Kiyoshi Aoki

Amorphization of the CeFe2 Laves phase compound by hydrogen absorption

Abstract Structural changes of the CeFe2 Laves phase compound during the hydrogen absorption and desorption process have been investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and magnetic property measurements. The transition from the crystalline to the amorphous state has been confirmed to occur below about 500 K, where the formation of the elemental hydride, CeH2, is virtually hindered. The whole crystalline phase does not change to the amorphous state in a uniform manner, but a part of the crystalline phase changes to the amorphous one which grows with further hydrogenation. The requirements for alloys to undergo hydrogen-induced amorphization were discussed. The proposed requirements are for alloys 1. (i) to have the Laves structure, 2. (ii) to contain hydride-forming elements 3. (iii) to have either a low melting point or a tendency to decompose into other intermetallic compounds.

Formation of amorphous aluminum tantalum nitride powders by mechanical alloying

Aluminum tantalum nitride amorphous alloy powders have been synthesized by a high energy ball mill under purified nitrogen gas (N2) flow at room temperature. The alloy powders were characterized by means of transmission electron microscopy, differential thermal analysis, and chemical analysis. An amorphous phase containing 18 at.u2009% N2 was obtained after 65 ks of the milling time. At the final stage of milling (72 ks) the crystallization temperature, Tx, and enthalpy change of crystallization, ΔHx, are 1115 K and −95 kJ mol−1, respectively.

Calorimetric and morphological studies of mechanically alloyed Al‐50 at. % transition metal prepared by the rod‐milling technique

The mode of amorphization and crystallization of mechanically alloyed Al‐TM (TM; Zr, Nb, and Ta) has been studied by means of differential thermal analysis, differential scanning calorimetry, optical metallography, scanning electron microscopy, and transmission electron microscopy. The mechanical alloying process via the rod‐milling technique is classified into three stages of milling. At the early stage of milling, the elemental powders of Al and TM are grown to form layered‐composite particles of a larger diameter as a result of cold welding. At the intermediate stage, a complete crystalline‐to‐amorphous transformation occurs at around 700 K, by heating the well‐arranged layered particles in a differential thermal analyzer under an Ar atmosphere. This transformation occurs due to a thermally assisted solid‐state amorphization between the layers of the elemental starting material in the composite particles. At the final stage of milling, an amorphous phase is formed by the mechanical driving force which ...

Thermally assisted solid state amorphization of rod milled Al50Nb50 alloy

Differential thermal analysis, differential scanning calorimetry, optical metallography, scanning electron microscopy, and transmission electron microscopy were used to study the amorphization and crystallization processes of mechanically alloyed Al50Nb50 powder prepared by the rod‐milling technique. The results have shown that the crystalline‐to‐amorphous transformation occurs in three stages. At the intermediate stage of milling, the transformation from crystalline into amorphous was conducted by heating the alloy to 800 K. This transformation has occurred due to a thermally assisted solid‐state amorphization between the layers of the elemental starting material of the composite particles. At the final stage of milling, the mechanical driving force which was generated by the rods causes the formation of a homogeneous and uniform amorphous alloy. The amorphization temperature, Ta, and the crystallization temperature, Tx, are determined to be 650 and 1105 K, respectively. Moreover, the enthalpy change of ...

Thermodynamics of hydrogen absorption in amorphous ZrNi alloys

Abstract The relative partial molar enthalpy and entropy for the absorption of hydrogen were obtained from the pressure-composition isotherms for both amorphous and crystalline Zrue5f8Ni alloys. The quantities of Δ S H and Δ S H in the amorphous state show the minimum at certain hydrogen content and then remain fairly constant as the hydrogen content is further increased. The characteristic variations in Δ S H for the amorphous Zrue5f8Ni alloys may be attributed to the changes in the local environmental structure around hydrogen atoms, leading to an increase in the configurational entropy.

Factors controlling hydrogen-induced amorphization of C15 Laves compounds

Abstract The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe2Hx (R = a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides AxB1−xHy. However, the radii of the holes in the corresponding crystalline hydrides c-RFe2Hx 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 AB2 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.

Effect of ball-to-powder weight ratio on the amorphization reaction of Al50Ta50 by ball milling

Abstract The present study was undertaken in order to elucidate the effect of the ball-to-powder weight ratio on the amorphization reaction of Al 50 Ta 50 alloy powders using the ball milling technique. As the ball-to-powder weight ratio increases, the rate of amorphization reaction increases drastically, but the content of iron contamination, which comes from the milling tools, increases sharply. A complete amorphous phase is obtained with a ball-to-powder ratio in the range between 36:1 and 108:1 under the present experimental conditions.

Preparation of AlXTa1−X amorphous alloy powder by mechanical alloying

AlXTa1−X amorphous alloys were produced in the range of 0.10<X<0.90 by mechanical alloying of pure elemental powders of Al and Ta in Ar gas atmosphere at ambient temperature. The crystallization temperature increases monotonically with increasing Ta content to reach to an extraordinarily high value of 1194 K for Al10 Ta90 amorphous alloy.

Hydrogen‐induced amorphization and its effect on magnetic properties of the Laves‐phase GdFe2 compound

The influence of hydrogenation on the structural change and magnetic properties of the Laves‐phase GdFe2 compound has been investigated. Crystalline c‐GdFe2 absorbs 4.4 hydrogen atoms per formula unit without any change in the crystal structure below 423 K. On the other hand, GdFe2 transforms to amorphous a‐GdFe2H3.6 by hydrogenation from 423 to 523 K, where the decomposition of GdFe2 into GdH2 and α‐Fe is prevented. The same amount of hydrogen (3.6 atoms per formula unit) dissolves in amorphous alloys produced by both hydrogenation and rapid quenching. Hydrogenation sharply reduces the Curie temperature of the crystalline alloy from 818 to 107 K, in contrast to the Curie temperature of 443 K for the hydrogen‐induced amorphous alloy. The temperature dependence of the magnetization of the hydrogen‐induced amorphous alloy is identical to that of the hydrogenated rapidly quenched amorphous alloy, suggesting similarity of their structures. The magnetization of the hydrogen‐induced amorphous alloy shows a broa...

Morphological and calorimetric studies on the

Amorphous Al50Zr50 alloy powders have been prepared by rod-milling technique using mechanical alloying (MA) method. The amorphization and crystallization processes of the alloyed powders were followed by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The results have shown that the formation of amorphous Al50Zr50 alloy powders occurs through three stages, agglomeration, disintegration, and homogenization. At the disintegration stage, the alloyed powders contain many fine layers of Al and Zr. An amorphous phase has been formed at about 880 K as a result of heating these layered particles in a thermal analyzer. The crystalline-to-amorphous transformation at this stage of milling is attributed to a thermally assisted solid-state amorphizing reaction. The present study corroborates the similarity of the amorphization process through the MA with the solid-state interdiffusion reaction in multilayered thin films. The amorphization temperature, Ta, and the activation energy of amorphization, Ea, are 675 and 156 kJ/mol, respectively. In addition, the enthalpy change of amorphization, ΔHa, was evaluated to be -3.5 kJ/mol. On the other hand, the crystallization temperature, Tx, and enthalpy change of crystallization, ΔHx, were 1000 K and −68 kJ/mol, respectively.