Amorphization of C15 Laves RFe 2 compounds by hydrogen absorption
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Amorphization of C15 Laves RFe2 compounds by hydrogen absorption
K. Aoki, M. Dilixiati and K. Ishikawa Department of Materials Science, Faculty of Engineering, Kitami Institute of Technology, 165 Koencho, Kitami, Hokkaido 090-8507, Japan ABSTRACT C15 Laves RFe2 (R= Y, Ce, Sm, Gd, Tb, Dy, Ho and Er) compounds were thermally analyzed to elucidate conditions of hydrogen-induced amorphization (HIA), i.e. amorphization of intermetallics by hydrogen absorption, using a pressure scanning differential calorimeter (PDSC) in a hydrogen atmosphere of 1.0 MPa. As the temperature increases, hydrogen absorption in the crystalline state, HIA, precipitation of RH2 in the amorphous phase and decomposition of the remaining amorphous phase into RH2+α-Fe occur exothermally for RFe2 (R=Y, Sm, Gd, Tb, Dy and Ho). HIA and precipitation of ErH2 occur simultaneously in ErFe2. On the contrary, hydrogen absorption of CeFe2 always leads to HIA, i.e. no hydrogen absorbed crystalline phase is formed. The mechanism of HIA in the C15 Laves phases RFe2 is discussed on the basis of the experimental results. INTRODUCTION Amorphous alloys having unique mechanical, physical and chemical properties are usually prepared by rapid quenching of molten alloys. About 20 years ago, a quite different method to make amorphous alloys, i.e. hydrogen-induced amorphization (HIA), was demonstrated in metastable Zr3Rh by Yeh et al. [1]. Subsequently, HIA has been found in many intermetallics having the C15, B82, C23, D019 and L12 structures and containing a large amount of a hydrideforming element [2-9]. HIA is scientifically interesting, because crystalline hydrides transform to corresponding amorphous ones in contrast to crystallization of conventional amorphous alloys. In addition, HIA is practically important, because it is closely related to disproportionation which controls the lifetime of hydrogen storage alloys. However, the mechanism of HIA is still uncertain. In order to understand the mechanism of HIA, it is useful to know conditions of HIA. Generally, HIA occurs slowly by isothermal annealing of intermetallics in a hydrogen atmosphere. However, HIA of the C15 Laves phases RM2 (R= rare earth metal, M= Fe, Co, Ni) proceeds quickly, so that the transformation from the crystalline to the amorphous hydride can be observed by the thermal analysis. In the present work, structural changes of the C15 Laves phases RFe2 in a hydrogen atmosphere of 1.0 MPa are investigated in order to elucidate the conditions of HIA using a pressure differential scanning calorimeter (PDSC). The mechanism of HIA in the C15 Laves phases RFe2 is discussed on the basis of the experimental results. EXPERIMENTAL PROCEDURE RFe2 (R= Y, Ce, Sm, Gd, Tb, Dy, Ho and Er) alloys were prepared using high purity metals by arc melting in an argon atmosphere. The ingots were homogenized at 973 K for 1 week in evacuated quartz tubes. The thermal analysis was carried out using a PDSC at a heating rate of 0.17 K/s and in a hydrogen atmosphere of 1.0 MPa. The origin of each ther
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