Pressure Dependence of Hydrogen-Induced Amorphization in C15 Laves Phases TbM₂ (M=Fe, Co and Ni)

Abstract

The hydrogen pressure dependence of structural changes in C15 Laves phases TbM₂ (M=Fe, Co and Ni) were investigated between 0.1 and 5.0 MPa H₂ in order to elucidate conditions and the mechanism of hydrogen induced amorphization (HIA). General thermal reactions occur on heating as follows; hydrogen absorption in the crystalline state, hydrogen-induced amorphization (HIA), precipitation of TbH₂ (or TbNi₅H_x) and decomposition of the remaining amorphous alloys. Exceptionally, a single-phase amorphous a-TbFe₂H_x was not formed below 0.2 MPa H₂ in TbFe₂, because HIA and precipitation of TbH₂ simultaneously occurred. On the other hand, a single-phase a-TbNi₂H_x was not formed below 1.0 MPa H₂ because of the low hydrogen contents in c-TbNi₂H_x. The present work indicates that hydrogen absorption more than 1.0 (H/M) is essential for the occurrence of HIA in the C15 Laves phases TbM₂.