Hydrogen induced amorphization of $Zr_3Al$ intermetallic compound ($Cu_3Au$ type, $L1_2$ structure) is investigated by means of X-ray diffraction, TEM, optical microscopy and hydrogen thermal desorption analysis. Amorphization by the hydrogenation is found to occur below 550K and 53 atm $H_2$. $ZrH_2$ is initially formed at 570K, and the hydrogenated $Zr_3Al$ transforms into the equilibrium phases ($Zr_2Al, ZrH_2$) at the higher temperature.
The hydrogen content at 523K and 53 atm $H_2$ is determined to be 4.8 in formula unit ratio ($a-Zr_3AlH_{4.8}$). The DSC curve of $a-Zr_3AlH_{4.8}$ shows very broad endothermic peak resulting from the hydrogen desorption together with the exothermic peak attributed to the precipitation of $ZrH_2$ phase.
It is observed from optical microscopy that the formation of amorphous phase takes place from the free surface of sample. The transition layer between $a-Zr_3AlH_{4.8}$ and $c-Zr_3Al$, which seems to be an intermediate stage from the cystalline to amorphous state, is observed. XRD patterns of the transition layer show that the original crystal structure remains in lattice expansion and that the ordered structure is maintained, which implies no appreciable change in long-range order. The lattice expansion of the transition layer by hydrogenation is about 2 vol%.
Based on the above experimental results, it is suggested that the amorphization occurs by the elastic instability caused by the volume expansion during hydrogenation.
