A study on hydrogen-induced amorphization in C15 Laves phase DyNi ₂ under different hydrogen pressures

Four exothermic reactions of (1) hydrogen absorption forming crystalline (c-) and amorphous (a-) DyNi₂H_x, (2) the transformation of c-DyNi₂H_x to a-DyNi₂H_x, i.e., hydrogen-induced amorphization (HIA), (3) the precipitation of DyNi₅ in a-DyNi₂H_x and (4) the decomposition of the remaining amorphous hydride into DyNi₅ and BiF₃-type DyH₃ were observed sequentially when the C15 Laves phase DyNi₂ was heated using a pressure differential scanning calorimeter (PDSC) at 2.0 MPa H ₂ or higher hydrogen pressures. On the other hand, (1) the formation of c- and a-DyNi₂H_x, (2) partial HIA and (3) the decomposition of the mixture of a- and c-DyNi₂H_x into DyNi₅ and DyH₃ (or DyH₂) were observed below 2.0 MPa H₂. Simultaneous formations of c- and a-DyNi ₂H_x at the first exothermic peak were explained by the local temperature increment resulting from the low activation energy for hydrogen absorption forming c-DyNi₂H_x. On the other hand, the formation of DyNi₅ during the decomposition was explainable in consideration of the large formation enthalpy of DyNi₂. Furthermore, the mechanism of the phase transformations of DyNi₂ heated in a hydrogen atmosphere was discussed on the basis of the experimental results such as the pressure dependence of T_p and T_p/T_m (the peak temperature/the melting point of DyNi₂), the activation energy E_A, and the enthalpy change ΔH.