Showing papers on "Laves phase published in 1999"

Hydrogen-absorbing alloys

Abstract: Improvement of hydrogen capacity in hydrogen-absorbing alloys has been achieved in recent years. Mg-based alloys which were synthesized by ball milling showed lower dehydrogenation temperatures than intermetallic Mg-based alloys. This technique is also effective for preparing a novel Mg-based amorphous alloy, MgNi, and its hydride. Besides conventional intermetallic compounds such as LaNi5, solid solution alloy, ‘Laves phase related BCC solid solution’ with body-centered-cubic structure showed a hydrogen capacity of 2.2 mass% at room temperature. Alanate, which is not an interstitial hydride, was found to react with gaseous hydrogen reversibly with a catalyst, and its hydrogen capacity was more than 3 mass%.

Protium Absorption-Desorption Properties of Ti–V–Cr Alloys with a BCC Structure

Abstract: Vanadium-based BCC alloys are known to absorb a high amount of protium. But, they do not desorb absorbed-protium completely. This paper aims to clarify the effects of alloy composition and heat-treatment on the protium absorption-desorption properties of Ti-(0-35) at%V-Cr alloys. A Ti-35V-40Cr alloy was chosen as the starting composition. The plateau pressure of the Ti-35V-(40+x)Cr (x=-3, 0, 3) alloys increased with increasing Cr content. For the alloys containing more than 15 at%V, a BCC phase was formed in the as-cast state, which was found to have an effective protium capacity of about 2.4 mass%H. However, a Laves phase was formed for alloys containing less than 10 at%V, which only had an effective protium capacity of about 1.8 mass%H. The effect of heat-treatment on the microstructures and protium absorption-desorption properties of the Ti-35V-40Cr alloy were studied. It was found that the optimum condition for heat-treatment was annealing at 1573 K for I min. The Ti-35V-40Cr alloy heat-treated under this condition exhibited the highest effective protium capacity of 2.6 mass%H. This effective protium capacity is higher than any other reported values at 313 K. However, further annealing at 1573 K led to a decrease in protium capacity, which may result from an increase in the amount of a Ti-rich phase. A Ti-10V-55.4Cr alloy, which contained a Laves phase in the as-cast state, became single-phase BCC after heat-treatment. Even though the alloy contains only 10 at%V, this heat-treated alloy showed an effective protium capacity of 2.5 mass%H, which is higher than any other reported values for BCC alloys.

Microstructure and mechanical properties of two-phase Cr–Cr2Nb, Cr–Cr2Zr and Cr–Cr2(Nb,Zr) alloys

Abstract: The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below. or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr-Cr 2 X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr 2 X Laves phase in the Cr-Nb and Cr-Nb-Zr alloys, but not in the Cr-Zr alloys. The binary Cr 2 Nb phase consisted of an extensively twinned ({111} twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr 2 X Laves phase present. Age hardening due to solid-state precipitation of Cr 2 X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr 2 X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.

A thermodynamic interpretation of the size-ratio limits for laves phase formation

Abstract: Laves phases with AB{sub 2} compositions are a common type of topologically close-packed structures, with three polytypes typically observed: cubic C15 (MgCu{sub 2}), hexagonal C14 (MgZn{sub 2}), and dihexagonal C36 (MgNi{sub 2}). There are over 360 binary Laves phases reported so far. Such an abundance of Laves phases has been attributed to the geometric principles for the ordered arrangement of atoms on lattice sites. Considering the great amount of research devoted to Laves phases, thermodynamic information of Laves phases is very limited and no review or compilation of such information is available, even though such information is imperative for understanding the bonding characteristics, phase stability, point defect mechanism, and mechanical properties of Laves phases. The fact that Laves phases can only be stabilized within an R{sub A}/R{sub B} range of 1.05 to 1.67 is obviously related to the geometric packing condition in the Laves phases. However, a systematic analysis of thermodynamic information, especially enthalpies of formation of Laves phases, may offer a new approach to understand this size-ratio effect in controlling Laves phase formation.

Quasielastic neutron scattering study of hydrogen motion in C14- and C15-type ZrCr2Hx

Abstract: In order to clarify the mechanisms of hydrogen diffusion in the cubic (C15) and hexagonal (C14) modifications of Laves phase ZrCr2, we have performed high-resolution quasielastic neutron scattering measurements on C15-ZrCr2H045 and C14-ZrCr2H05 over the temperature range 10-340 K It is found that in both systems the diffusive motion of hydrogen can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial (ZrCr2) sites and the slower hopping from one hexagon to the other The experimental results are analysed to determine the hydrogen hopping rates and the tracer diffusion coefficients as functions of temperature The motional parameters of hydrogen in the C15- and C14-type samples are found to be close to each other Comparison of the motional parameters of hydrogen in ZrCr2Hx and in the related C15-type TaV2Hx shows that the localized H motion in ZrCr2 is slower, whereas the long-range H diffusion is much faster than in TaV2 These features are consistent with the difference between intersite distances in the hydrogen sublattices of ZrCr2Hx and TaV2Hx

Separation of the 4 f -spin, 4 f -orbital, and conduction-electron magnetization from exotic thermomagnetic behavior for ferromagnetic Sm intermetallics

Abstract: The thermomagnetic curves of the ferromagnetic Sm intermetallics, polycrystalline samples of the CsCl-type compounds SmZn and SmCd, and a single crystal of the cubic Laves phase compound ${\mathrm{SmAl}}_{2}$, have been measured in a temperature range from 2 to 300 K, and the exotic behavior has been well explained with the model of ${\mathrm{Sm}}^{3+}$ in a metallic matrix. The $M\ensuremath{-}T$ curves in the ordered states for these compounds, just like those of ferrimagnets, reflect the different temperature dependences of the spin moment and the orbital one, which is due mainly to the thermal variation of the admixture among the J multiplets of the $4f$ electrons through the exchange field. Utilizing this property, the separation of the $4f$-spin, $4f$-orbital, and conduction-electron component from the total ordered moment has been successfully done. Some microscopic properties of these compounds, such as the difference between the conduction-electron polarization effect in the ordered state and that in the paramagnetic one, have been also suggested through the analysis.

New magnetic phenomena in TbNi2

Abstract: Structural investigations at room temperature revealed that TbNi2 does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at TR = 14 K below the Curie temperature of TC = 36±0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above TR and TC was performed. The analysis of these data showed that this transition at TR is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.

Hysteresis and magnetostriction of TbxDyyHo1−x−yFe1.95 [112] dendritic rods

Abstract: The magnetization and magnetostriction of a variety of 3/16-in.-diam Laves phase rods of TbxDyyHo1−x−yFe1.95 grown in the form of [112] oriented dendritic compounds were measured as a function of applied magnetic field −3000

Structure and magnetostriction of PrxDy1-xFe2 and Pr0.4Dy0.6(Fe1-yMy)(2) alloys (M = Co, Ni)

Abstract: The structure, Curie temperature and magnetostriction of PrxDy1−xFe2 (0⩽x⩽0.5) and Pr0.4Dy0.6(Fe1−yMy)2 (0⩽y⩽0.6) alloys (M=Co, Ni) have been investigated using optical microscopy, x-ray diffraction, ac initial susceptibility and standard strain gauge techniques. The matrix of homogenized PrxDy1−xFe2 alloys is a cubic Laves phase (Pr, Dy)Fe2 with MgCu2-type structure, with a small amount of second phase (Pr, Dy)Fe3 when x⩽0.2. The amount of (Pr, Dy)Fe3 phase increases with the increase of Pr content, and it becomes the main phase when x=0.4. When x=0.5, the matrix is found to be the (Pr, Dy)2Fe17 phase coexisting with a small amount of phases (Pr, Dy)Fe2, (Pr, Dy)Fe3 and rare-earth rich phases. For Pr0.4Dy0.6(Fe1−yCoy)2 alloys, the amount of (Pr, Dy)(Fe, Co)2 phase increases with increasing Co content and the phase (Pr, Dy)(Fe, Co)2 becomes the main phase when y=0.6. However, the substitution of Ni for Fe up to 60 at % Ni in Pr0.4Dy0.6Fe2 alloys does not favor the formation of the cubic Laves phase (Pr, D...

On the anomalous pinning properties of the C15 Laves phase superconductor CeRu2

Abstract: We report the anomalous field (H) dependence of pinning force density F P as deduced from our magnetization measurements on the single-crystal and polycrystalline samples of CeRu 2 . It is shown that, contrary to a single maximum in other hard type II superconductors, CeRu 2 samples exhibit two distinct peak features in their F P -H plots. Our results show that the scaling of F P with the applied magnetic field at different temperatures cannot be described by the conventional Kramer model based on crossover from a line-breaking regime to a flux-line lattice shearing regime.

Phase relation and microstructure of the Nb–Cr–W alloy system

Abstract: The phase diagram of the Nb–Cr–W alloy system at 1623 K is constructed by means of X-ray diffraction, optical microscopy and transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy. The NbCr2 Laves phase equilibrates directly with b.c.c. phases with wider range of compositions without forming any intermediate phases. The element W is soluble by about 8at.% in the NbCr2 Laves phase at 1623 K. The Laves phase field extends in a direction such that W substitutes for the Nb site in NbCr2. The metastable C36 and C14 Laves phases in addition to the C15 Laves phase are identified in the as-cast material of the ternary Laves alloy by high resolution transmission electron microscopic observation. It is suggested that the addition of W to NbCr2 reduces the stability of the C15 Laves phase. The alloying behavior of element W in the C15 Laves phase or the retained C36 Laves phase, whether it substitutes for Nb site or Cr site, depends upon the prepared alloy compositions.

Ab initio studies of the formation of a Y1-xNi2 superstructure with ordered Y vacancies

Abstract: Ab initio total-energy calculations have been performed to study the structural stability of Y1-xNi2. In the literature (Villars P and Calvert L D 1985 Pearson's Handbook of Crystallographic Data for Intermetallic Phases (Materials Park, OH: American Society for Metals)) YNi2 is often considered to show the cubic Laves phase structure, but x-ray diffraction experiments of Latroche et al ( J. Less-Common Met. 161 L27) showed that YNi2 crystallizes in a superstructure of C15 with ordered Y vacancies with a stoichiometry of approximately Y0.95Ni2. The total-energy calculations for the superstructure and for the ideal C15 structure, as well as for the neighbouring phases in the Y-Ni phase diagram YNi and YNi3, confirm that the formation of the superstructure with Y vacancies is favoured against the formation of the pure C15 compound YNi2. The calculated relaxation of the atoms around the vacancies is also in good agreement with the experimental results (Latroche et al), demonstrating that the relaxation of strains in the Y sublattice is the driving mechanism for formation of vacancies. In addition, the electronic properties of the vacancy superstructure have been examined.