In order to study the amorphous structure, the hydrogen desorption behavior was investigated in the hydrogenated amorphous Zr-Ni alloys by the hydrogen thermal desorption technique. The study of thermal desorption of hydrogen from amorphous Zr-Ni alloys provided evidence for a distribution of hydrogen among sites of different energy caused by the disorder in an amorphous metal. The amorphous Zr-Ni alloys had at least two kinds of hydrogen occupation sites of different chemical environment.
The different types of sites for hydrogen were identified as $Zr_4$, $Zr_3Ni$, $Zr_2Ni_2$ tetrahedral sites. The relation of hydrogen concentration between the different types of tetrahedral sites, $Zr_{4-n}Ni_n$(n=1,2,3,4) was thought to be consistent with the Harris statistical model, but the maximum hydrogen concentration for Zr-rich amorphous Zr-Ni alloys was suggested to have the value, $2.0 ≤ (H/M)^{max} ≤ 2.5$. The dissolution of hydrogen in the amorphous Zr-Ni alloys was exothermic and pressure-concentration isotherms show positive deviations from Sievert's law at temperatures of 423K and 523K for hydrogen pressures upto 1 atm which was related to the distribution of hydrogen atoms in sites of different energies.
In an amorphous $Zr_{36}Ni_{64}$ alloy, there might be a hydrogen induced rearrargement of metal atoms during hydrogenation, which could be suggested from the experimental phenomena of hydrogen desorption peak separation with hydrogenation time at the hydrogenation condition ; T=523K, P=1 atm $H_2$. Hydrogen desorption peak separation was thought to indicate the generation of new type of interstitial sites for hydrogen, which was suggested to be $Zr_3Ni_2$ hexahedral sites from the analyses by TEM observation, RDF calculation and hydrogen thermal analysis with annealing treatment and rehydrogenation after desorbing hydrogen to the temperature, 723 K. Hydrogen induced atom rearrangement seemed to be the reversible movement within 1 atomic distance and the result from the relation of the CSRO of the amorphous Zr-Ni system with large interaction force between the hydrogen and zirconium atoms.
The thermal stability of amorphous $Zr_xNi_{100-x}$ (x=50,64,75) alloys was very deteriorated with the hydrogen dissolution. In the temperature range (523 - 623 K) far below the crystallization temperature (673-743 K) of the amorphous $Zr_xNi_{100-x}$ alloys, crystallization into $ZrH_2$ phase occurred. The activation energy of the formation of $ZrH_2$ phase in the amorphous $Zr_xNi_{100-x}$:H (x=50,64,75) alloys was obtained as 135.0 ± 10 kJ/mol, which was very smaller than that of the crystallization of amorphous $Zr_{64}Ni_{36}$ alloy, 346.8kJ/mol.
The mechanism of the formation of $ZrH_2$ phase was suggested to be related with the diffusion of Ni atoms on the basis of the similarity of the activation energy, 135.0 ± 10 kJ/mol obtained in this study with that for diffusion of Ni atoms in the amorphous Zr-Ni alloy, 106.8KJ/mol, reported by other researchers, which was different from that of crystallization of the amorphous $Zr_xNi_{100-x}$ (x=50,64,75) alloys, interdiffusion of Zr and Ni atoms.
The amorphous $Zr_{25}Ti_{25}Ni_{50}$ alloy, substituted Ti for Zr in amorphous $Zr_{50}Ni_{50}$ alloy had the good resistance against the deterioration of the thermal stability with hydrogenation. It was thought to give the explanation for the improvement of the thermal stability that the diffusivity of Ni in α-Ti was far smaller that of Ni in α-Zr by 1-2 order.
