FORMATION AND CRYSTALLIZATION OF HYDROGEN-INDUCED AMORPHOUS RNI2HX (R = SM, GD, DY, HO, ER) ALLOYS

The formation ranges of amorphous and crystalline hydrides by hydrogenation of C15 Laves compounds RNi2 (R = Sm, Gd, Dy, Ho, Er) were examined by X-ray diffractometry (XRD) as a function of both the hydrogenation temperature and the hydrogen pressure. An amorphous single phase was formed below about 500 K independent of the hydrogen pressure for SmNi2. On the other hand, it was formed in the range of 300-523 K and 2.0-5.0 MPa H-2 for HoNi2, and in the range of 400-500 K and 4.0-5.0 MPa H-2 for ErNi2. The crystallization behavior of the hydrogen-induced amorphous a-RNi2H(x) alloys was investigated by differential scanning calorimetry (DSC) and XRD. The a-RNi2H(x) alloys decompose into c-RNi5 + c-RH2 for R=Sm and Gd, while they decompose into c-RNi3 + c-RH2 for R = Dy, Ho and Er. The activation energies for crystallization in these amorphous alloys were almost constant at about 150 kJ/mol in spite of their different crystallization behavior. The enthalpy changes for crystallization were about 3.7 kJ/mol for all the amorphous alloys except for a-ErNi2Hx. The crystallization temperatures of these amorphous alloys ranged from 628 to 672 K. The formation process of the hydrogen-induced amorphous a-RNi2H(x) (R = Gd, Dy) alloys was studied by XRD and DSC as a function of the hydrogenation time. Two kinds of the amorphization processes were observed. Amorphization of DyNi2 progressed by the nucleation and growth of the hydrogen absorbed amorphous phase. On the contrary, amorphization of GdNi2 progressed by the polymorphous transformation from the hydrogen absorbed Laves compound to the amorphous phase. The Johnson-Mehl-Avrami (JMA) plots for the process of amorphization gave the numerical exponent n = 3.9 for DyNi2 and n = 0.5 for GdNi2, respectively. The difference in the process of hydrogen-induced amorphization in these compounds was discussed.