Hydrogen-Induced Amorphization in Intermetallic Compounds

This article briefly reviews our recent studies on the hydrogen-induced amorphization (HIA), i.e., the transformation from the crystalline to the amorphous state by hydrogen absorption, of the intermetallic compounds. The experimental evidence of amorphization, the amorphizing alloy systems, the progress and the mechanism of HIA, and the influence of HIA on the magnetic properties of the intermetallic compounds are described

Hydrogenation of Amorphous and Crystalline RE-Ni Alloys

Amorphous alloys with the combination of a rare earth metal(RE=Y, La, Ce, Pr and Sm) and nickel were prepared around the composition at an eutectic point by the melt-quenching technique. Amorphous Y-Ni and Sm-Ni alloys absorbed a large amount of hydrogen even in the amorphous state, while the the other alloys in La-Ni, Ce-Ni and Pr-Ni systems were decomposed to a mixture of crystalline phase and hydride during hydrogenation at 323 K. An amorphous SmNi_2 phase was also synthesized by reaction of hydrogen gas with the crystalline SmNi_2 compound below about 500 K. The hydrogen absorption capacity of the melt-quenched amorphous Sm-Ni alloys was smaller than those of both the corresponding crystalline alloys and the amorphous SmNi_2 alloy obtained by reaction of hydrogen

Crystallization of Amorphous GdFe_2H_x Alloys Prepared by Hydrogen Absorption

Amorphous a-GdFe_2H_x alloys were prepared by two kinds of technique, i.e., the newly developed hydrogen-induced amorphization (HIA) of the Laves phase c-GdFe_2 and hyrogenation of the rapidly quenched amorphous a-Gd_Fe_ alloy. The formation of a GdFe_2H_x by hydrogen absorption was found to occur between 473 K and 573 K where the decomposition of c-GdFe_2 into the elemental hydride GdH_2 and α-Fe is suppressed. On the other hand, the rapidly quenched a-Gd_Fe_ alloy could absorb hydrogen in the amorphous state below 523 K. The crystallization behavior of the hydrogen-induced amorphous a-GdFe_2H_ alloy was similar to that of the hydrogenated amorphous a-Gd_Fe_H_ alloy. That is, the DSC curves of the a-GdFe_2H_x alloys showed a broad endothermic peak resulting partial desorption of hydrogen together with two exothermic peaks. The first exothermic peak was associated with the crystallization of the amorphous phase into GdH_2 and α-Fe, and the second one was attributed to the growth of them. On the other hand, a-Gd_Fe_ crystallized polymorphously to the Laves phase c-GdFe_2 showing an exothermic peak

Nitrogen-Induced Amorphization of BCC Type Solid Solution Alloys by Mechanical Alloying

Mixtures of elemental metal powders consisting of the group 5A and 6A metals, which form the bcc type solid solution alloys, have been mechanically alloyed using the planetary ball mill in an argon and nitrogen atmosphere. The products were characterized by X-ray diffractometry, transmission electron microscopy, differential scanning calorimetry and chemical analysis. The bcc type solid solution alloys were produced by mechanical alloying (MA) in an argon atmosphere. On the contrary, amorphization by the additive effects obtained by combining MA with nitrogenation was demonstrated in the alloys consisting of the group 5A and 5A metals, the group 6A and 6A metals, and Ta_Mo_. The factors controlling nitrogen-induced amorphization during MA and the formation conditions of the amorphous alloys were discussed

Hydrogen-Induced Amorphization in Zr_3Al Compound with L1_2 Structure

The process of hydrogen-induced amorphization in the L1_2 type intermetallic compound Zr_3Al has been investigated by x-ray diffraction and transmission electron microscopy. Then, it has been compared with that of the isomorphous Zr_3In. Zr_3Al amorphizes below about 500 K where the decomposition of it into ZrH_2 and Al is substantially suppressed in contrast to Zr_3In. Almost complete disordering precedes the onset of amorphization in the same way as irradiation-induced amorphization of this compound. The DSC result suggests that hydrogen atoms in a-Zr_3AlH_4 are trapped tightly in comparison with those in a-RM_2H_x (R=a rare earth metal, M=Fe, Co, Ni) prepared by hydrogenation

Hydrogen-Induced Amorphization of Nd_3Ga and Sm_3Ga Compounds with D0_<19> Structure

Structural changes of c-R_3Ga ( R=Nd, Sm ) compounds having the D0_ structure during the hydrogen absorption process were investigated by x-ray diffraction, electron microscopy, and the thermal analysis. By hydrogenation of c-Nd_3Ga and c-Sm_3Ga compounds, an amorphous single phase a-Nd_3GaH_ and a-Sm_3GaH_ was formed between 300 K and 400 K, respectively. However, hydrogenation of the compounds gave rise to the transformation from the D0_ phase to c-Nd_3GaH_x and c-Sm_3GaH_x having the fcc structure above 450 K. Crystallization of the hydrogen-induced amorphous alloys was studied by differential scanning calorimetry from which the crystallization temperature, the enthalpy change, and the activation energy of crystallization were determined

Hydrogenation of Amorphous and Crystalline Gd-Co Alloys

Amorphous Gd-Co alloys were prepared over a wide composition range from 45 to 70 at % Gd by melt-quenching. These alloys absorbed a large amount of hydrogen in the amorphous state below 423 K. The hydrogen absorption capacity for the amorphous alloys was lower than that for the corresponding crystalline alloys and it increased with increasing Gd content in the alloy. The number of hydrogen atom absorbed per one gadolinium atom was approximately two with regardless to the alloy composition. On the other hand, the Laves phase compound GdCo_2 was changed to an amorphous phase after reaction with hydrogen below about 700 K, among all of intermetallic compounds formed in Gd-Co system. This amorphous alloy did not show a pressure plateau in the pressure-composition isotherm, in the same manner as other melt-quenched amorphous alloys