Hydrogen induced transformations in Zr–Cu–Ni–Al quasicrystals

Zr-based quasicrystalline alloys can store hydrogen up to a content close to the highest value known for crystalline materials, but there are questions regarding their structural stability during hydrogen charging. Hydrogenation of Zr69.5Cu12Ni11Al7.5 quasicrystals was observed to increase the (quasi-)lattice constant, but at higher contents also to cause microstructural changes, as indicated by the weakening of the contrast of the quasicrystals in transmission electron microscopy as well as a disappearance of the weak diffraction spots in the corresponding diffraction pattern. A more detailed investigation reveals the formation of approximants at hydrogen contents above H/M=0.4 and a variation in the order of approximants from lower to higher order with increasing amount of hydrogen. In addition, amorphization was observed starting with the nucleation and growth of amorphous hydrides at the interfaces between approximants and the amorphous matrix. The reason for amorphization might be the change in the ratio e/a and/or the expansion of the quasilattice above a critical value. Pd coating can hinder the formation of ZrO2 layers, thus allowing not only faster hydrogen absorption, but also hydrogen desorption. However, only partial desorption occurred prior to the decomposition of the quasicrystalline phase. Only hydrogenation up to about H/M=0.4, probably below the content necessary for the formation of approximants, seems to be fully reversible.