HYDROGEN-INDUCED AMORPHIZATION OF YNI2 EN HANCED BY REACTIVE MECHANICAL GRINDING

The C15 Laves phase YNi2, which becomes amorphous YNi2Hx by hydrogenat ion, was mechanically ground under various hydrogen partial pressures up to 1.0 MPa to investigate the effect of the mechanical grinding (MG ) on the hydrogen-induced amorphization (HIA) processes. Furthermore, the phase separation processes during dehydriding reaction were also e xamined. The results obtained are summarized as follows. Under the ini tial hydrogen pressure of 1.0 MPa, a single phase of amorphous YNi2Hx is observed by grinding only for 180 min, while such an amorphization can hardly occur even after hydrogenation for 10080 min without grindi ng. However the thermal stabilities of the amorphous phase and the dis solved hydrogen do not change by MG. On the other hand, when ground un der the initial partial hydrogen pressure of 0.2 MPa, YNi2Hx is separa ted into two phases. The one is the alpha-phase, in which the solubili ty of a hydrogen is larger than that obtained by hydriding YNi2 withou t MG under the same condition, and the other is the alpha'-phase. The solubility of a hydrogen in or-phase gradually decreases with increasi ng the MG time, while the alpha'-phase is transformed into the amorpho us phase upon further grinding. After grinding it for 1080 min, the al pha-phase is changed into YNi5. Excess Y left in the phase transformat ion will be dissolved into the amorphous phase and it will react with hydrogen to form YH2. These differences of the amorphization processes depending on the initial hydrogen pressures can be understood by cons idering the free energy variation in each phase by MG.