Influence of palladium on hydrogenation and crystallization of Zr–Cu–Ni–Al glasses

Hydrogenation of glassy Zr–Cu–(Ni, Pd)–Al alloys was performed electrochemically in a 2:1 glycerin–phosphoric acid electrolyte. In comparison to Zr–Cu–Ni–Al quasicrystals the absorption kinetics in amorphous Zr–Cu–Ni–Al were found to be slower, but the storage capacity is similar. Desorption is probably hindered by the formation of thin ZrO2 layers. Partial replacement of Ni by Pd in an amorphous Zr68.5Cu13Ni11Al7.5 alloy was found to accelerate absorption kinetics. The observed increase in length during hydrogenation indicates an interesting new result: instead of a continuous increase of the specific volume per hydrogen atom, a decrease from rather high value at very low hydrogen content is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty ones. Thermal stability was studied by X-ray diffraction and TEM. Amorphous Zr68.5Cu13Ni10Pd1Al7.5 and Zr68.5Cu13Ni5.5Pd5.5Al7.5 was found to transform into a nanocrystalline microstructure of an icosahedral quasicrystalline phase. Zr68.5Cu13Pd11Al7.5 transform mainly into tetragonal Zr2(Cu, Pd) and an unidentified phase. Above a hydrogen content of H/M=0.05, the formation of quasicrystals is replaced by a tetragonal phase with lattice parameters close to those of Zr2Ni. At high hydrogen contents (about H/M=1.0) phase separation is assumed to take place followed by the formation of nanocrystalline ZrH2.