Pressure Dependence of Hydrogen-Induced Amorphization in C15 Laves Phase TbFe<SUB>2</SUB>

Abstract

The pressure dependence of structural changes of C15 Laves phase TbFe 2 heated in a hydrogen atmosphere was investigated using a pressure differential scanning calorimeter (PDSC), a powder X-ray diffractometer (XRD), a differential scanning calorimeter (DSC) and a hydrogen analyzer. Hydrogen absorption in the crystalline state, hydrogen-induced amorphization (HIA), precipitation of TbH 2 and decomposition of the remaining amorphous alloy into α-Fe and TbH 2 occurred exothermally with increasing temperature above 0.5 MPa H 2 , independent of the heating rate. HIA and precipitation of TbH 2 occurred simultaneously in 0.2 MPa H 2 (0.33 K/s) and in 0.1 MPa H 2 (0.17 K/s). On the contrary, hydrogen absorbed crystalline c-TbFe 2 H 3.5 decomposed directly into α-Fe and TbH 2 in 0.1 MPa H 2 (0.33 K/s). That is, no amorphous phase was formed at the lower hydrogen pressure and at a high heating rate. The reason why HIA occurs above the critical hydrogen pressure and below the critical heating rate was discussed on the basis of the hydrogen content absorbed in the crystalline state of TbFe 2 .

Process and mechanism of hydrogen-induced amorphization in C15 Laves phases RFe2

“…Recently, the present authors have shown that C14 Laves phase NdMn 2 with the atomic size ratio R Nd /R Mn = 1.45 amorphizes also by hydrogenation [17]. Furthermore, it has been reported that structural changes in C15 Laves phases RFe 2 depend on the kind of R [12,13]. That is, hydrogen absorption, HIA, the precipitation of RH 3 and the decomposition of the remaining amorphous phase occur in RFe 2 (R = Sm, Gd, Tb, Dy and Ho).…”

“…HIA has been observed in the intermetallic compounds A x B 1−x consisting of hydride forming (A) and non-hydride forming metals (B) with the crystal structures such as C15, B8 2 , C23, D0 19 and L1 2 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Among these, structural changes in the C15 Laves phases RFe 2 (R = rare earth metals) are particularly interesting, because both a crystalline and an amorphous hydride form depending on the hydrogenation temperature [12]. One of the present authors (Aoki) investigated hydrogen-induced structural changes in C15 Laves phases AB 2 and demonstrated that HIA occurs in the compounds having the atomic size ratio larger than 1.37 [16].…”

Pressure Dependence of Hydrogen-Induced Transformations in C15 Laves Phase DyCo<SUB>2</SUB>

“…It has been reported that crystalline hydrides c-RM 2 H x can desorb hydrogen in the crystalline state and their Ar-DSC curves show only endothermic peaks at relatively low temperatures without exception. 14,15,18,19) The Ar-DSC curve of DyCo 2 heated to above the first The sample heated to above the first exothermic peak (to 400 K) (a,b), the second peak (to 623 K) (c), the third peak (to 723 K) (d) and to the fourth peak (to 798 K) (e).…”

“…Furthermore, T p of HIA for C15 Laves RFe 2 (R ¼ Tb, Er) shows also a same strong negative pressure dependence. [15][16][17][18] It has been proposed that HIA of C15 Laves RM 2 occurs by the short-range diffusion of R and M atoms. Therefore, it is interesting to compare the pressure dependence of HIA for DyCo 2 with that for DyFe 2 .…”

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

“…Among amorphizing compounds, structural changes in the C15 Laves compounds RFe 2 (R = rare earth metals) are particularly interesting, because a crystalline and an amorphous hydride are formed depending on the hydrogenation temperature [7]. More recently, it has been reported that HIA in both TbFe 2 and ErFe 2 occurs at a given hydrogen pressure or higher pressures [12][13][14][15], but that HIA in CeFe 2 does ocevacuated quartz tube. After the homogenization, the ingot was crushed into 100 mesh in acetone to prevent oxidation and ignition.…”