Influence of hydrogen on formation and stability of Zr-based quasicrystals

Hydrogenation of quasicrystalline Zr69.5Cu12Ni11Al7.5 was performed electro chemically in a 2:1 glycerine-phosphoric acid electrolyte. Hydrogen absorpt ion/desorption as well as the influence of hydrogen on the formation and st ability was studied by means of X-ray diffraction (XRD), transmission elect ron microscopy (TEM?) and thermal desorption spectroscopy (TDS). In compari son to amorphous Zr-Cu-Ni-Al the absorption kinetics in quasicrystalline Zr -Cu-Ni-Al were found to be faster. Desorption is hindered in both materials probably due to the formation of thin ZrO2 layers. Only partial desorption of hydrogen was observed by means of TDS to occur prior to the decompositi on of the quasicrystalline phase. The observed increase in length during hy drogen charging indicates an interesting new result. Instead of the continu ous increase of the specific volume per hydrogen atom at very low hydrogen concentration typical for the filling up of larger trapping sites, a decrea se from a rather high value is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty one s. Above a hydrogen content of H/M=0.05 the formation of Zr-Cu-Ni-Al quasicrys tals is replaced by tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiA l2. At high hydrogen contents (about H/M=1.0) phase separation is assumed t o take place followed by the formation of nanocrystalline ZrH2. Icosahedral Zr69.5Cu12Ni11Al7.5 was found to decompose through a discontinuous transfo rmation by complex precipitation reactions; it transforms mainly into tetra gonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2. Hydrogenation of quas icrystals at low concentrations does not change the phases formed during de composition. At hydrogen contents higher than H/M=1.1 decomposition of quas icrystals starts with the precipitation of tetragonal ZrH2-x. (C) 2000 Else vier Science B.V. All rights reserved.