Pressure dependence of hydrogen-induced transformations in C15 Laves phase DyFe 2 studied by pressure differential scanning calorimetry

Abstract

By thermal analysis of DyFe 2: (1) hydrogen absorption; (2) hydrogen-induced amorphization (HIA); (3) the precipitation of BiF 3-type DyH 3; 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 DyFe 2) for hydrogen absorption, HIA, the precipitation of DyH 3 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 DyH 3 shows a small and positive one. As a consequence of such pressure dependence, HIA overlaps with the precipitation of DyH 3 at 0.2 MPa H 2, while the crystalline hydride decomposes directly into α-Fe and DyH 3 at 0.1 MPa H 2. The activation energy E A for hydrogen absorption, HIA, the precipitation of DyH 3 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.