Amorphous phase formation by hydrogen absorption

Abstract

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 D0 19 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 TbFe 2 compounds investigated by DTA, DSC, XRD, TEM, Mössbauer spectroscopy, magnetic measurements and hydrogen analysis, the factors controlling HIA are presented. DTA curves of C15 Laves RFe 2 (R=a rare-earth metal) in a hydrogen atmosphere show exothermic peaks resulting from hydrogen absorption, hydrogen-induced amorphization, precipitation of RH 2 and crystallization of amorphous phases. As the melting temperature T m of the C15 Laves phase RFe 2 increases, hydrogen absorption temperature T h, HIA temperature T a and crystallization temperature T x increase. On the contrary, T h/ T m, T a/ T m and T x/ T m 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 TbFe 2 during heating in a hydrogen atmosphere are evaluated by DSC. Finally the mechanism of HIA for the C15 Laves RM 2 phases is discussed on the basis of the experimental data.