**1. 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.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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