TRANSFORMATION CHARACTERISTICS OF HYDROGEN-INDUCED AMORPHIZATION OF THE ORDERED ZR3AL PHASE

Hydrogen absorption into the ordered Zr3Al intermetallic compound (Cu3Au type, L1(2) structure) leads to an amorphization at below 550 K. As the reaction temperature increases, the hydrogenated Zr3Al decomposes into the equilibrium phases (ZrH2 + Zr2Al). The amorphous samples are studied by X-ray diffraction, differential scanning calorimetry, optical microscopy, scanning electron microscopy and high-resolution transmission electron microscopy to clarify the transformation mechanism. During DSC scanning, ZrH2 phase is crystallized from amorphous matrix at first. The evolution of hydrogen from amorphous phase begins at low temperature at about 313 K, and continues until the full crystallization of amorphous phase into ZrH2 + Zr2Al. It is observed by optical microscopy that the formation of amorphous phase takes place from the free surface of the sample with a discrete a/c (amorphous/crystal) boundary layer. From SEM observation, many cracks are found at the front of the advancing amorphous phase, which are thought to be caused by the hydrogen absorption. This result clearly indicates the accumulation of the elastic strain by hydrogen absorption. High-resolution TEM on the partially hydrogenated samples show that the spatially continuous amorphous phase and the unreacted crystalline islands is coexisted with a sharp a/c phase boundary, and the mode of the nucleation of amorphous phase is heterogeneous. Electron and X-ray diffraction patterns in this region reveal that the ordered crystalline structure is maintained with a lattice expansion of about 2 vol.%, and there is no indication of the chemical disordering. This volume expansion is considered to be a critical limit for the maintenance of the crystalline structure. Based on the observed experimental results, it is suggested that the amorphization is mainly caused by the elastic strain during hydrogenation.