In order to study the criteria and the mechanism of hydrogen induced amorphization in rare earth / transition intermetallic compounds, especially in $RM_2$ Laves phases (R = rare earth element, M=transition metals), the structural changes during hydrogen absorption and desorption are investigated by means of X-ray diffraction, DSC (differential scanning calorimetry), hydrogen desorption test, TEM and Sievert's type hydrogenation apparatus.
It has been recently discovered that amorphous alloys can be prepared by solid state reaction. Formation of an amorphous alloy by solid state reaction has been received considerable attention because it is the new method for the preparation of amorphous alloys. Among the solid state reaction methods, the amorphization due to hydrogen absorption (hydrogen induced amorphization) is first demonstrated in rapidly solidified metastable $Zr_3Rh$ by Yeh et al. Later, Masumoto et al. have also observed the phenomena of hydrogen induced amorphization in $RM_2$ Laves phases. The general criteria for hydrogen induced amorphization, suggested by other researchers, are the negative heat of mixing between hydrogen atom and metal atoms and the fast mobility of the metallic constituents for the inhibition of the nucleation and growth of more stable crystalline counterparts. Even though, the general criteria are proposed, the amorphization mechanism and criteria are still uncertain. In order to understand the mechanism and the criteria of the amorphization, it is necessary to know the activation energy, driving force, hydrogen occupation sites after and before the amorphizadtion and the dependence on the crystal structure, stability of a compound and amount of the absorbed hydrogen.
To study the dependence of hydrogen induced amorphization on the crystal structure, stability of a compound and the amount of the absorbed hydrogen, La/Ni systems are selected. In La/Ni systems, various intermetallic compounds such as $LaNi_5$, $La_2Ni_7$, $Lani_3$ $LaNi_2$, LaNix (X=1.5), LaNi, etc. exist and their crystal structure and their stability (it is easily estimated in phase diagram by their melting or decomposition temperature) are markedly different from each other.
In order to study the mechanism of hydrogen induced amorphization, a kinetic study of the amorphization must be performed because the activation energy of the amorphization, which can be measured by a kinetic study, are closely related to the mechanism. For this purpose, $ErNi_2$ Laves phase is selected. Generally, hydrogen induced amorphization occurs immediately when hydrogen is absorbed in the compound. Therefore, the activation every in this compound will include that of the penetration of hydrogen into the surface (for example, activation energy for chemisorption, the jumping of the chemisobed hydrogen into the bulk, etc.). Thus, for the hydrogen induced amorphization kinetic study, the selection of a compound which has no problem of hydrogen penetration into the surface is very important. $ErNi_2$ Laves phase been known that the amorphization proceeds after hydrogen absorption. Thus, $ErNi_2$ is suitable for the study on the amorphization kinetics.
The phenomena of hydrogen induced amorphization in $RFe_2$, $RNi_2$ and $RCo_2$ have been reported. Finally, in this work, by selecting transition metals other than Fe, Co and Ni, which can construct $RM_2$ Laves phases with rare earth element and have the same period with that of Fe, Co and Ni, the phenomena of hydrogen induced amorphization are investigated. Only Mn satisfy the above condition. And then the factors affecting hydrogen induced amorphization are discussed by collecting the reported data.
From the results of the above work, following conclusions can be made; (1) The amorphization of a compound is dependent upon the stability of the compound. Amorphous phase is a metastable one during the phase decomposition into the most stable crystalline phases(crystalline elemental hydreide and other crystalline counterparts). Therefore, the compound with high stability does not decompose into the elemental hydride at a moderate hydrogenation condition. And the compound with a low stability easily decomposes into elemental hydride so that the amorphization does not occur. A compound with a moderate stability is amenable to the amorphization. This fact is confirmed in La/Ni system and $RMn_2$ Laves phases. (2) The mechanism of hydrogen induced amorphization is not related to the motion of hydrogen atom but related to the lattice distortion caused by the motion of a smaller atom in a compound so that the surroundings of the hydrogen atom in the amorphous state is similar to configuration of hydrogen atom in elemental hydride. This fact is confirmed from the amorphization kinetic study. (3) In $RM_2$ Laves phases, two types of the amorphization has been found; the one is the compounds which show a crystalline $RM_2H_X$ prior to amorphization and the other is the compounds which does not show crystalline $RM_2H_X$. The existence of a crystalline $RM_2H_X$ prior to the amorphization depends on the stability of a compound. Only the compound with a high stability show a crystalline $RM_2H_X$. Two types of the amorphization can be explained as an extent of lattice expansion due to hydrogen absorption, which can be estimated by Young's modulus of the compounds. Compounds with high modulus show the existence of crystalline $RM_2H_X$, due to small lattice expansion with hydrogen absorption. And compounds with low modulus do not show the crystalline $RM_2H_X$, due to large lattice expansion which induces the amorphization with no nucleation barrier.
