Pressure dependence of hydrogen-induced amorphization in C15 laves phases TbM2 (M=Fe, Co and Ni)

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 H-2 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 TbNi5Hx) and decomposition of the remaining amorphous alloys. Exceptionally, a single-phase amorphous a-TbFe2Hx was not formed below 0.2 MPa H-2 in TbFe2, because HIA and precipitation of TbH2 simultaneously occurred. On the other hand, a single-phase a-TbNi2Hx was not formed below 1.0 MPa H-2 because of the low hydrogen contents in c-TbNi2Hx. 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.