Amorphous phase formation by hydrogen absorption

This article describes our recent investigations on the amorphous phase formation by hydrogen absorption, i.e. hydrogen-induced amorphization (HIA) of intermetallic compounds. HIA for D019 and C15 Laves compounds is demonstrated at first. Next, chemical compositions and crystal structures of amorphizing compounds are reviewed. Subsequently, we focus on HIA of the C15 Laves compounds. HIA of C15 Laves phase TbFe2 compounds investigated by DTA, DSC, XRD, TEM, Mِssbauer spectroscopy, magnetic measurements and hydrogen analysis, the factors controlling HIA are presented. DTA curves of C15 Laves RFe2 (R=a rare-earth metal) in a hydrogen atmosphere show exothermic peaks resulting from hydrogen absorption, hydrogen-induced amorphization, precipitation of RH2 and crystallization of amorphous phases. As the melting temperature Tm of the C15 Laves phase RFe2 increases, hydrogen absorption temperature Th, HIA temperature Ta and crystallization temperature Tx increase. On the contrary, Th/Tm, Ta/Tm and Tx/Tm are nearly constant, being 0.28, 0.4 and 0.5, respectively, suggesting that HIA occurs by the diffusion of metallic atoms over a short distance. In addition, the enthalpy change and the activation energy for the thermal reactions of TbFe2 during heating in a hydrogen atmosphere are evaluated by DSC. Finally the mechanism of HIA for the C15 Laves RM2 phases is discussed on the basis of the experimental data.