Hydrogen-induced Amorphization Research Articles

Differential thermal analysis of hydrogen-induced amorphization in C15 Laves compounds RFe2

The C15 Laves compounds of RFe2 (R = rare earth metal, Ce, Sm, Gd, Tb, Dy, Ho and Er) were thermal-analyzed by heating in a hydrogen atmosphere of 0.10 to 5.00 MPa at a heating rate of 20 K min−1. Four kinds of exothermic peaks are observed, which represent hydrogenation, amorphization, crystallization and decomposition. The hydrogen pressure plays an important role in hydrogeninduced amorphization (HIA). HIA for each compound can be observed only when the hydrogen pressure is above a critical level. Increasing hydrogen pressure lowers the amorphization temperature and increases the crystallization temperature. In addition, the amorphization temperature and the critical pressure of HIA as well as the crystallization temperature of the corresponding amorphous alloys increase with increase of the atomic number of R.

Hydrogen-induced amorphization in CeAl alloys

Hydrogen-induced amorphization (HIA), i.e., formation of an amorphous phase by hydrogen absorption of intermetallic compounds, has been investigated in the last decade as a novel preparation method of amorphous alloys without liquid quenching. HIA is reported in many compounds, which are almost C15 Laves RM{sub 2} compounds (R = rare earth metal, M = ferromagnetic metal of Fe, Co, Ni). This leads to a question whether HIA is a popular phenomenon limited just to C15 Laves RM{sub 2} compounds. Recently, the present authors have shown that HIA occurs in many R-Al compounds (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er as well as Y). Furthermore, the behavior of amorphization from crystalline state and the behavior of crystallization from the corresponding amorphous state in R-Al alloys are fairly different from those in Laves RM{sub 2} compounds. For a more detail study as presented in this paper, Ce-Al alloys have been chosen because they are taken as representative. In the Ce-Al alloys, there are five intermetallic compounds. As CeAl{sub 3} and Ce{sub 3}Al{sub 11} seldom absorb hydrogen, this study will focus on Ce{sub 3}Al, CeAl and CeAl{sub 2}.

Pulsed microwave plasma assisted chemical vapour deposition of diamond: J. Laimer, S. Matsumoto (Technical University, Vienna, Austria). Int. J. Refractory Metals Hard Mater., vol 14, no 1/3, 1996, 179–184

Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

Synthesis of silicides by mechanical alloying: K. Kobayashi, K. Miwa (National Industrial Research Institute, Nagoya, Japoan). J. Japan Soc. Powder Powder Metall., vol 42, no 1, 1995, 103–107. (In Japanese.)

Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

The thermodynamic characterization of the H-induced amorphization of CeFe 2 and the determination of the energy site distribution using reaction calorimetry

The enthalpy of reaction for the hydrogen-induced amorphization of CeFe 2 has been determined as a function of n = H/CeFe 2. At 323 K the enthalpy for this amorphization reaction is constant equal to −37 kJ/mol 1 2 H 2 from n = 0 to 1.8; it then declines in magnitude to about −24 kJ/mol 1 2 H 2 at n = 3.0. The hydrogen pressure is too small to measure at 323 K except for n ⩽ 2.5. Hydrogen can be partially removed from the amorphized hydride at, e.g. 573 K, permitting the determination of the reaction enthalpy for the reintroduction of the hydrogen into the hypostoichiometric amorphous hydride. Enthalpies could be determined using this technique from about n = 2 to 3. From the relationship between these calorimetrically measured ΔH H values and n for the reintroduction of hydrogen into the hypostoichiometric hydride, i.e. n < 3.0, a part of the energy distribution function for the occupation of the interstices with hydrogen can be determined. The distribution function can be closely described by a Gaussian function with σ = 14 kJ/mol 1 2 H 2 with an average value of ΔH av H = −54 kJ/mol 1 2 H 2. If the distribution is indeed a Gaussian, the ΔH for the following reaction can be obtained, i.e. ▪.

反応性メカニカル・ゲラインディングにより促進されたYNi&lt;SUB&gt;2&lt;/SUB&gt;の水素誘起非晶質化

The C15 Laves phase YNi2, which becomes amorphous YNi2Hx by hydrogenation, was mechanically ground under various hydrogen partial pressures up to 1.0 MPa to investigate the effect of the mechanical grinding (MG) on the hydrogen-induced amorphization (HIA) processes. Furthermore, the phase separation processes during dehydriding reaction were also examined. The results obtained are summarized as follows. Under the initial hydrogen pressure of 1.0 MPa, a single phase of amorphous YNi2Hx is observed by grinding only for 180 min, while such an amorphization can hardly occur even after hydrogenation for 10080 min without grinding. However the thermal stabilities of the amorphous phase and the dissolved hydrogen do not change by MG. On the other hand, when ground under the initial partial hydrogen pressure of 0.2 MPa, YNi2Hx is separated into two phases. The one is the α-phase, in which the solubility of a hydrogen is larger than that obtained by hydriding YNi2 without MG under the same condition, and the other is the α′-phase. The solubility of a hydrogen in α-phase gradually decreases with increasing the MG time, while the α′-phase is transformed into the amorphous phase upon further grinding. After grinding it for 1080 min, the α-phase is changed into YNi5. Excess Y left in the phase transformation will be dissolved into the amorphous phase and it will react with hydrogen to form YH2. These differences of the amorphization processes depending on the initial hydrogen pressures can be understood by considering the free energy variation in each phase by MG.

Hydrogen-induced amorphization of intermetallics

This paper describes our investigations on hydrogen-induced amorphization (HIA) in intermetallics caused by hydrogenation. The crystal structures and chemical compositions of amorphizing intermetallics are presented at first. In addition, the formation conditions and the formation processes of amorphous alloys, the mechanism of HIA, the factors controlling the occurrence of HIA and the relation between the structures of the hydrogen-induced amorphous alloys and the stability of C15 Laves phases are given.

Hydrogen-induced amorphization of titanium-based intermetallic compounds with the D019 structure

Structural changes of titanium-based intermetallic compounds with the D019 structure, i.e. Ti3Ga, Ti3In, Ti3Sn and Ti3Al caused by hydrogenation have been investigated by X-ray diffractometry and transmission electron microscopy. Crystallization and hydrogen desorption of the hydrogen-induced amorphous alloys have been examined by differential scanning calorimetry and hydrogen analysis. Both Ti3Ga and Ti3In amorphize by hydrogenation at room temperature, while Ti3Al and Ti3Sn absorb hydrogen in the crystalline state. However, Ti3Al alloyed with Zr or Hf amorphize by hydrogenation below about 500 K. The mechanism and the factors controlling hydrogen-induced amorphization of the D019 compounds are discussed.

Hydrogen-induced structural changes of Fe2Ce alloys

The amorphization kinetics of the Fe2Ce Laves phase were studied, utilizing X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermal H desorption measurements. Additionally, the local H diffusion process in this ferromagnetic compound was investigated by means of magnetic susceptibility and magnetic after-effect (MAE) measurements. Fe2Ce can easily be amorphized (pH2 = 5 × 103to1 × 105Pa, T < 400K); irreversible hydrogen-induced amorphization takes place in a narrow phase boundary which migrates from the surface into the sample. Furthermore, cyclic H-charging of amorphous Fe2Ce below its crystallization temperature reveals a severe degradation in H-storage capacity from 53 at.% to 18 at.% after only three cycles. This degradation is attributed to a hydrogen-induced phase separation into crystalline CeH2 and α-Fe which is enhanced by the high mobility of Fe atoms.

Hydrogen diffusion in DyFe2 and its effect on magnetic properties

For the DyFe2 Laves phase intermetallic compound, the nature of hydrogen-induced amorphization, structural changes and the dynamic behavior of hydrogen atoms were studied by means of X-ray diffraction, magnetic after-effect and thermal desorption spectrometry. DyFe2 can be amorphized by hydrogen charging within the narrow temperature range between 523 K and 593 K, even at a low hydrogen pressure of 0.1 MPa. At lower temperatures, DyFe2 forms a crystalline hydride, whereas it decomposes into α-Fe and DyH2 at higher temperatures. From the relaxation spectra obtained in magnetic after-effect measurements, it is found that hydrogen atoms in the crystalline hydride occupy the tetrahedral interstitial sites consisting of DyFe3 and Dy2Fe2 with activation energies of short-range diffusion determined as 0.24 eV and 0.35 eV respectively. For the amorphous hydride, hydrogen atoms additionally occupy Dy3Fe sites with an activation energy of 0.43 eV, and the distribution of activation energies becomes broader.

Differential thermal analysis of hydrogen-induced amorphization in C15 Laves compounds RCo 2

Structural changes of the C15 Laves compounds RCo 2 (Rrare earth metals) during differential thermal analysis (DTA) in a hydrogen atmosphere have been investigated by X-ray diffractometry, transmission electron microscopy and differential scanning calorimetry. The results of TbCo 2 and CeCo 2 are exemplified. The DTA curves of the RCo 2 compounds exhibit three or four exothermic peaks resulting from hydrogen absorption, hydrogen-induced amorphization, crystallization and grain growth. As the atomic number of R increases, the amorphization temperature T a increases, but the ratio T a/ T m, where T m is the melting point of the original compound, is nearly constant at 0.4. The mechanisms of hydrogen-induced amorphization in the RCo 2 compounds is discussed.

Formation of the amorphous phase in ErCo 2 C15 Laves compounds by hydrogen absorption

The hydrogen-induced amorphization of an ErCo 2 C15 Laves compound has been observed by X-ray diffraction measurements, transmission election microscopy and scanning election microscopy. Kinetic studies on the hydrogen-induced amorphization in ErCo 2 have been performed by the internal standard method from X-ray intensities. The activation energy for the amorphization in ErCo 2 has been calculated to be 26 ± 1.4 kcal/mol. It is suggested that the amorphization mechanism is related to the short range motion of Co atoms. From the investigations of the amorphization temperatures of Er(Co 0.9M 0.1) 2 compounds (M = Cr, Mn, Fe, Ni, Cu), the amorphization temperature has been determined to be proportional to the atomic radius of a substituting transition element. This proportionality indicates that the atomic radius which is an important parameter in diffusion is the factor affecting the hydrogen-induced amorphization.

Variations of the amorphization temperature and particle size by hydrogen absorption in Er(Co1−xFex)2 pseudo-binary laves compounds

In this article, the amorphization temperatures and particle sizes of the pseudo-binary Er(Co[sub 1[minus]x]Fe[sub x])[sub 2] compounds are investigated as a function of the alloy composition, and related with hydrogen content and elastic modulus. Previously, it was reported that the ternary Er-Co-Fe system formed a homogeneous single phase with C15 cubic structure over the whole concentration range and absorbed a large amount of hydrogen at ambient conditions. Hydrogen concentration in Er-Co-Fe system had a maximum at the composition of x = 0.6, Er(Co[sub 0.4]Fe[sub 0.6])[sub 2]. Thus, the possibility exists of hydrogen-induced amorphization, and also of an effect of hydrogen concentration on the amorphization behavior.

A correlation between stability of compounds and structure of hydrogen-induced amorphous alloys in GdM 2 (M = Fe, Co, Ni)

Enthalpy changes ΔH for hydrogen absorption and desorption, hydrogen-induced amorphization (HIA) in C15 Laves phases c-GdM 2 (M = Fe, Co, Ni) and crystallization of the resultant amorphous alloys a-GdM 2H x have been measured by differential scanning calorimetry (DSC). Furthermore, structures of a-GdM 2H x have been investigated by X-ray diffractometry (XRD) to correlate their structures with the stabilities of the original compounds. Both a-GdFe 2H 3.0 and a-GdCo 2H 3.4 prepared from the less stable compounds show the strong tendency for the same kinds of metal atoms to cluster and crystallize to GdH 2 and Fe(Co). Correspondingly, the sum of ΔH for all reactions become equal to that for the formation of GdH 2. On the contrary, a-GdNi 2H 3.5 prepared from the stable compound shows a rather homogeneous structure and crystallizes to GdH 2 and GdH 2 and GdNi 5. As a result, the sum of ΔH shows a large negative value in comparison with that for the formation of GdH 2.

Local structure evolution in hydrogen-induced amorphization of YFe 2 and YNi 2 Laves phases

Although both YFe 2 and YNi 2 crystalline compounds have a C15 Laves phase structure, YFe 2 forms crystalline YFe 2D 4.3, while YNi 2 is transformed into amorphous YNi 2D 3.6 on absorbing deuterium atoms under a high pressure of D 2 gas at room temperature. The characteristic evolution in the short-range structure of metallic atoms and the chemical environment around a deuterium atom during the hydrogen-induced solid state amorphizing process are examined by X-ray and neutron diffraction.

Differential scanning calorimetric study of hydrogen–induced amorphization in C15 Laves phase GdCo2

Differential scanning calorimetry and x-ray diffractometry have been used to monitor reactions of the C15 Laves phase GdCo2 during constant-rate heating in a hydrogen atmosphere and to determine the enthalpy change of each reaction. The values of the enthalpy change ΔH of hydrogen absorption, hydrogen-induced amorphization and crystallization are evaluated to be −67.2, −10.2, and −9.8 kJ/mol Gd, respectively. On the other hand, ΔH of hydrogen desorption is +31.4 kJ/mol Gd. The amorphous a-GdCo2Hx alloy crystallizes to GdH2 and Co. Therefore, the total value of ΔH of the above reactions along with the formation of GdCo2, −103 kJ/mol Gd, becomes nearly equal to that of formation of GdH2, −101 kJ/mol Gd.

Hydrogen-induced amorphization of intermetallic compound Ti 3Ga with DO 19 structure

Hydrogen-induced amorphization of intermetallic compound Ti 3Ga with DO 19 structure

The mechanism of hydrogen-induced amorphization in intermetallic compounds

In this article, the amorphization mechanisms of intermetallic compounds by hydrogen absorption are discussed on the basis of reported experimental results. It is found that there are two different modes for amorphization. One is a gradual and thermally activated process, while the other is a strain-induced reaction during hydrogen absorption. As factors affecting the amorphization modes, the hydrogen concentration and the elastic property of the compound are discussed. Of these two factors the elastic property of the compound plays a key role on the amorphization. When an elastic strain exceeding the elastic limit was imposed on the compound, the crystal lattice was not deformed plastically but collapsed into the amorphous state because the brittleness of the compound did not permit plastic deformation.

Factors controlling hydrogen-induced amorphization of C15 Laves compounds

The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe 2H x (R = a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides A x B 1− x H y . However, the radii of the holes in the corresponding crystalline hydrides c-RFe 2H x are smaller than this value. Thus, the higher stabilities of the amorphous hydrides which are considered to be the origin for the driving force of hydrogen-induced amorphization (HIA) in the C15 Laves compounds are interpreted on the basis of the differences in the hole sizes. Furthermore, the occurrence of HIA in C15 AB 2 Laves compounds has been analyzed in terms of the tetrahedral hole size, the stability of the compounds, the atomic size ratio, the valence electron concentration and the contraction or the expansion of the constituent elements and the holes. The atomic size ratio is the single most important factor controlling the occurrence of HIA in these compounds, and the compounds with the ratio above 1.37 are amorphized by hydrogenation.

Infrared spectra of platinum-impregnated V 2O 5 powders on hydrogen insertion

Infrared spectra were measured in the wavenumber range 400–4000 cm −1 during the heating of platinum-impregnated V 2O 5 powders from room temperature to 250 °C in hydrogen gas (0.58 MPa) or on H x V 2O 5 ( x=0–3.27) by a Fourier transform IR diffuse reflectance spectrometer. It is found that the O-H stretching vibration mode at 3372 cm −1 becomes enlarged as the temperature or hydrogen concentration is increased, whereas the V-O stretching vibration modes tend to disappear, and that a new IR band assigned to the O-H bending mode appears at 920 cm −1; no clear evidence for the existence of V-H hydride bonds is detected. These spectral changes suggest a hydrogen-induced amorphization through bond rearrangements. In fact, the Bragg reflection lines due to V 2O 5 tend to be invisible with increasing amount of absorbed hydrogen. The hydrogen-induced amorphization in V 2O 5 is discussed in some detail in the light of information about the structural changes in V 2O 5 on hydrogen insertion and after heat treatment.