogenation have been observed for the qc-glass

*<sup>d</sup>* (1)

(reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).

122 New Advances in Hydrogenation Processes - Fundamentals and Applications

employing the following relationship [42–44]:

spots [17, 41].

**Figure 12.** Variation in 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 (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).


**Table 4.** Values of hardness (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 (reprinted with kind permission from Ref. [23], Copyright 2013, Elsevier).
