Pressure dependence of hydrogen-induced transformations in C15 Laves phase DyFe2 studied by pressure differential scanning calorimetry

By thermal analysis of DyFe2: (1) hydrogen absorption; (2) hydrogen-induced amorphization (HIA); (3) the precipitation of BiF3-type DyH3; and (4) the decomposition of the remaining amorphous hydride occur exothermically with increasing temperature at 1.0 MPa H-2. T-p/T-m (the peak temperature/the melting temperature of DyFe2) for hydrogen absorption, HIA, the precipitation of DyH3 and the decomposition of the amorphous hydride are 0.28, 0.36, 0.43 and 0.48, respectively, which are closely related with kinetics of the transformations. The peak temperature T-p for HIA shows a large and negative pressure dependence, but that for the precipitation of DyH3 shows a small and positive one. As a consequence of such pressure dependence, HIA overlaps with the precipitation of DyH3 at 0.2 MPa H-2, while the crystalline hydride decomposes directly into alpha-Fe and DyH3 at 0.1 MPa H-2. The activation energy E-A for hydrogen absorption, HIA, the precipitation of DyH3 and the decomposition of the amorphous hydride are calculated to be 56, 75, 308 and 162 kJ/mol Dy, respectively, which are closely related with the mechanism of HIA. The mechanism of HIA is discussed on the basis of the experimental results. (C) 2004 Elsevier B.V. All rights reserved.